osu!taiko penalty for frequent rhythm changes with long gaps (#37200)

Co-authored-by: Jay Lawton <jaime@merkids.com>
Co-authored-by: James Wilson <tsunyoku@gmail.com>

Templates/Rulesets/ruleset-empty/osu.Game.Rulesets.EmptyFreeform/EmptyFreeformDifficultyCalculator.cs

@@ -19,13 +19,13 @@ namespace osu.Game.Rulesets.EmptyFreeform
         {
         }
 
-        protected override DifficultyAttributes CreateDifficultyAttributes(IBeatmap beatmap, Mod[] mods, Skill[] skills, double clockRate)
+        protected override DifficultyAttributes CreateDifficultyAttributes(IBeatmap beatmap, Mod[] mods, Skill[] skills)
         {
             return new DifficultyAttributes(mods, 0);
         }
 
-        protected override IEnumerable<DifficultyHitObject> CreateDifficultyHitObjects(IBeatmap beatmap, double clockRate) => Enumerable.Empty<DifficultyHitObject>();
+        protected override IEnumerable<DifficultyHitObject> CreateDifficultyHitObjects(IBeatmap beatmap, Mod[] mods) => Enumerable.Empty<DifficultyHitObject>();
 
-        protected override Skill[] CreateSkills(IBeatmap beatmap, Mod[] mods, double clockRate) => Array.Empty<Skill>();
+        protected override Skill[] CreateSkills(IBeatmap beatmap, Mod[] mods) => Array.Empty<Skill>();
     }
 }

Templates/Rulesets/ruleset-example/osu.Game.Rulesets.Pippidon/PippidonDifficultyCalculator.cs

@@ -19,13 +19,13 @@ namespace osu.Game.Rulesets.Pippidon
         {
         }
 
-        protected override DifficultyAttributes CreateDifficultyAttributes(IBeatmap beatmap, Mod[] mods, Skill[] skills, double clockRate)
+        protected override DifficultyAttributes CreateDifficultyAttributes(IBeatmap beatmap, Mod[] mods, Skill[] skills)
         {
             return new DifficultyAttributes(mods, 0);
         }
 
-        protected override IEnumerable<DifficultyHitObject> CreateDifficultyHitObjects(IBeatmap beatmap, double clockRate) => Enumerable.Empty<DifficultyHitObject>();
+        protected override IEnumerable<DifficultyHitObject> CreateDifficultyHitObjects(IBeatmap beatmap, Mod[] mods) => Enumerable.Empty<DifficultyHitObject>();
 
-        protected override Skill[] CreateSkills(IBeatmap beatmap, Mod[] mods, double clockRate) => Array.Empty<Skill>();
+        protected override Skill[] CreateSkills(IBeatmap beatmap, Mod[] mods) => Array.Empty<Skill>();
     }
 }

Templates/Rulesets/ruleset-scrolling-empty/osu.Game.Rulesets.EmptyScrolling/EmptyScrollingDifficultyCalculator.cs

@@ -19,13 +19,13 @@ namespace osu.Game.Rulesets.EmptyScrolling
         {
         }
 
-        protected override DifficultyAttributes CreateDifficultyAttributes(IBeatmap beatmap, Mod[] mods, Skill[] skills, double clockRate)
+        protected override DifficultyAttributes CreateDifficultyAttributes(IBeatmap beatmap, Mod[] mods, Skill[] skills)
         {
             return new DifficultyAttributes(mods, 0);
         }
 
-        protected override IEnumerable<DifficultyHitObject> CreateDifficultyHitObjects(IBeatmap beatmap, double clockRate) => Enumerable.Empty<DifficultyHitObject>();
+        protected override IEnumerable<DifficultyHitObject> CreateDifficultyHitObjects(IBeatmap beatmap, Mod[] mods) => Enumerable.Empty<DifficultyHitObject>();
 
-        protected override Skill[] CreateSkills(IBeatmap beatmap, Mod[] mods, double clockRate) => Array.Empty<Skill>();
+        protected override Skill[] CreateSkills(IBeatmap beatmap, Mod[] mods) => Array.Empty<Skill>();
     }
 }

Templates/Rulesets/ruleset-scrolling-example/osu.Game.Rulesets.Pippidon/PippidonDifficultyCalculator.cs

@@ -19,13 +19,13 @@ namespace osu.Game.Rulesets.Pippidon
         {
         }
 
-        protected override DifficultyAttributes CreateDifficultyAttributes(IBeatmap beatmap, Mod[] mods, Skill[] skills, double clockRate)
+        protected override DifficultyAttributes CreateDifficultyAttributes(IBeatmap beatmap, Mod[] mods, Skill[] skills)
         {
             return new DifficultyAttributes(mods, 0);
         }
 
-        protected override IEnumerable<DifficultyHitObject> CreateDifficultyHitObjects(IBeatmap beatmap, double clockRate) => Enumerable.Empty<DifficultyHitObject>();
+        protected override IEnumerable<DifficultyHitObject> CreateDifficultyHitObjects(IBeatmap beatmap, Mod[] mods) => Enumerable.Empty<DifficultyHitObject>();
 
-        protected override Skill[] CreateSkills(IBeatmap beatmap, Mod[] mods, double clockRate) => Array.Empty<Skill>();
+        protected override Skill[] CreateSkills(IBeatmap beatmap, Mod[] mods) => Array.Empty<Skill>();
     }
 }

osu.Game.Rulesets.Catch/Difficulty/CatchDifficultyCalculator.cs

@@ -15,6 +15,7 @@ using osu.Game.Rulesets.Difficulty;
 using osu.Game.Rulesets.Difficulty.Preprocessing;
 using osu.Game.Rulesets.Difficulty.Skills;
 using osu.Game.Rulesets.Mods;
+using osu.Game.Utils;
 
 namespace osu.Game.Rulesets.Catch.Difficulty
 {
@@ -22,8 +23,6 @@ namespace osu.Game.Rulesets.Catch.Difficulty
     {
         private const double difficulty_multiplier = 4.59;
 
-        private float halfCatcherWidth;
-
         public override int Version => 20251020;
 
         public CatchDifficultyCalculator(IRulesetInfo ruleset, IWorkingBeatmap beatmap)
@@ -31,7 +30,7 @@ namespace osu.Game.Rulesets.Catch.Difficulty
         {
         }
 
-        protected override DifficultyAttributes CreateDifficultyAttributes(IBeatmap beatmap, Mod[] mods, Skill[] skills, double clockRate)
+        protected override DifficultyAttributes CreateDifficultyAttributes(IBeatmap beatmap, Mod[] mods, Skill[] skills)
         {
             if (beatmap.HitObjects.Count == 0)
                 return new CatchDifficultyAttributes { Mods = mods };
@@ -46,12 +45,19 @@ namespace osu.Game.Rulesets.Catch.Difficulty
             return attributes;
         }
 
-        protected override IEnumerable<DifficultyHitObject> CreateDifficultyHitObjects(IBeatmap beatmap, double clockRate)
+        protected override IEnumerable<DifficultyHitObject> CreateDifficultyHitObjects(IBeatmap beatmap, Mod[] mods)
         {
             CatchHitObject? lastObject = null;
 
             List<DifficultyHitObject> objects = new List<DifficultyHitObject>();
 
+            double clockRate = ModUtils.CalculateRateWithMods(mods);
+
+            float halfCatcherWidth = Catcher.CalculateCatchWidth(beatmap.Difficulty) * 0.5f;
+
+            // For circle sizes above 5.5, reduce the catcher width further to simulate imperfect gameplay.
+            halfCatcherWidth *= 1 - (Math.Max(0, beatmap.Difficulty.CircleSize - 5.5f) * 0.0625f);
+
             // In 2B beatmaps, it is possible that a normal Fruit is placed in the middle of a JuiceStream.
             foreach (var hitObject in CatchBeatmap.GetPalpableObjects(beatmap.HitObjects))
             {
@@ -68,16 +74,11 @@ namespace osu.Game.Rulesets.Catch.Difficulty
             return objects;
         }
 
-        protected override Skill[] CreateSkills(IBeatmap beatmap, Mod[] mods, double clockRate)
+        protected override Skill[] CreateSkills(IBeatmap beatmap, Mod[] mods)
         {
-            halfCatcherWidth = Catcher.CalculateCatchWidth(beatmap.Difficulty) * 0.5f;
-
-            // For circle sizes above 5.5, reduce the catcher width further to simulate imperfect gameplay.
-            halfCatcherWidth *= 1 - (Math.Max(0, beatmap.Difficulty.CircleSize - 5.5f) * 0.0625f);
-
             return new Skill[]
             {
-                new Movement(mods, halfCatcherWidth, clockRate),
+                new Movement(mods),
             };
         }
 

osu.Game.Rulesets.Catch/Difficulty/Evaluators/MovementEvaluator.cs

@@ -11,12 +11,16 @@ namespace osu.Game.Rulesets.Catch.Difficulty.Evaluators
     {
         private const double direction_change_bonus = 21.0;
 
-        public static double EvaluateDifficultyOf(DifficultyHitObject current, double catcherSpeedMultiplier)
+        public static double EvaluateDifficultyOf(DifficultyHitObject current)
         {
             var catchCurrent = (CatchDifficultyHitObject)current;
             var catchLast = (CatchDifficultyHitObject)current.Previous(0);
             var catchLastLast = (CatchDifficultyHitObject)current.Previous(1);
 
+            // In catch, clockrate adjustments do not only affect the timings of hitobjects,
+            // but also the speed of the player's catcher, which has an impact on difficulty
+            double catcherSpeedMultiplier = current.ClockRate;
+
             double weightedStrainTime = catchCurrent.StrainTime + 13 + (3 / catcherSpeedMultiplier);
 
             double distanceAddition = (Math.Pow(Math.Abs(catchCurrent.DistanceMoved), 1.3) / 510);
@@ -40,6 +44,30 @@ namespace osu.Game.Rulesets.Catch.Difficulty.Evaluators
                                     / (CatchDifficultyHitObject.NORMALIZED_HALF_CATCHER_WIDTH * 6) / sqrtStrain;
             }
 
+            // Linear spacing nerf.
+            double linearSpacingCount = 0;
+
+            for (int i = 0; i < Math.Min(current.Index, 10); i++)
+            {
+                var catchPrevObj = (CatchDifficultyHitObject)catchCurrent.Previous(i);
+
+                // Only same direction movements matter as they do not take any additional inputs.
+                if (Math.Sign(catchCurrent.DistanceMoved) != Math.Sign(catchPrevObj.DistanceMoved) || catchCurrent.DistanceMoved == 0 || catchPrevObj.DistanceMoved == 0)
+                    break;
+
+                double currentSpacing = Math.Abs(catchCurrent.DistanceMoved / catchCurrent.StrainTime);
+                double prevSpacing = Math.Abs(catchPrevObj.DistanceMoved / catchPrevObj.StrainTime);
+
+                double relativeDifference = Math.Abs(currentSpacing / prevSpacing - 1);
+
+                if (relativeDifference > 0.05)
+                    break;
+
+                linearSpacingCount++;
+            }
+
+            distanceAddition *= Math.Pow(0.7, linearSpacingCount);
+
             // Bonus for edge dashes.
             if (catchCurrent.LastObject.DistanceToHyperDash <= 20.0f)
             {

osu.Game.Rulesets.Catch/Difficulty/Skills/Movement.cs

@@ -17,28 +17,14 @@ namespace osu.Game.Rulesets.Catch.Difficulty.Skills
 
         protected override int SectionLength => 750;
 
-        protected readonly float HalfCatcherWidth;
-
-        /// <summary>
-        /// The speed multiplier applied to the player's catcher.
-        /// </summary>
-        private readonly double catcherSpeedMultiplier;
-
-        public Movement(Mod[] mods, float halfCatcherWidth, double clockRate)
+        public Movement(Mod[] mods)
             : base(mods)
         {
-            HalfCatcherWidth = halfCatcherWidth;
-
-            // In catch, clockrate adjustments do not only affect the timings of hitobjects,
-            // but also the speed of the player's catcher, which has an impact on difficulty
-            // TODO: Support variable clockrates caused by mods such as ModTimeRamp
-            //  (perhaps by using IApplicableToRate within the CatchDifficultyHitObject constructor to set a catcher speed for each object before processing)
-            catcherSpeedMultiplier = clockRate;
         }
 
         protected override double StrainValueOf(DifficultyHitObject current)
         {
-            return MovementEvaluator.EvaluateDifficultyOf(current, catcherSpeedMultiplier);
+            return MovementEvaluator.EvaluateDifficultyOf(current);
         }
     }
 }

osu.Game.Rulesets.Mania/Difficulty/ManiaDifficultyCalculator.cs

@@ -19,6 +19,7 @@ using osu.Game.Rulesets.Mania.Scoring;
 using osu.Game.Rulesets.Mods;
 using osu.Game.Rulesets.Objects;
 using osu.Game.Rulesets.Scoring;
+using osu.Game.Utils;
 
 namespace osu.Game.Rulesets.Mania.Difficulty
 {
@@ -36,7 +37,7 @@ namespace osu.Game.Rulesets.Mania.Difficulty
             isForCurrentRuleset = beatmap.BeatmapInfo.Ruleset.MatchesOnlineID(ruleset);
         }
 
-        protected override DifficultyAttributes CreateDifficultyAttributes(IBeatmap beatmap, Mod[] mods, Skill[] skills, double clockRate)
+        protected override DifficultyAttributes CreateDifficultyAttributes(IBeatmap beatmap, Mod[] mods, Skill[] skills)
         {
             if (beatmap.HitObjects.Count == 0)
                 return new ManiaDifficultyAttributes { Mods = mods };
@@ -62,11 +63,13 @@ namespace osu.Game.Rulesets.Mania.Difficulty
             return 1;
         }
 
-        protected override IEnumerable<DifficultyHitObject> CreateDifficultyHitObjects(IBeatmap beatmap, double clockRate)
+        protected override IEnumerable<DifficultyHitObject> CreateDifficultyHitObjects(IBeatmap beatmap, Mod[] mods)
         {
             var sortedObjects = beatmap.HitObjects.ToArray();
             int totalColumns = ((ManiaBeatmap)beatmap).TotalColumns;
 
+            double clockRate = ModUtils.CalculateRateWithMods(mods);
+
             LegacySortHelper<HitObject>.Sort(sortedObjects, Comparer<HitObject>.Create((a, b) => (int)Math.Round(a.StartTime) - (int)Math.Round(b.StartTime)));
 
             List<DifficultyHitObject> objects = new List<DifficultyHitObject>();
@@ -88,7 +91,7 @@ namespace osu.Game.Rulesets.Mania.Difficulty
         // Sorting is done in CreateDifficultyHitObjects, since the full list of hitobjects is required.
         protected override IEnumerable<DifficultyHitObject> SortObjects(IEnumerable<DifficultyHitObject> input) => input;
 
-        protected override Skill[] CreateSkills(IBeatmap beatmap, Mod[] mods, double clockRate) => new Skill[]
+        protected override Skill[] CreateSkills(IBeatmap beatmap, Mod[] mods) => new Skill[]
         {
             new Strain(mods, ((ManiaBeatmap)Beatmap).TotalColumns)
         };

osu.Game.Rulesets.Osu.Tests/OsuDifficultyCalculatorTest.cs

@@ -34,6 +34,30 @@ namespace osu.Game.Rulesets.Osu.Tests
         public void TestClassicMod(double expectedStarRating, int expectedMaxCombo, string name)
             => Test(expectedStarRating, expectedMaxCombo, name, new OsuModClassic());
 
+        [TestCase(239, "diffcalc-test")]
+        [TestCase(54, "zero-length-sliders")]
+        [TestCase(4, "very-fast-slider")]
+        public void TestOffsetChanges(int expectedMaxCombo, string name)
+        {
+            const double offset_iterations = 400;
+            var beatmap = GetBeatmap(name);
+
+            var attributes = CreateDifficultyCalculator(beatmap).Calculate();
+            double expectedStarRating = attributes.StarRating;
+
+            for (int i = 0; i < offset_iterations; i++)
+            {
+                foreach (var beatmapHitObject in beatmap.Beatmap.HitObjects)
+                    beatmapHitObject.StartTime++;
+
+                attributes = CreateDifficultyCalculator(beatmap).Calculate();
+
+                // Platform-dependent math functions (Pow, Cbrt, Exp, etc) may result in minute differences.
+                Assert.That(attributes.StarRating, Is.EqualTo(expectedStarRating).Within(0.00001));
+                Assert.That(attributes.MaxCombo, Is.EqualTo(expectedMaxCombo));
+            }
+        }
+
         protected override DifficultyCalculator CreateDifficultyCalculator(IWorkingBeatmap beatmap) => new OsuDifficultyCalculator(new OsuRuleset().RulesetInfo, beatmap);
 
         protected override Ruleset CreateRuleset() => new OsuRuleset();

osu.Game.Rulesets.Osu/Difficulty/Evaluators/Aim/AgilityEvaluator.cs

@@ -0,0 +1,42 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System;
+using osu.Game.Rulesets.Difficulty.Preprocessing;
+using osu.Game.Rulesets.Osu.Difficulty.Preprocessing;
+using osu.Game.Rulesets.Osu.Objects;
+
+namespace osu.Game.Rulesets.Osu.Difficulty.Evaluators.Aim
+{
+    public static class AgilityEvaluator
+    {
+        private const double distance_cap = OsuDifficultyHitObject.NORMALISED_DIAMETER * 1.2; // 1.2 circles distance between centers
+
+        /// <summary>
+        /// Evaluates the difficulty of fast aiming
+        /// </summary>
+        public static double EvaluateDifficultyOf(DifficultyHitObject current)
+        {
+            if (current.BaseObject is Spinner)
+                return 0;
+
+            var osuCurrObj = (OsuDifficultyHitObject)current;
+            var osuPrevObj = current.Index > 0 ? (OsuDifficultyHitObject)current.Previous(0) : null;
+
+            double travelDistance = osuPrevObj?.LazyTravelDistance ?? 0;
+            double distance = travelDistance + osuCurrObj.LazyJumpDistance;
+
+            double distanceScaled = Math.Min(distance, distance_cap) / distance_cap;
+
+            double agilityDifficulty = distanceScaled * 1000 / osuCurrObj.AdjustedDeltaTime;
+
+            agilityDifficulty *= Math.Pow(osuCurrObj.SmallCircleBonus, 1.5);
+
+            agilityDifficulty *= highBpmBonus(osuCurrObj.AdjustedDeltaTime);
+
+            return agilityDifficulty;
+        }
+
+        private static double highBpmBonus(double ms) => 1 / (1 - Math.Pow(0.2, ms / 1000));
+    }
+}

osu.Game.Rulesets.Osu/Difficulty/Evaluators/Aim/FlowAimEvaluator.cs

@@ -0,0 +1,126 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System;
+using osu.Game.Rulesets.Difficulty.Preprocessing;
+using osu.Game.Rulesets.Difficulty.Utils;
+using osu.Game.Rulesets.Osu.Difficulty.Preprocessing;
+using osu.Game.Rulesets.Osu.Objects;
+using osuTK;
+
+namespace osu.Game.Rulesets.Osu.Difficulty.Evaluators.Aim
+{
+    public static class FlowAimEvaluator
+    {
+        private const double velocity_change_multiplier = 0.52;
+
+        /// <summary>
+        /// Evaluates difficulty of "flow aim" - aiming pattern where player doesn't stop their cursor on every object and instead "flows" through them.
+        /// </summary>
+        public static double EvaluateDifficultyOf(DifficultyHitObject current, bool withSliderTravelDistance)
+        {
+            if (current.BaseObject is Spinner || current.Index <= 1 || current.Previous(0).BaseObject is Spinner)
+                return 0;
+
+            var osuCurrObj = (OsuDifficultyHitObject)current;
+            var osuLastObj = (OsuDifficultyHitObject)current.Previous(0);
+            var osuLastLastObj = (OsuDifficultyHitObject)current.Previous(1);
+
+            double currDistance = withSliderTravelDistance ? osuCurrObj.LazyJumpDistance : osuCurrObj.JumpDistance;
+            double prevDistance = withSliderTravelDistance ? osuLastObj.LazyJumpDistance : osuLastObj.JumpDistance;
+
+            double currVelocity = currDistance / osuCurrObj.AdjustedDeltaTime;
+
+            if (osuLastObj.BaseObject is Slider && withSliderTravelDistance)
+            {
+                // If the last object is a slider, then we extend the travel velocity through the slider into the current object.
+                double sliderDistance = osuLastObj.LazyTravelDistance + osuCurrObj.LazyJumpDistance;
+                currVelocity = Math.Max(currVelocity, sliderDistance / osuCurrObj.AdjustedDeltaTime);
+            }
+
+            double prevVelocity = prevDistance / osuLastObj.AdjustedDeltaTime;
+
+            double flowDifficulty = currVelocity;
+
+            // Apply high circle size bonus to the base velocity.
+            // We use reduced CS bonus here because the bonus was made for an evaluator with a different d/t scaling
+            flowDifficulty *= Math.Sqrt(osuCurrObj.SmallCircleBonus);
+
+            // Rhythm changes are harder to flow
+            flowDifficulty *= 1 + Math.Min(0.25,
+                Math.Pow((Math.Max(osuCurrObj.AdjustedDeltaTime, osuLastObj.AdjustedDeltaTime) - Math.Min(osuCurrObj.AdjustedDeltaTime, osuLastObj.AdjustedDeltaTime)) / 50, 4));
+
+            if (osuCurrObj.Angle != null && osuLastObj.Angle != null)
+            {
+                double angleDifference = Math.Abs(osuCurrObj.Angle.Value - osuLastObj.Angle.Value);
+                double angleDifferenceAdjusted = Math.Sin(angleDifference / 2) * 180.0;
+                double angularVelocity = angleDifferenceAdjusted / (osuCurrObj.AdjustedDeltaTime * 0.1);
+
+                // Low angular velocity flow (angles are consistent) is easier to follow than erratic flow
+                flowDifficulty *= 0.8 + Math.Sqrt(angularVelocity / 270.0);
+            }
+
+            // If all three notes are overlapping - don't reward bonuses as you don't have to do additional movement
+            double overlappedNotesWeight = 1;
+
+            if (current.Index > 2)
+            {
+                double o1 = calculateOverlapFactor(osuCurrObj, osuLastObj);
+                double o2 = calculateOverlapFactor(osuCurrObj, osuLastLastObj);
+                double o3 = calculateOverlapFactor(osuLastObj, osuLastLastObj);
+
+                overlappedNotesWeight = 1 - o1 * o2 * o3;
+            }
+
+            if (osuCurrObj.Angle != null)
+            {
+                // Acute angles are also hard to flow
+                flowDifficulty += currVelocity *
+                                  SnapAimEvaluator.CalcAngleAcuteness(osuCurrObj.Angle.Value) *
+                                  overlappedNotesWeight;
+            }
+
+            if (Math.Max(prevVelocity, currVelocity) != 0)
+            {
+                if (withSliderTravelDistance)
+                {
+                    currVelocity = currDistance / osuCurrObj.AdjustedDeltaTime;
+                }
+
+                // Scale with ratio of difference compared to 0.5 * max dist.
+                double distRatio = DifficultyCalculationUtils.Smoothstep(Math.Abs(prevVelocity - currVelocity) / Math.Max(prevVelocity, currVelocity), 0, 1);
+
+                // Reward for % distance up to 125 / strainTime for overlaps where velocity is still changing.
+                double overlapVelocityBuff = Math.Min(OsuDifficultyHitObject.NORMALISED_DIAMETER * 1.25 / Math.Min(osuCurrObj.AdjustedDeltaTime, osuLastObj.AdjustedDeltaTime),
+                    Math.Abs(prevVelocity - currVelocity));
+
+                flowDifficulty += overlapVelocityBuff *
+                                  distRatio *
+                                  overlappedNotesWeight *
+                                  velocity_change_multiplier;
+            }
+
+            if (osuCurrObj.BaseObject is Slider && withSliderTravelDistance)
+            {
+                // Include slider velocity to make velocity more consistent with snap
+                flowDifficulty += osuCurrObj.TravelDistance / osuCurrObj.TravelTime;
+            }
+
+            // Final velocity is being raised to a power because flow difficulty scales harder with both high distance and time, and we want to account for that
+            flowDifficulty = Math.Pow(flowDifficulty, 1.45);
+
+            // Reduce difficulty for low spacing since spacing below radius is always to be flowed
+            return flowDifficulty * DifficultyCalculationUtils.Smootherstep(currDistance, 0, OsuDifficultyHitObject.NORMALISED_RADIUS);
+        }
+
+        private static double calculateOverlapFactor(OsuDifficultyHitObject first, OsuDifficultyHitObject second)
+        {
+            var firstBase = (OsuHitObject)first.BaseObject;
+            var secondBase = (OsuHitObject)second.BaseObject;
+            double objectRadius = firstBase.Radius;
+
+            double distance = Vector2.Distance(firstBase.StackedPosition, secondBase.StackedPosition);
+            return Math.Clamp(1 - Math.Pow(Math.Max(distance - objectRadius, 0) / objectRadius, 2), 0, 1);
+        }
+    }
+}

osu.Game.Rulesets.Osu/Difficulty/Evaluators/Aim/SnapAimEvaluator.cs

@@ -0,0 +1,220 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System;
+using osu.Framework.Extensions.ObjectExtensions;
+using osu.Game.Rulesets.Difficulty.Preprocessing;
+using osu.Game.Rulesets.Difficulty.Utils;
+using osu.Game.Rulesets.Osu.Difficulty.Preprocessing;
+using osu.Game.Rulesets.Osu.Objects;
+
+namespace osu.Game.Rulesets.Osu.Difficulty.Evaluators.Aim
+{
+    public static class SnapAimEvaluator
+    {
+        private const double wide_angle_multiplier = 9.67;
+        private const double acute_angle_multiplier = 2.41;
+        private const double slider_multiplier = 1.5;
+        private const double velocity_change_multiplier = 0.9;
+        private const double wiggle_multiplier = 1.02; // WARNING: Increasing this multiplier beyond 1.02 reduces difficulty as distance increases. Refer to the desmos link above the wiggle bonus calculation
+        private const double maximum_repetition_nerf = 0.15;
+        private const double maximum_vector_influence = 0.5;
+
+        /// <summary>
+        /// Evaluates the difficulty of aiming the current object, based on:
+        /// <list type="bullet">
+        /// <item><description>cursor velocity to the current object,</description></item>
+        /// <item><description>angle difficulty,</description></item>
+        /// <item><description>sharp velocity increases,</description></item>
+        /// <item><description>and slider difficulty.</description></item>
+        /// </list>
+        /// </summary>
+        public static double EvaluateDifficultyOf(DifficultyHitObject current, bool withSliderTravelDistance)
+        {
+            if (current.BaseObject is Spinner || current.Index <= 1 || current.Previous(0).BaseObject is Spinner)
+                return 0;
+
+            var osuCurrObj = (OsuDifficultyHitObject)current;
+            var osuLastObj = (OsuDifficultyHitObject)current.Previous(0);
+            var osuLast2Obj = (OsuDifficultyHitObject)current.Previous(2);
+
+            const int radius = OsuDifficultyHitObject.NORMALISED_RADIUS;
+            const int diameter = OsuDifficultyHitObject.NORMALISED_DIAMETER;
+
+            // Calculate the velocity to the current hitobject, which starts with a base distance / time assuming the last object is a hitcircle.
+            double currDistance = withSliderTravelDistance ? osuCurrObj.LazyJumpDistance : osuCurrObj.JumpDistance;
+            double currVelocity = currDistance / osuCurrObj.AdjustedDeltaTime;
+
+            // But if the last object is a slider, then we extend the travel velocity through the slider into the current object.
+            if (osuLastObj.BaseObject is Slider && withSliderTravelDistance)
+            {
+                double sliderDistance = osuLastObj.LazyTravelDistance + osuCurrObj.LazyJumpDistance;
+                currVelocity = Math.Max(currVelocity, sliderDistance / osuCurrObj.AdjustedDeltaTime);
+            }
+
+            double prevDistance = withSliderTravelDistance ? osuLastObj.LazyJumpDistance : osuLastObj.JumpDistance;
+            double prevVelocity = prevDistance / osuLastObj.AdjustedDeltaTime;
+
+            double snapDifficulty = currVelocity; // Start difficulty with regular velocity.
+
+            // Penalize angle repetition.
+            snapDifficulty *= vectorAngleRepetition(osuCurrObj, osuLastObj);
+
+            if (osuCurrObj.Angle != null && osuLastObj.Angle != null)
+            {
+                double currAngle = osuCurrObj.Angle.Value;
+                double lastAngle = osuLastObj.Angle.Value;
+
+                // Rewarding angles, take the smaller velocity as base.
+                double velocityInfluence = Math.Min(currVelocity, prevVelocity);
+
+                double acuteAngleBonus = 0;
+
+                if (Math.Max(osuCurrObj.AdjustedDeltaTime, osuLastObj.AdjustedDeltaTime) < 1.25 * Math.Min(osuCurrObj.AdjustedDeltaTime, osuLastObj.AdjustedDeltaTime)) // If rhythms are the same.
+                {
+                    acuteAngleBonus = CalcAngleAcuteness(currAngle);
+
+                    // Penalize angle repetition. It is important to do it _before_ multiplying by anything because we compare raw acuteness here
+                    acuteAngleBonus *= 0.08 + 0.92 * (1 - Math.Min(acuteAngleBonus, Math.Pow(CalcAngleAcuteness(lastAngle), 3)));
+
+                    // Apply acute angle bonus for BPM above 300 1/2 and distance more than one diameter
+                    acuteAngleBonus *= velocityInfluence * DifficultyCalculationUtils.Smootherstep(DifficultyCalculationUtils.MillisecondsToBPM(osuCurrObj.AdjustedDeltaTime, 2), 300, 400) *
+                                       DifficultyCalculationUtils.Smootherstep(currDistance, 0, diameter * 2);
+                }
+
+                double wideAngleBonus = calcAngleWideness(currAngle);
+
+                // Penalize angle repetition. It is important to do it _before_ multiplying by velocity because we compare raw wideness here
+                wideAngleBonus *= 0.25 + 0.75 * (1 - Math.Min(wideAngleBonus, Math.Pow(calcAngleWideness(lastAngle), 3)));
+
+                // Rescaling velocity for the wide angle bonus
+                const double wide_angle_time_scale = 1.45;
+                double wideAngleCurrVelocity = currDistance / Math.Pow(osuCurrObj.AdjustedDeltaTime, wide_angle_time_scale);
+                double wideAnglePrevVelocity = prevDistance / Math.Pow(osuLastObj.AdjustedDeltaTime, wide_angle_time_scale);
+
+                if (osuLastObj.BaseObject is Slider && withSliderTravelDistance)
+                {
+                    double sliderDistance = osuLastObj.LazyTravelDistance + osuCurrObj.LazyJumpDistance;
+                    wideAngleCurrVelocity = Math.Max(wideAngleCurrVelocity, sliderDistance / Math.Pow(osuCurrObj.AdjustedDeltaTime, wide_angle_time_scale));
+                }
+
+                wideAngleBonus *= Math.Min(wideAngleCurrVelocity, wideAnglePrevVelocity);
+
+                if (osuLast2Obj != null)
+                {
+                    // If objects just go back and forth through a middle point - don't give as much wide bonus
+                    // Use Previous(2) and Previous(0) because angles calculation is done prevprev-prev-curr, so any object's angle's center point is always the previous object
+                    var lastBaseObject = (OsuHitObject)osuLastObj.BaseObject;
+                    var last2BaseObject = (OsuHitObject)osuLast2Obj.BaseObject;
+
+                    float distance = (last2BaseObject.StackedPosition - lastBaseObject.StackedPosition).Length;
+
+                    if (distance < 1)
+                    {
+                        wideAngleBonus *= 1 - 0.55 * (1 - distance);
+                    }
+                }
+
+                // Add in acute angle bonus or wide angle bonus, whichever is larger.
+                snapDifficulty += Math.Max(acuteAngleBonus * acute_angle_multiplier, wideAngleBonus * wide_angle_multiplier);
+
+                // Apply wiggle bonus for jumps that are [radius, 3*diameter] in distance, with < 110 angle
+                // https://www.desmos.com/calculator/dp0v0nvowc
+                double wiggleBonus = velocityInfluence
+                                     * DifficultyCalculationUtils.Smootherstep(currDistance, radius, diameter)
+                                     * Math.Pow(DifficultyCalculationUtils.ReverseLerp(currDistance, diameter * 3, diameter), 1.8)
+                                     * DifficultyCalculationUtils.Smootherstep(currAngle, double.DegreesToRadians(110), double.DegreesToRadians(60))
+                                     * DifficultyCalculationUtils.Smootherstep(prevDistance, radius, diameter)
+                                     * Math.Pow(DifficultyCalculationUtils.ReverseLerp(prevDistance, diameter * 3, diameter), 1.8)
+                                     * DifficultyCalculationUtils.Smootherstep(lastAngle, double.DegreesToRadians(110), double.DegreesToRadians(60));
+
+                snapDifficulty += wiggleBonus * wiggle_multiplier;
+            }
+
+            if (Math.Max(prevVelocity, currVelocity) != 0)
+            {
+                if (withSliderTravelDistance)
+                {
+                    // We want to use just the object jump without slider velocity when awarding differences
+                    currVelocity = currDistance / osuCurrObj.AdjustedDeltaTime;
+                }
+
+                // Scale with ratio of difference compared to 0.5 * max dist.
+                double distRatio = DifficultyCalculationUtils.Smoothstep(Math.Abs(prevVelocity - currVelocity) / Math.Max(prevVelocity, currVelocity), 0, 1);
+
+                // Reward for % distance up to 125 / strainTime for overlaps where velocity is still changing.
+                double overlapVelocityBuff = Math.Min(diameter * 1.25 / Math.Min(osuCurrObj.AdjustedDeltaTime, osuLastObj.AdjustedDeltaTime), Math.Abs(prevVelocity - currVelocity));
+
+                double velocityChangeBonus = overlapVelocityBuff * distRatio;
+
+                // Penalize for rhythm changes.
+                velocityChangeBonus *= Math.Pow(Math.Min(osuCurrObj.AdjustedDeltaTime, osuLastObj.AdjustedDeltaTime) / Math.Max(osuCurrObj.AdjustedDeltaTime, osuLastObj.AdjustedDeltaTime), 2);
+
+                snapDifficulty += velocityChangeBonus * velocity_change_multiplier;
+            }
+
+            // Reward sliders based on velocity.
+            if (osuCurrObj.BaseObject is Slider && withSliderTravelDistance)
+            {
+                double sliderBonus = osuCurrObj.TravelDistance / osuCurrObj.TravelTime;
+                snapDifficulty += (sliderBonus < 1 ? sliderBonus : Math.Pow(sliderBonus, 0.75)) * slider_multiplier;
+            }
+
+            // Apply high circle size bonus
+            snapDifficulty *= osuCurrObj.SmallCircleBonus;
+
+            snapDifficulty *= highBpmBonus(osuCurrObj.AdjustedDeltaTime);
+
+            return snapDifficulty;
+        }
+
+        private static double highBpmBonus(double ms) => 1 / (1 - Math.Pow(0.03, Math.Pow(ms / 1000, 0.65)));
+
+        private static double vectorAngleRepetition(OsuDifficultyHitObject current, OsuDifficultyHitObject previous)
+        {
+            if (current.Angle == null || previous.Angle == null)
+                return 1;
+
+            const double note_limit = 6;
+
+            double constantAngleCount = 0;
+
+            for (int index = 0; index < note_limit; index++)
+            {
+                var loopObj = (OsuDifficultyHitObject)current.Previous(index);
+
+                if (loopObj.IsNull())
+                    break;
+
+                // Only consider vectors in the same jump section, stopping to change rhythm ruins momentum
+                if (Math.Max(current.AdjustedDeltaTime, loopObj.AdjustedDeltaTime) > 1.1 * Math.Min(current.AdjustedDeltaTime, loopObj.AdjustedDeltaTime))
+                    break;
+
+                if (loopObj.NormalisedVectorAngle.IsNotNull() && current.NormalisedVectorAngle.IsNotNull())
+                {
+                    double angleDifference = Math.Abs(current.NormalisedVectorAngle.Value - loopObj.NormalisedVectorAngle.Value);
+                    // Refer to this desmos for tuning, constants need to be precise so that values stay within the range of 0 and 1.
+                    // https://www.desmos.com/calculator/a8jesv5sv2
+                    constantAngleCount += Math.Cos(8 * Math.Min(double.DegreesToRadians(11.25), angleDifference));
+                }
+            }
+
+            double vectorRepetition = Math.Pow(Math.Min(0.5 / constantAngleCount, 1), 2);
+
+            double stackFactor = DifficultyCalculationUtils.Smootherstep(current.LazyJumpDistance, 0, OsuDifficultyHitObject.NORMALISED_DIAMETER);
+
+            double currAngle = current.Angle.Value;
+            double lastAngle = previous.Angle.Value;
+
+            double angleDifferenceAdjusted = Math.Cos(2 * Math.Min(double.DegreesToRadians(45), Math.Abs(currAngle - lastAngle) * stackFactor));
+
+            double baseNerf = 1 - maximum_repetition_nerf * CalcAngleAcuteness(lastAngle) * angleDifferenceAdjusted;
+
+            return Math.Pow(baseNerf + (1 - baseNerf) * vectorRepetition * maximum_vector_influence * stackFactor, 2);
+        }
+
+        private static double calcAngleWideness(double angle) => DifficultyCalculationUtils.Smoothstep(angle, double.DegreesToRadians(40), double.DegreesToRadians(140));
+
+        public static double CalcAngleAcuteness(double angle) => DifficultyCalculationUtils.Smoothstep(angle, double.DegreesToRadians(140), double.DegreesToRadians(40));
+    }
+}

osu.Game.Rulesets.Osu/Difficulty/Evaluators/AimEvaluator.cs

@@ -1,172 +0,0 @@
-// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
-// See the LICENCE file in the repository root for full licence text.
-
-using System;
-using osu.Game.Rulesets.Difficulty.Preprocessing;
-using osu.Game.Rulesets.Difficulty.Utils;
-using osu.Game.Rulesets.Osu.Difficulty.Preprocessing;
-using osu.Game.Rulesets.Osu.Objects;
-
-namespace osu.Game.Rulesets.Osu.Difficulty.Evaluators
-{
-    public static class AimEvaluator
-    {
-        private const double wide_angle_multiplier = 1.5;
-        private const double acute_angle_multiplier = 2.55;
-        private const double slider_multiplier = 1.35;
-        private const double velocity_change_multiplier = 0.75;
-        private const double wiggle_multiplier = 1.02;
-
-        /// <summary>
-        /// Evaluates the difficulty of aiming the current object, based on:
-        /// <list type="bullet">
-        /// <item><description>cursor velocity to the current object,</description></item>
-        /// <item><description>angle difficulty,</description></item>
-        /// <item><description>sharp velocity increases,</description></item>
-        /// <item><description>and slider difficulty.</description></item>
-        /// </list>
-        /// </summary>
-        public static double EvaluateDifficultyOf(DifficultyHitObject current, bool withSliderTravelDistance)
-        {
-            if (current.BaseObject is Spinner || current.Index <= 1 || current.Previous(0).BaseObject is Spinner)
-                return 0;
-
-            var osuCurrObj = (OsuDifficultyHitObject)current;
-            var osuLastObj = (OsuDifficultyHitObject)current.Previous(0);
-            var osuLastLastObj = (OsuDifficultyHitObject)current.Previous(1);
-            var osuLast2Obj = (OsuDifficultyHitObject)current.Previous(2);
-
-            const int radius = OsuDifficultyHitObject.NORMALISED_RADIUS;
-            const int diameter = OsuDifficultyHitObject.NORMALISED_DIAMETER;
-
-            // Calculate the velocity to the current hitobject, which starts with a base distance / time assuming the last object is a hitcircle.
-            double currVelocity = osuCurrObj.LazyJumpDistance / osuCurrObj.AdjustedDeltaTime;
-
-            // But if the last object is a slider, then we extend the travel velocity through the slider into the current object.
-            if (osuLastObj.BaseObject is Slider && withSliderTravelDistance)
-            {
-                double travelVelocity = osuLastObj.TravelDistance / osuLastObj.TravelTime; // calculate the slider velocity from slider head to slider end.
-                double movementVelocity = osuCurrObj.MinimumJumpDistance / osuCurrObj.MinimumJumpTime; // calculate the movement velocity from slider end to current object
-
-                currVelocity = Math.Max(currVelocity, movementVelocity + travelVelocity); // take the larger total combined velocity.
-            }
-
-            // As above, do the same for the previous hitobject.
-            double prevVelocity = osuLastObj.LazyJumpDistance / osuLastObj.AdjustedDeltaTime;
-
-            if (osuLastLastObj.BaseObject is Slider && withSliderTravelDistance)
-            {
-                double travelVelocity = osuLastLastObj.TravelDistance / osuLastLastObj.TravelTime;
-                double movementVelocity = osuLastObj.MinimumJumpDistance / osuLastObj.MinimumJumpTime;
-
-                prevVelocity = Math.Max(prevVelocity, movementVelocity + travelVelocity);
-            }
-
-            double wideAngleBonus = 0;
-            double acuteAngleBonus = 0;
-            double sliderBonus = 0;
-            double velocityChangeBonus = 0;
-            double wiggleBonus = 0;
-
-            double aimStrain = currVelocity; // Start strain with regular velocity.
-
-            if (osuCurrObj.Angle != null && osuLastObj.Angle != null)
-            {
-                double currAngle = osuCurrObj.Angle.Value;
-                double lastAngle = osuLastObj.Angle.Value;
-
-                // Rewarding angles, take the smaller velocity as base.
-                double angleBonus = Math.Min(currVelocity, prevVelocity);
-
-                if (Math.Max(osuCurrObj.AdjustedDeltaTime, osuLastObj.AdjustedDeltaTime) < 1.25 * Math.Min(osuCurrObj.AdjustedDeltaTime, osuLastObj.AdjustedDeltaTime)) // If rhythms are the same.
-                {
-                    acuteAngleBonus = calcAcuteAngleBonus(currAngle);
-
-                    // Penalize angle repetition.
-                    acuteAngleBonus *= 0.08 + 0.92 * (1 - Math.Min(acuteAngleBonus, Math.Pow(calcAcuteAngleBonus(lastAngle), 3)));
-
-                    // Apply acute angle bonus for BPM above 300 1/2 and distance more than one diameter
-                    acuteAngleBonus *= angleBonus *
-                                       DifficultyCalculationUtils.Smootherstep(DifficultyCalculationUtils.MillisecondsToBPM(osuCurrObj.AdjustedDeltaTime, 2), 300, 400) *
-                                       DifficultyCalculationUtils.Smootherstep(osuCurrObj.LazyJumpDistance, diameter, diameter * 2);
-                }
-
-                wideAngleBonus = calcWideAngleBonus(currAngle);
-
-                // Penalize angle repetition.
-                wideAngleBonus *= 1 - Math.Min(wideAngleBonus, Math.Pow(calcWideAngleBonus(lastAngle), 3));
-
-                // Apply full wide angle bonus for distance more than one diameter
-                wideAngleBonus *= angleBonus * DifficultyCalculationUtils.Smootherstep(osuCurrObj.LazyJumpDistance, 0, diameter);
-
-                // Apply wiggle bonus for jumps that are [radius, 3*diameter] in distance, with < 110 angle
-                // https://www.desmos.com/calculator/dp0v0nvowc
-                wiggleBonus = angleBonus
-                              * DifficultyCalculationUtils.Smootherstep(osuCurrObj.LazyJumpDistance, radius, diameter)
-                              * Math.Pow(DifficultyCalculationUtils.ReverseLerp(osuCurrObj.LazyJumpDistance, diameter * 3, diameter), 1.8)
-                              * DifficultyCalculationUtils.Smootherstep(currAngle, double.DegreesToRadians(110), double.DegreesToRadians(60))
-                              * DifficultyCalculationUtils.Smootherstep(osuLastObj.LazyJumpDistance, radius, diameter)
-                              * Math.Pow(DifficultyCalculationUtils.ReverseLerp(osuLastObj.LazyJumpDistance, diameter * 3, diameter), 1.8)
-                              * DifficultyCalculationUtils.Smootherstep(lastAngle, double.DegreesToRadians(110), double.DegreesToRadians(60));
-
-                if (osuLast2Obj != null)
-                {
-                    // If objects just go back and forth through a middle point - don't give as much wide bonus
-                    // Use Previous(2) and Previous(0) because angles calculation is done prevprev-prev-curr, so any object's angle's center point is always the previous object
-                    var lastBaseObject = (OsuHitObject)osuLastObj.BaseObject;
-                    var last2BaseObject = (OsuHitObject)osuLast2Obj.BaseObject;
-
-                    float distance = (last2BaseObject.StackedPosition - lastBaseObject.StackedPosition).Length;
-
-                    if (distance < 1)
-                    {
-                        wideAngleBonus *= 1 - 0.35 * (1 - distance);
-                    }
-                }
-            }
-
-            if (Math.Max(prevVelocity, currVelocity) != 0)
-            {
-                // We want to use the average velocity over the whole object when awarding differences, not the individual jump and slider path velocities.
-                prevVelocity = (osuLastObj.LazyJumpDistance + osuLastLastObj.TravelDistance) / osuLastObj.AdjustedDeltaTime;
-                currVelocity = (osuCurrObj.LazyJumpDistance + osuLastObj.TravelDistance) / osuCurrObj.AdjustedDeltaTime;
-
-                // Scale with ratio of difference compared to 0.5 * max dist.
-                double distRatio = DifficultyCalculationUtils.Smoothstep(Math.Abs(prevVelocity - currVelocity) / Math.Max(prevVelocity, currVelocity), 0, 1);
-
-                // Reward for % distance up to 125 / strainTime for overlaps where velocity is still changing.
-                double overlapVelocityBuff = Math.Min(diameter * 1.25 / Math.Min(osuCurrObj.AdjustedDeltaTime, osuLastObj.AdjustedDeltaTime), Math.Abs(prevVelocity - currVelocity));
-
-                velocityChangeBonus = overlapVelocityBuff * distRatio;
-
-                // Penalize for rhythm changes.
-                velocityChangeBonus *= Math.Pow(Math.Min(osuCurrObj.AdjustedDeltaTime, osuLastObj.AdjustedDeltaTime) / Math.Max(osuCurrObj.AdjustedDeltaTime, osuLastObj.AdjustedDeltaTime), 2);
-            }
-
-            if (osuLastObj.BaseObject is Slider)
-            {
-                // Reward sliders based on velocity.
-                sliderBonus = osuLastObj.TravelDistance / osuLastObj.TravelTime;
-            }
-
-            aimStrain += wiggleBonus * wiggle_multiplier;
-            aimStrain += velocityChangeBonus * velocity_change_multiplier;
-
-            // Add in acute angle bonus or wide angle bonus, whichever is larger.
-            aimStrain += Math.Max(acuteAngleBonus * acute_angle_multiplier, wideAngleBonus * wide_angle_multiplier);
-
-            // Apply high circle size bonus
-            aimStrain *= osuCurrObj.SmallCircleBonus;
-
-            // Add in additional slider velocity bonus.
-            if (withSliderTravelDistance)
-                aimStrain += sliderBonus * slider_multiplier;
-
-            return aimStrain;
-        }
-
-        private static double calcWideAngleBonus(double angle) => DifficultyCalculationUtils.Smoothstep(angle, double.DegreesToRadians(40), double.DegreesToRadians(140));
-
-        private static double calcAcuteAngleBonus(double angle) => DifficultyCalculationUtils.Smoothstep(angle, double.DegreesToRadians(140), double.DegreesToRadians(40));
-    }
-}

osu.Game.Rulesets.Osu/Difficulty/Evaluators/FlashlightEvaluator.cs

@@ -2,8 +2,12 @@
 // See the LICENCE file in the repository root for full licence text.
 
 using System;
+using System.Collections.Generic;
+using System.Linq;
 using osu.Game.Rulesets.Difficulty.Preprocessing;
+using osu.Game.Rulesets.Mods;
 using osu.Game.Rulesets.Osu.Difficulty.Preprocessing;
+using osu.Game.Rulesets.Osu.Mods;
 using osu.Game.Rulesets.Osu.Objects;
 
 namespace osu.Game.Rulesets.Osu.Difficulty.Evaluators
@@ -28,7 +32,7 @@ namespace osu.Game.Rulesets.Osu.Difficulty.Evaluators
         /// <item><description>and whether the hidden mod is enabled.</description></item>
         /// </list>
         /// </summary>
-        public static double EvaluateDifficultyOf(DifficultyHitObject current, bool hidden)
+        public static double EvaluateDifficultyOf(DifficultyHitObject current, IReadOnlyList<Mod> mods)
         {
             if (current.BaseObject is Spinner)
                 return 0;
@@ -40,7 +44,7 @@ namespace osu.Game.Rulesets.Osu.Difficulty.Evaluators
             double smallDistNerf = 1.0;
             double cumulativeStrainTime = 0.0;
 
-            double result = 0.0;
+            double flashlightDifficulty = 0.0;
 
             OsuDifficultyHitObject lastObj = osuCurrent;
 
@@ -66,9 +70,9 @@ namespace osu.Game.Rulesets.Osu.Difficulty.Evaluators
                     double stackNerf = Math.Min(1.0, (currentObj.LazyJumpDistance / scalingFactor) / 25.0);
 
                     // Bonus based on how visible the object is.
-                    double opacityBonus = 1.0 + max_opacity_bonus * (1.0 - osuCurrent.OpacityAt(currentHitObject.StartTime, hidden));
+                    double opacityBonus = 1.0 + max_opacity_bonus * (1.0 - osuCurrent.OpacityAt(currentHitObject.StartTime, mods.OfType<OsuModHidden>().Any(m => !m.OnlyFadeApproachCircles.Value)));
 
-                    result += stackNerf * opacityBonus * scalingFactor * jumpDistance / cumulativeStrainTime;
+                    flashlightDifficulty += stackNerf * opacityBonus * scalingFactor * jumpDistance / cumulativeStrainTime;
 
                     if (currentObj.Angle != null && osuCurrent.Angle != null)
                     {
@@ -81,14 +85,14 @@ namespace osu.Game.Rulesets.Osu.Difficulty.Evaluators
                 lastObj = currentObj;
             }
 
-            result = Math.Pow(smallDistNerf * result, 2.0);
+            flashlightDifficulty = Math.Pow(smallDistNerf * flashlightDifficulty, 2.0);
 
             // Additional bonus for Hidden due to there being no approach circles.
-            if (hidden)
-                result *= 1.0 + hidden_bonus;
+            if (mods.OfType<OsuModHidden>().Any())
+                flashlightDifficulty *= 1.0 + hidden_bonus;
 
             // Nerf patterns with repeated angles.
-            result *= min_angle_multiplier + (1.0 - min_angle_multiplier) / (angleRepeatCount + 1.0);
+            flashlightDifficulty *= min_angle_multiplier + (1.0 - min_angle_multiplier) / (angleRepeatCount + 1.0);
 
             double sliderBonus = 0.0;
 
@@ -108,9 +112,9 @@ namespace osu.Game.Rulesets.Osu.Difficulty.Evaluators
                     sliderBonus /= (osuSlider.RepeatCount + 1);
             }
 
-            result += sliderBonus * slider_multiplier;
+            flashlightDifficulty += sliderBonus * slider_multiplier;
 
-            return result;
+            return flashlightDifficulty;
         }
     }
 }

osu.Game.Rulesets.Osu/Difficulty/Evaluators/ReadingEvaluator.cs

@@ -0,0 +1,272 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System;
+using System.Collections.Generic;
+using osu.Framework.Extensions.ObjectExtensions;
+using osu.Game.Rulesets.Difficulty.Preprocessing;
+using osu.Game.Rulesets.Difficulty.Utils;
+using osu.Game.Rulesets.Osu.Difficulty.Preprocessing;
+using osu.Game.Rulesets.Osu.Objects;
+
+namespace osu.Game.Rulesets.Osu.Difficulty.Evaluators
+{
+    public static class ReadingEvaluator
+    {
+        private const double reading_window_size = 3000; // 3 seconds
+        private const double distance_influence_threshold = OsuDifficultyHitObject.NORMALISED_DIAMETER * 1.5; // 1.5 circles distance between centers
+        private const double hidden_multiplier = 0.28;
+        private const double density_multiplier = 2.4;
+        private const double density_difficulty_base = 2.5;
+        private const double preempt_balancing_factor = 140000;
+        private const double preempt_starting_point = 500; // AR 9.66 in milliseconds
+        private const double minimum_angle_relevancy_time = 2000; // 2 seconds
+        private const double maximum_angle_relevancy_time = 200;
+
+        public static double EvaluateDifficultyOf(DifficultyHitObject current, bool hidden)
+        {
+            if (current.BaseObject is Spinner || current.Index == 0)
+                return 0;
+
+            var currObj = (OsuDifficultyHitObject)current;
+            var nextObj = (OsuDifficultyHitObject)current.Next(0);
+
+            double velocity = Math.Max(1, currObj.LazyJumpDistance / currObj.AdjustedDeltaTime); // Only allow velocity to buff
+
+            double currentVisibleObjectDensity = retrieveCurrentVisibleObjectDensity(currObj);
+            double pastObjectDifficultyInfluence = getPastObjectDifficultyInfluence(currObj);
+
+            double constantAngleNerfFactor = getConstantAngleNerfFactor(currObj);
+
+            double noteDensityDifficulty = calculateDensityDifficulty(nextObj, velocity, constantAngleNerfFactor, pastObjectDifficultyInfluence, currentVisibleObjectDensity);
+
+            double hiddenDifficulty = hidden
+                ? calculateHiddenDifficulty(currObj, pastObjectDifficultyInfluence, currentVisibleObjectDensity, velocity, constantAngleNerfFactor)
+                : 0;
+
+            double preemptDifficulty = calculatePreemptDifficulty(velocity, constantAngleNerfFactor, currObj.Preempt);
+
+            double readingDifficulty = DifficultyCalculationUtils.Norm(1.5, preemptDifficulty, hiddenDifficulty, noteDensityDifficulty);
+
+            // Having less time to process information is harder
+            readingDifficulty *= highBpmBonus(currObj.AdjustedDeltaTime);
+
+            return readingDifficulty;
+        }
+
+        /// <summary>
+        /// Calculates the density difficulty of the current object and how hard it is to aim it because of it based on:
+        /// <list type="bullet">
+        /// <item><description>cursor velocity to the current object,</description></item>
+        /// <item><description>how many times the current object's angle was repeated,</description></item>
+        /// <item><description>density of objects visible when the current object appears,</description></item>
+        /// <item><description>density of objects visible when the current object needs to be clicked,</description></item>
+        /// /// </list>
+        /// </summary>
+        private static double calculateDensityDifficulty(OsuDifficultyHitObject? nextObj, double velocity, double constantAngleNerfFactor,
+                                                         double pastObjectDifficultyInfluence, double currentVisibleObjectDensity)
+        {
+            // Consider future densities too because it can make the path the cursor takes less clear
+            double futureObjectDifficultyInfluence = Math.Sqrt(currentVisibleObjectDensity);
+
+            if (nextObj != null)
+            {
+                // Reduce difficulty if movement to next object is small
+                futureObjectDifficultyInfluence *= DifficultyCalculationUtils.Smootherstep(nextObj.LazyJumpDistance, 15, distance_influence_threshold);
+            }
+
+            // Value higher note densities exponentially
+            double noteDensityDifficulty = Math.Pow(pastObjectDifficultyInfluence + futureObjectDifficultyInfluence, 1.7) * 0.4 * constantAngleNerfFactor * velocity;
+
+            // Award only denser than average maps.
+            noteDensityDifficulty = Math.Max(0, noteDensityDifficulty - density_difficulty_base);
+
+            // Apply a soft cap to general density reading to account for partial memorization
+            noteDensityDifficulty = Math.Pow(noteDensityDifficulty, 0.45) * density_multiplier;
+
+            return noteDensityDifficulty;
+        }
+
+        /// <summary>
+        /// Calculates the difficulty of aiming the current object when the approach rate is very high based on:
+        /// <list type="bullet">
+        /// <item><description>cursor velocity to the current object,</description></item>
+        /// <item><description>how many times the current object's angle was repeated,</description></item>
+        /// <item><description>how many milliseconds elapse between the approach circle appearing and touching the inner circle</description></item>
+        /// </list>
+        /// </summary>
+        private static double calculatePreemptDifficulty(double velocity, double constantAngleNerfFactor, double preempt)
+        {
+            // Arbitrary curve for the base value preempt difficulty should have as approach rate increases.
+            // https://www.desmos.com/calculator/c175335a71
+            double preemptDifficulty = Math.Pow((preempt_starting_point - preempt + Math.Abs(preempt - preempt_starting_point)) / 2, 2.5) / preempt_balancing_factor;
+
+            preemptDifficulty *= constantAngleNerfFactor * velocity;
+
+            return preemptDifficulty;
+        }
+
+        /// <summary>
+        /// Calculates the difficulty of aiming the current object when the hidden mod is active based on:
+        /// <list type="bullet">
+        /// <item><description>cursor velocity to the current object,</description></item>
+        /// <item><description>time the current object spends invisible,</description></item>
+        /// <item><description>density of objects visible when the current object appears,</description></item>
+        /// <item><description>density of objects visible when the current object needs to be clicked,</description></item>
+        /// <item><description>how many times the current object's angle was repeated,</description></item>
+        /// <item><description>if the current object is perfectly stacked to the previous one</description></item>
+        /// </list>
+        /// </summary>
+        private static double calculateHiddenDifficulty(OsuDifficultyHitObject currObj, double pastObjectDifficultyInfluence, double currentVisibleObjectDensity, double velocity,
+                                                        double constantAngleNerfFactor)
+        {
+            // Higher preempt means that time spent invisible is higher too, we want to reward that
+            double preemptFactor = Math.Pow(currObj.Preempt, 2.2) * 0.01;
+
+            // Account for both past and current densities
+            double densityFactor = Math.Pow(currentVisibleObjectDensity + pastObjectDifficultyInfluence, 3.3) * 3;
+
+            double hiddenDifficulty = (preemptFactor + densityFactor) * constantAngleNerfFactor * velocity * 0.01;
+
+            // Apply a soft cap to general HD reading to account for partial memorization
+            hiddenDifficulty = Math.Pow(hiddenDifficulty, 0.4) * hidden_multiplier;
+
+            var previousObj = (OsuDifficultyHitObject)currObj.Previous(0);
+
+            // Buff perfect stacks only if current note is completely invisible at the time you click the previous note.
+            if (currObj.LazyJumpDistance == 0 && currObj.OpacityAt(previousObj.BaseObject.StartTime, true) == 0 && previousObj.StartTime > currObj.StartTime - currObj.Preempt)
+                hiddenDifficulty += hidden_multiplier * 2500 / Math.Pow(currObj.AdjustedDeltaTime, 1.5); // Perfect stacks are harder the less time between notes
+
+            return hiddenDifficulty;
+        }
+
+        private static double getPastObjectDifficultyInfluence(OsuDifficultyHitObject currObj)
+        {
+            double pastObjectDifficultyInfluence = 0;
+
+            foreach (var loopObj in retrievePastVisibleObjects(currObj))
+            {
+                double loopDifficulty = currObj.OpacityAt(loopObj.BaseObject.StartTime, false);
+
+                // When aiming an object small distances mean previous objects may be cheesed, so it doesn't matter whether they were arranged confusingly.
+                loopDifficulty *= DifficultyCalculationUtils.Smootherstep(loopObj.LazyJumpDistance, 15, distance_influence_threshold);
+
+                // Account less for objects close to the max reading window
+                double timeBetweenCurrAndLoopObj = currObj.StartTime - loopObj.StartTime;
+                double timeNerfFactor = getTimeNerfFactor(timeBetweenCurrAndLoopObj);
+
+                loopDifficulty *= timeNerfFactor;
+                pastObjectDifficultyInfluence += loopDifficulty;
+            }
+
+            return pastObjectDifficultyInfluence;
+        }
+
+        // Returns a list of objects that are visible on screen at the point in time the current object becomes visible.
+        private static IEnumerable<OsuDifficultyHitObject> retrievePastVisibleObjects(OsuDifficultyHitObject current)
+        {
+            for (int i = 0; i < current.Index; i++)
+            {
+                OsuDifficultyHitObject hitObject = (OsuDifficultyHitObject)current.Previous(i);
+
+                if (hitObject.IsNull() ||
+                    current.StartTime - hitObject.StartTime > reading_window_size ||
+                    hitObject.StartTime < current.StartTime - current.Preempt) // Current object not visible at the time object needs to be clicked
+                    break;
+
+                yield return hitObject;
+            }
+        }
+
+        // Returns the density of objects visible at the point in time the current object needs to be clicked capped by the reading window.
+        private static double retrieveCurrentVisibleObjectDensity(OsuDifficultyHitObject current)
+        {
+            double visibleObjectCount = 0;
+
+            OsuDifficultyHitObject? hitObject = (OsuDifficultyHitObject)current.Next(0);
+
+            while (hitObject != null)
+            {
+                if (hitObject.StartTime - current.StartTime > reading_window_size ||
+                    current.StartTime < hitObject.StartTime - hitObject.Preempt) // Object not visible at the time current object needs to be clicked.
+                    break;
+
+                double timeBetweenCurrAndLoopObj = hitObject.StartTime - current.StartTime;
+                double timeNerfFactor = getTimeNerfFactor(timeBetweenCurrAndLoopObj);
+
+                visibleObjectCount += hitObject.OpacityAt(current.BaseObject.StartTime, false) * timeNerfFactor;
+
+                hitObject = (OsuDifficultyHitObject?)hitObject.Next(0);
+            }
+
+            return visibleObjectCount;
+        }
+
+        // Returns a factor of how often the current object's angle has been repeated in a certain time frame.
+        // It does this by checking the difference in angle between current and past objects and sums them based on a range of similarity.
+        // https://www.desmos.com/calculator/eb057a4822
+        private static double getConstantAngleNerfFactor(OsuDifficultyHitObject current)
+        {
+            double constantAngleCount = 0;
+            int index = 0;
+            double currentTimeGap = 0;
+
+            OsuDifficultyHitObject loopObjPrev0 = current;
+            OsuDifficultyHitObject? loopObjPrev1 = null;
+            OsuDifficultyHitObject? loopObjPrev2 = null;
+
+            while (currentTimeGap < minimum_angle_relevancy_time)
+            {
+                var loopObj = (OsuDifficultyHitObject)current.Previous(index);
+
+                if (loopObj.IsNull())
+                    break;
+
+                // Account less for objects that are close to the time limit.
+                double longIntervalFactor = 1 - DifficultyCalculationUtils.ReverseLerp(loopObj.AdjustedDeltaTime, maximum_angle_relevancy_time, minimum_angle_relevancy_time);
+
+                if (loopObj.Angle.IsNotNull() && current.Angle.IsNotNull())
+                {
+                    double angleDifference = Math.Abs(current.Angle.Value - loopObj.Angle.Value);
+                    double angleDifferenceAlternating = Math.PI;
+
+                    if (loopObjPrev0.Angle != null && loopObjPrev1?.Angle != null && loopObjPrev2?.Angle != null)
+                    {
+                        angleDifferenceAlternating = Math.Abs(loopObjPrev1.Angle.Value - loopObj.Angle.Value);
+                        angleDifferenceAlternating += Math.Abs(loopObjPrev2.Angle.Value - loopObjPrev0.Angle.Value);
+
+                        double weight = 1.0;
+
+                        // Be sure that one of the angles is very sharp, when other is wide
+                        weight *= DifficultyCalculationUtils.ReverseLerp(Math.Min(loopObj.Angle.Value, loopObjPrev0.Angle.Value) * 180 / Math.PI, 20, 5);
+                        weight *= DifficultyCalculationUtils.ReverseLerp(Math.Max(loopObj.Angle.Value, loopObjPrev0.Angle.Value) * 180 / Math.PI, 60, 120);
+
+                        // Lerp between max angle difference and rescaled alternating difference, with more harsh scaling compared to normal difference
+                        angleDifferenceAlternating = double.Lerp(Math.PI, 0.1 * angleDifferenceAlternating, weight);
+                    }
+
+                    double stackFactor = DifficultyCalculationUtils.Smootherstep(loopObj.LazyJumpDistance, 0, OsuDifficultyHitObject.NORMALISED_RADIUS);
+
+                    constantAngleCount += Math.Cos(3 * Math.Min(double.DegreesToRadians(30), Math.Min(angleDifference, angleDifferenceAlternating) * stackFactor)) * longIntervalFactor;
+                }
+
+                currentTimeGap = current.StartTime - loopObj.StartTime;
+                index++;
+
+                loopObjPrev2 = loopObjPrev1;
+                loopObjPrev1 = loopObjPrev0;
+                loopObjPrev0 = loopObj;
+            }
+
+            return Math.Clamp(2 / constantAngleCount, 0.2, 1);
+        }
+
+        // Returns a nerfing factor for when objects are very distant in time, affecting reading less.
+        private static double getTimeNerfFactor(double deltaTime)
+        {
+            return Math.Clamp(2 - deltaTime / (reading_window_size / 2), 0, 1);
+        }
+
+        private static double highBpmBonus(double ms) => 1 / (1 - Math.Pow(0.8, ms / 1000));
+    }
+}

osu.Game.Rulesets.Osu/Difficulty/Evaluators/Speed/RhythmEvaluator.cs

@@ -8,15 +8,16 @@ using osu.Game.Rulesets.Difficulty.Preprocessing;
 using osu.Game.Rulesets.Difficulty.Utils;
 using osu.Game.Rulesets.Osu.Difficulty.Preprocessing;
 using osu.Game.Rulesets.Osu.Objects;
+using osu.Game.Rulesets.Scoring;
 
-namespace osu.Game.Rulesets.Osu.Difficulty.Evaluators
+namespace osu.Game.Rulesets.Osu.Difficulty.Evaluators.Speed
 {
     public static class RhythmEvaluator
     {
         private const int history_time_max = 5 * 1000; // 5 seconds
         private const int history_objects_max = 32;
-        private const double rhythm_overall_multiplier = 1.0;
-        private const double rhythm_ratio_multiplier = 15.0;
+        private const double rhythm_overall_multiplier = 0.95;
+        private const double rhythm_ratio_multiplier = 26.0;
 
         /// <summary>
         /// Calculates a rhythm multiplier for the difficulty of the tap associated with historic data of the current <see cref="OsuDifficultyHitObject"/>.
@@ -26,11 +27,9 @@ namespace osu.Game.Rulesets.Osu.Difficulty.Evaluators
             if (current.BaseObject is Spinner)
                 return 0;
 
-            var currentOsuObject = (OsuDifficultyHitObject)current;
-
             double rhythmComplexitySum = 0;
 
-            double deltaDifferenceEpsilon = ((OsuDifficultyHitObject)current).HitWindowGreat * 0.3;
+            double deltaDifferenceEpsilon = ((OsuDifficultyHitObject)current).HitWindow(HitResult.Great) * 0.3;
 
             var island = new Island(deltaDifferenceEpsilon);
             var previousIsland = new Island(deltaDifferenceEpsilon);
@@ -57,6 +56,8 @@ namespace osu.Game.Rulesets.Osu.Difficulty.Evaluators
             for (int i = rhythmStart; i > 0; i--)
             {
                 OsuDifficultyHitObject currObj = (OsuDifficultyHitObject)current.Previous(i - 1);
+                if (currObj.BaseObject is Spinner)
+                    continue;
 
                 // scales note 0 to 1 from history to now
                 double timeDecay = (history_time_max - (current.StartTime - currObj.StartTime)) / history_time_max;
@@ -64,44 +65,56 @@ namespace osu.Game.Rulesets.Osu.Difficulty.Evaluators
 
                 double currHistoricalDecay = Math.Min(noteDecay, timeDecay); // either we're limited by time or limited by object count.
 
-                // Use custom cap value to ensure that that at this point delta time is actually zero
+                // Use custom cap value to ensure that at this point delta time is actually zero
                 double currDelta = Math.Max(currObj.DeltaTime, 1e-7);
                 double prevDelta = Math.Max(prevObj.DeltaTime, 1e-7);
                 double lastDelta = Math.Max(lastObj.DeltaTime, 1e-7);
 
+                // Make sure to always have the current island initialised - if we don't do it here it will only initialise on the next rhythm change
+                if (island.Delta == int.MaxValue)
+                    island = new Island((int)currDelta, deltaDifferenceEpsilon);
+
                 // calculate how much current delta difference deserves a rhythm bonus
                 // this function is meant to reduce rhythm bonus for deltas that are multiples of each other (i.e 100 and 200)
                 double deltaDifference = Math.Max(prevDelta, currDelta) / Math.Min(prevDelta, currDelta);
 
-                // Take only the fractional part of the value since we're only interested in punishing multiples
-                double deltaDifferenceFraction = deltaDifference - Math.Truncate(deltaDifference);
-
-                double currRatio = 1.0 + rhythm_ratio_multiplier * Math.Min(0.5, DifficultyCalculationUtils.SmoothstepBellCurve(deltaDifferenceFraction));
-
                 // reduce ratio bonus if delta difference is too big
                 double differenceMultiplier = Math.Clamp(2.0 - deltaDifference / 8.0, 0.0, 1.0);
 
                 double windowPenalty = Math.Min(1, Math.Max(0, Math.Abs(prevDelta - currDelta) - deltaDifferenceEpsilon) / deltaDifferenceEpsilon);
 
-                double effectiveRatio = windowPenalty * currRatio * differenceMultiplier;
+                double effectiveRatio = getEffectiveRatio(deltaDifference) * windowPenalty * differenceMultiplier;
+
+                // if previous object is a slider it might be easier to tap since you don't have to do a whole tapping motion
+                // while a full deltatime might end up some weird ratio the "unpress->tap" motion might be simple
+                // for example a slider-circle-circle pattern should be evaluated as a regular triple and not as a single->double
+                if (prevObj.BaseObject is Slider)
+                {
+                    double sliderLazyEndDelta = currObj.MinimumJumpTime;
+                    double sliderLazyDeltaDifference = Math.Max(sliderLazyEndDelta, currDelta) / Math.Min(sliderLazyEndDelta, currDelta);
+
+                    double sliderRealEndDelta = currObj.LastObjectEndDeltaTime;
+                    double sliderRealDeltaDifference = Math.Max(sliderRealEndDelta, currDelta) / Math.Min(sliderRealEndDelta, currDelta);
+
+                    double sliderEffectiveRatio = Math.Min(getEffectiveRatio(sliderLazyDeltaDifference), getEffectiveRatio(sliderRealDeltaDifference));
+                    effectiveRatio = Math.Min(sliderEffectiveRatio, effectiveRatio);
+                }
+
+                bool isSpeedingUp = prevDelta > currDelta + deltaDifferenceEpsilon;
+
+                if (Math.Abs(prevDelta - currDelta) < deltaDifferenceEpsilon)
+                {
+                    // island is still progressing
+                    island.AddDelta((int)currDelta);
+                }
 
                 if (firstDeltaSwitch)
                 {
-                    if (Math.Abs(prevDelta - currDelta) < deltaDifferenceEpsilon)
-                    {
-                        // island is still progressing
-                        island.AddDelta((int)currDelta);
-                    }
-                    else
+                    if (Math.Abs(prevDelta - currDelta) > deltaDifferenceEpsilon)
                     {
                         // bpm change is into slider, this is easy acc window
                         if (currObj.BaseObject is Slider)
-                            effectiveRatio *= 0.125;
-
-                        // bpm change was from a slider, this is easier typically than circle -> circle
-                        // unintentional side effect is that bursts with kicksliders at the ends might have lower difficulty than bursts without sliders
-                        if (prevObj.BaseObject is Slider)
-                            effectiveRatio *= 0.3;
+                            effectiveRatio *= 0.5;
 
                         // repeated island polarity (2 -> 4, 3 -> 5)
                         if (island.IsSimilarPolarity(previousIsland))
@@ -116,6 +129,9 @@ namespace osu.Game.Rulesets.Osu.Difficulty.Evaluators
                         if (previousIsland.DeltaCount == island.DeltaCount)
                             effectiveRatio *= 0.5;
 
+                        if (isSpeedingUp)
+                            effectiveRatio *= 0.65;
+
                         var islandCount = islandCounts.FirstOrDefault(x => x.Island.Equals(island));
 
                         if (islandCount != default)
@@ -134,7 +150,10 @@ namespace osu.Game.Rulesets.Osu.Difficulty.Evaluators
                         }
                         else
                         {
-                            islandCounts.Add((island, 1));
+                            if (island.DeltaCount > 0)
+                            {
+                                islandCounts.Add((island, 1));
+                            }
                         }
 
                         // scale down the difficulty if the object is doubletappable
@@ -176,10 +195,18 @@ namespace osu.Game.Rulesets.Osu.Difficulty.Evaluators
                 prevObj = currObj;
             }
 
-            double rhythmDifficulty = Math.Sqrt(4 + rhythmComplexitySum * rhythm_overall_multiplier) / 2.0; // produces multiplier that can be applied to strain. range [1, infinity) (not really though)
-            rhythmDifficulty *= 1 - currentOsuObject.GetDoubletapness((OsuDifficultyHitObject)current.Next(0));
+            // If the current island is long we don't want the sum to have as big of an effect
+            rhythmComplexitySum *= DifficultyCalculationUtils.ReverseLerp(island.DeltaCount, 22, 3);
 
-            return rhythmDifficulty;
+            return Math.Sqrt(4 + rhythmComplexitySum * rhythm_overall_multiplier) / 2.0; // produces multiplier that can be applied to strain. range [1, infinity) (not really though);
+        }
+
+        private static double getEffectiveRatio(double deltaDifference)
+        {
+            // Take only the fractional part of the value since we're only interested in punishing multiples
+            double deltaDifferenceFraction = deltaDifference - Math.Truncate(deltaDifference);
+
+            return 1.0 + rhythm_ratio_multiplier * Math.Min(0.5, DifficultyCalculationUtils.SmoothstepBellCurve(deltaDifferenceFraction));
         }
 
         private class Island : IEquatable<Island>
@@ -211,9 +238,12 @@ namespace osu.Game.Rulesets.Osu.Difficulty.Evaluators
 
             public bool IsSimilarPolarity(Island other)
             {
-                // TODO: consider islands to be of similar polarity only if they're having the same average delta (we don't want to consider 3 singletaps similar to a triple)
-                //       naively adding delta check here breaks _a lot_ of maps because of the flawed ratio calculation
-                return DeltaCount % 2 == other.DeltaCount % 2;
+                // single delta islands shouldn't be compared
+                if (DeltaCount <= 1 || other.DeltaCount <= 1)
+                    return false;
+
+                return Math.Abs(Delta - other.Delta) < deltaDifferenceEpsilon &&
+                       DeltaCount % 2 == other.DeltaCount % 2;
             }
 
             public bool Equals(Island? other)

osu.Game.Rulesets.Osu/Difficulty/Evaluators/Speed/SpeedEvaluator.cs

@@ -2,47 +2,39 @@
 // See the LICENCE file in the repository root for full licence text.
 
 using System;
-using System.Collections.Generic;
-using System.Linq;
 using osu.Game.Rulesets.Difficulty.Preprocessing;
 using osu.Game.Rulesets.Difficulty.Utils;
-using osu.Game.Rulesets.Mods;
 using osu.Game.Rulesets.Osu.Difficulty.Preprocessing;
-using osu.Game.Rulesets.Osu.Mods;
 using osu.Game.Rulesets.Osu.Objects;
+using osu.Game.Rulesets.Scoring;
 
-namespace osu.Game.Rulesets.Osu.Difficulty.Evaluators
+namespace osu.Game.Rulesets.Osu.Difficulty.Evaluators.Speed
 {
     public static class SpeedEvaluator
     {
-        private const double single_spacing_threshold = OsuDifficultyHitObject.NORMALISED_DIAMETER * 1.25; // 1.25 circles distance between centers
         private const double min_speed_bonus = 200; // 200 BPM 1/4th
         private const double speed_balancing_factor = 40;
-        private const double distance_multiplier = 0.8;
 
         /// <summary>
         /// Evaluates the difficulty of tapping the current object, based on:
         /// <list type="bullet">
         /// <item><description>time between pressing the previous and current object,</description></item>
-        /// <item><description>distance between those objects,</description></item>
         /// <item><description>and how easily they can be cheesed.</description></item>
         /// </list>
         /// </summary>
-        public static double EvaluateDifficultyOf(DifficultyHitObject current, IReadOnlyList<Mod> mods)
+        public static double EvaluateDifficultyOf(DifficultyHitObject current)
         {
             if (current.BaseObject is Spinner)
                 return 0;
 
-            // derive strainTime for calculation
             var osuCurrObj = (OsuDifficultyHitObject)current;
-            var osuPrevObj = current.Index > 0 ? (OsuDifficultyHitObject)current.Previous(0) : null;
 
             double strainTime = osuCurrObj.AdjustedDeltaTime;
             double doubletapness = 1.0 - osuCurrObj.GetDoubletapness((OsuDifficultyHitObject?)osuCurrObj.Next(0));
 
             // Cap deltatime to the OD 300 hitwindow.
             // 0.93 is derived from making sure 260bpm OD8 streams aren't nerfed harshly, whilst 0.92 limits the effect of the cap.
-            strainTime /= Math.Clamp((strainTime / osuCurrObj.HitWindowGreat) / 0.93, 0.92, 1);
+            strainTime /= Math.Clamp((strainTime / osuCurrObj.HitWindow(HitResult.Great)) / 0.93, 0.92, 1);
 
             // speedBonus will be 0.0 for BPM < 200
             double speedBonus = 0.0;
@@ -51,26 +43,15 @@ namespace osu.Game.Rulesets.Osu.Difficulty.Evaluators
             if (DifficultyCalculationUtils.MillisecondsToBPM(strainTime) > min_speed_bonus)
                 speedBonus = 0.75 * Math.Pow((DifficultyCalculationUtils.BPMToMilliseconds(min_speed_bonus) - strainTime) / speed_balancing_factor, 2);
 
-            double travelDistance = osuPrevObj?.TravelDistance ?? 0;
-            double distance = travelDistance + osuCurrObj.MinimumJumpDistance;
-
-            // Cap distance at single_spacing_threshold
-            distance = Math.Min(distance, single_spacing_threshold);
-
-            // Max distance bonus is 1 * `distance_multiplier` at single_spacing_threshold
-            double distanceBonus = Math.Pow(distance / single_spacing_threshold, 3.95) * distance_multiplier;
-
-            // Apply reduced small circle bonus because flow aim difficulty on small circles doesn't scale as hard as jumps
-            distanceBonus *= Math.Sqrt(osuCurrObj.SmallCircleBonus);
-
-            if (mods.OfType<OsuModAutopilot>().Any())
-                distanceBonus = 0;
-
             // Base difficulty with all bonuses
-            double difficulty = (1 + speedBonus + distanceBonus) * 1000 / strainTime;
+            double speedDifficulty = (1 + speedBonus) * 1000 / strainTime;
+
+            speedDifficulty *= highBpmBonus(osuCurrObj.AdjustedDeltaTime);
 
             // Apply penalty if there's doubletappable doubles
-            return difficulty * doubletapness;
+            return speedDifficulty * doubletapness;
         }
+
+        private static double highBpmBonus(double ms) => 1 / (1 - Math.Pow(0.3, ms / 1000));
     }
 }

osu.Game.Rulesets.Osu/Difficulty/OsuDifficultyAttributes.cs

@@ -45,6 +45,12 @@ namespace osu.Game.Rulesets.Osu.Difficulty
         [JsonProperty("flashlight_difficulty")]
         public double FlashlightDifficulty { get; set; }
 
+        /// <summary>
+        /// The difficulty corresponding to the reading skill.
+        /// </summary>
+        [JsonProperty("reading_difficulty")]
+        public double ReadingDifficulty { get; set; }
+
         /// <summary>
         /// Describes how much of <see cref="AimDifficulty"/> is contributed to by hitcircles or sliders.
         /// A value closer to 1.0 indicates most of <see cref="AimDifficulty"/> is contributed by hitcircles.
@@ -75,6 +81,9 @@ namespace osu.Game.Rulesets.Osu.Difficulty
         [JsonProperty("speed_difficult_strain_count")]
         public double SpeedDifficultStrainCount { get; set; }
 
+        [JsonProperty("reading_difficult_note_count")]
+        public double ReadingDifficultNoteCount { get; set; }
+
         [JsonProperty("nested_score_per_object")]
         public double NestedScorePerObject { get; set; }
 
@@ -84,11 +93,6 @@ namespace osu.Game.Rulesets.Osu.Difficulty
         [JsonProperty("maximum_legacy_combo_score")]
         public double MaximumLegacyComboScore { get; set; }
 
-        /// <summary>
-        /// The beatmap's drain rate. This doesn't scale with rate-adjusting mods.
-        /// </summary>
-        public double DrainRate { get; set; }
-
         /// <summary>
         /// The number of hitcircles in the beatmap.
         /// </summary>
@@ -111,6 +115,7 @@ namespace osu.Game.Rulesets.Osu.Difficulty
 
             yield return (ATTRIB_ID_AIM, AimDifficulty);
             yield return (ATTRIB_ID_SPEED, SpeedDifficulty);
+            yield return (ATTRIB_ID_READING, ReadingDifficulty);
             yield return (ATTRIB_ID_DIFFICULTY, StarRating);
 
             if (ShouldSerializeFlashlightDifficulty())
@@ -127,6 +132,7 @@ namespace osu.Game.Rulesets.Osu.Difficulty
             yield return (ATTRIB_ID_NESTED_SCORE_PER_OBJECT, NestedScorePerObject);
             yield return (ATTRIB_ID_LEGACY_SCORE_BASE_MULTIPLIER, LegacyScoreBaseMultiplier);
             yield return (ATTRIB_ID_MAXIMUM_LEGACY_COMBO_SCORE, MaximumLegacyComboScore);
+            yield return (ATTRIB_ID_READING_DIFFICULT_NOTE_COUNT, ReadingDifficultNoteCount);
         }
 
         public override void FromDatabaseAttributes(IReadOnlyDictionary<int, double> values, IBeatmapOnlineInfo onlineInfo)
@@ -135,6 +141,7 @@ namespace osu.Game.Rulesets.Osu.Difficulty
 
             AimDifficulty = values[ATTRIB_ID_AIM];
             SpeedDifficulty = values[ATTRIB_ID_SPEED];
+            ReadingDifficulty = values[ATTRIB_ID_READING];
             StarRating = values[ATTRIB_ID_DIFFICULTY];
             FlashlightDifficulty = values.GetValueOrDefault(ATTRIB_ID_FLASHLIGHT);
             SliderFactor = values[ATTRIB_ID_SLIDER_FACTOR];
@@ -147,7 +154,7 @@ namespace osu.Game.Rulesets.Osu.Difficulty
             NestedScorePerObject = values[ATTRIB_ID_NESTED_SCORE_PER_OBJECT];
             LegacyScoreBaseMultiplier = values[ATTRIB_ID_LEGACY_SCORE_BASE_MULTIPLIER];
             MaximumLegacyComboScore = values[ATTRIB_ID_MAXIMUM_LEGACY_COMBO_SCORE];
-            DrainRate = onlineInfo.DrainRate;
+            ReadingDifficultNoteCount = values[ATTRIB_ID_READING_DIFFICULT_NOTE_COUNT];
             HitCircleCount = onlineInfo.CircleCount;
             SliderCount = onlineInfo.SliderCount;
             SpinnerCount = onlineInfo.SpinnerCount;

osu.Game.Rulesets.Osu/Difficulty/OsuDifficultyCalculator.cs

@@ -8,6 +8,7 @@ using osu.Game.Beatmaps;
 using osu.Game.Rulesets.Difficulty;
 using osu.Game.Rulesets.Difficulty.Preprocessing;
 using osu.Game.Rulesets.Difficulty.Skills;
+using osu.Game.Rulesets.Difficulty.Utils;
 using osu.Game.Rulesets.Mods;
 using osu.Game.Rulesets.Osu.Difficulty.Preprocessing;
 using osu.Game.Rulesets.Osu.Difficulty.Skills;
@@ -16,13 +17,12 @@ using osu.Game.Rulesets.Osu.Mods;
 using osu.Game.Rulesets.Osu.Objects;
 using osu.Game.Rulesets.Osu.Scoring;
 using osu.Game.Rulesets.Scoring;
+using osu.Game.Utils;
 
 namespace osu.Game.Rulesets.Osu.Difficulty
 {
     public class OsuDifficultyCalculator : DifficultyCalculator
     {
-        private const double star_rating_multiplier = 0.0265;
-
         public override int Version => 20251020;
 
         public OsuDifficultyCalculator(IRulesetInfo ruleset, IWorkingBeatmap beatmap)
@@ -46,7 +46,7 @@ namespace osu.Game.Rulesets.Osu.Difficulty
             return (79.5 - hitWindowGreat) / 6;
         }
 
-        protected override DifficultyAttributes CreateDifficultyAttributes(IBeatmap beatmap, Mod[] mods, Skill[] skills, double clockRate)
+        protected override DifficultyAttributes CreateDifficultyAttributes(IBeatmap beatmap, Mod[] mods, Skill[] skills)
         {
             if (beatmap.HitObjects.Count == 0)
                 return new OsuDifficultyAttributes { Mods = mods };
@@ -55,24 +55,30 @@ namespace osu.Game.Rulesets.Osu.Difficulty
             var aimWithoutSliders = skills.OfType<Aim>().Single(a => !a.IncludeSliders);
             var speed = skills.OfType<Speed>().Single();
             var flashlight = skills.OfType<Flashlight>().SingleOrDefault();
+            var reading = skills.OfType<Reading>().Single();
+
+            double aimDifficultyValue = aim.DifficultyValue();
+            double aimNoSlidersDifficultyValue = aimWithoutSliders.DifficultyValue();
+            double speedDifficultyValue = speed.DifficultyValue();
+            double readingDifficultyValue = reading.DifficultyValue();
+
+            double aimDifficultStrainCount = aim.CountTopWeightedStrains(aimDifficultyValue);
+            double speedDifficultStrainCount = speed.CountTopWeightedObjectDifficulties(speedDifficultyValue);
+            double readingDifficultNoteCount = reading.CountTopWeightedObjectDifficulties(readingDifficultyValue);
 
             double speedNotes = speed.RelevantNoteCount();
 
-            double aimDifficultStrainCount = aim.CountTopWeightedStrains();
-            double speedDifficultStrainCount = speed.CountTopWeightedStrains();
-
-            double aimNoSlidersTopWeightedSliderCount = aimWithoutSliders.CountTopWeightedSliders();
-            double aimNoSlidersDifficultStrainCount = aimWithoutSliders.CountTopWeightedStrains();
+            double aimNoSlidersTopWeightedSliderCount = aimWithoutSliders.CountTopWeightedSliders(aimNoSlidersDifficultyValue);
+            double aimNoSlidersDifficultStrainCount = aimWithoutSliders.CountTopWeightedStrains(aimNoSlidersDifficultyValue);
 
             double aimTopWeightedSliderFactor = aimNoSlidersTopWeightedSliderCount / Math.Max(1, aimNoSlidersDifficultStrainCount - aimNoSlidersTopWeightedSliderCount);
 
-            double speedTopWeightedSliderCount = speed.CountTopWeightedSliders();
+            double speedTopWeightedSliderCount = speed.CountTopWeightedSliders(speedDifficultyValue);
             double speedTopWeightedSliderFactor = speedTopWeightedSliderCount / Math.Max(1, speedDifficultStrainCount - speedTopWeightedSliderCount);
 
             double difficultSliders = aim.GetDifficultSliders();
 
-            double approachRate = CalculateRateAdjustedApproachRate(beatmap.Difficulty.ApproachRate, clockRate);
-            double overallDifficulty = CalculateRateAdjustedOverallDifficulty(beatmap.Difficulty.OverallDifficulty, clockRate);
+            double overallDifficulty = CalculateRateAdjustedOverallDifficulty(beatmap.Difficulty.OverallDifficulty, ModUtils.CalculateRateWithMods(mods));
 
             int hitCircleCount = beatmap.HitObjects.Count(h => h is HitCircle);
             int sliderCount = beatmap.HitObjects.Count(h => h is Slider);
@@ -80,19 +86,15 @@ namespace osu.Game.Rulesets.Osu.Difficulty
 
             int totalHits = beatmap.HitObjects.Count;
 
-            double drainRate = beatmap.Difficulty.DrainRate;
+            double sliderFactor = aimDifficultyValue > 0
+                ? OsuRatingCalculator.CalculateDifficultyRating(aimNoSlidersDifficultyValue) / OsuRatingCalculator.CalculateDifficultyRating(aimDifficultyValue)
+                : 1;
 
-            double aimDifficultyValue = aim.DifficultyValue();
-            double aimNoSlidersDifficultyValue = aimWithoutSliders.DifficultyValue();
-            double speedDifficultyValue = speed.DifficultyValue();
-
-            double mechanicalDifficultyRating = calculateMechanicalDifficultyRating(aimDifficultyValue, speedDifficultyValue);
-            double sliderFactor = aimDifficultyValue > 0 ? OsuRatingCalculator.CalculateDifficultyRating(aimNoSlidersDifficultyValue) / OsuRatingCalculator.CalculateDifficultyRating(aimDifficultyValue) : 1;
-
-            var osuRatingCalculator = new OsuRatingCalculator(mods, totalHits, approachRate, overallDifficulty, mechanicalDifficultyRating, sliderFactor);
+            var osuRatingCalculator = new OsuRatingCalculator(totalHits, overallDifficulty);
 
             double aimRating = osuRatingCalculator.ComputeAimRating(aimDifficultyValue);
             double speedRating = osuRatingCalculator.ComputeSpeedRating(speedDifficultyValue);
+            double readingRating = osuRatingCalculator.ComputeReadingRating(readingDifficultyValue);
 
             double flashlightRating = 0.0;
 
@@ -100,21 +102,18 @@ namespace osu.Game.Rulesets.Osu.Difficulty
                 flashlightRating = osuRatingCalculator.ComputeFlashlightRating(flashlight.DifficultyValue());
 
             double sliderNestedScorePerObject = LegacyScoreUtils.CalculateNestedScorePerObject(beatmap, totalHits);
-            double legacyScoreBaseMultiplier = LegacyScoreUtils.CalculateDifficultyPeppyStars(beatmap);
+            double legacyScoreBaseMultiplier = LegacyScoreUtils.CalculateDifficultyPeppyStars(WorkingBeatmap.Beatmap);
 
             var simulator = new OsuLegacyScoreSimulator();
             var scoreAttributes = simulator.Simulate(WorkingBeatmap, beatmap);
 
-            double baseAimPerformance = OsuStrainSkill.DifficultyToPerformance(aimRating);
-            double baseSpeedPerformance = OsuStrainSkill.DifficultyToPerformance(speedRating);
+            double baseAimPerformance = OsuPerformanceCalculator.DifficultyToPerformance(aimRating);
+            double baseSpeedPerformance = HarmonicSkill.DifficultyToPerformance(speedRating);
+            double baseReadingPerformance = HarmonicSkill.DifficultyToPerformance(readingRating);
             double baseFlashlightPerformance = Flashlight.DifficultyToPerformance(flashlightRating);
+            double baseCognitionPerformance = SumCognitionDifficulty(baseReadingPerformance, baseFlashlightPerformance);
 
-            double basePerformance =
-                Math.Pow(
-                    Math.Pow(baseAimPerformance, 1.1) +
-                    Math.Pow(baseSpeedPerformance, 1.1) +
-                    Math.Pow(baseFlashlightPerformance, 1.1), 1.0 / 1.1
-                );
+            double basePerformance = DifficultyCalculationUtils.Norm(OsuPerformanceCalculator.PERFORMANCE_NORM_EXPONENT, baseAimPerformance, baseSpeedPerformance, baseCognitionPerformance);
 
             double starRating = calculateStarRating(basePerformance);
 
@@ -127,12 +126,13 @@ namespace osu.Game.Rulesets.Osu.Difficulty
                 SpeedDifficulty = speedRating,
                 SpeedNoteCount = speedNotes,
                 FlashlightDifficulty = flashlightRating,
+                ReadingDifficulty = readingRating,
                 SliderFactor = sliderFactor,
                 AimDifficultStrainCount = aimDifficultStrainCount,
                 SpeedDifficultStrainCount = speedDifficultStrainCount,
+                ReadingDifficultNoteCount = readingDifficultNoteCount,
                 AimTopWeightedSliderFactor = aimTopWeightedSliderFactor,
                 SpeedTopWeightedSliderFactor = speedTopWeightedSliderFactor,
-                DrainRate = drainRate,
                 MaxCombo = beatmap.GetMaxCombo(),
                 HitCircleCount = hitCircleCount,
                 SliderCount = sliderCount,
@@ -145,28 +145,29 @@ namespace osu.Game.Rulesets.Osu.Difficulty
             return attributes;
         }
 
-        private double calculateMechanicalDifficultyRating(double aimDifficultyValue, double speedDifficultyValue)
+        public static double SumCognitionDifficulty(double reading, double flashlight)
         {
-            double aimValue = OsuStrainSkill.DifficultyToPerformance(OsuRatingCalculator.CalculateDifficultyRating(aimDifficultyValue));
-            double speedValue = OsuStrainSkill.DifficultyToPerformance(OsuRatingCalculator.CalculateDifficultyRating(speedDifficultyValue));
+            if (reading <= 0)
+                return flashlight;
 
-            double totalValue = Math.Pow(Math.Pow(aimValue, 1.1) + Math.Pow(speedValue, 1.1), 1 / 1.1);
+            if (flashlight <= 0)
+                return reading;
 
-            return calculateStarRating(totalValue);
+            // Nerf flashlight value in cognition sum when reading is greater than flashlight
+            return DifficultyCalculationUtils.Norm(OsuPerformanceCalculator.PERFORMANCE_NORM_EXPONENT, reading, flashlight * Math.Clamp(flashlight / reading, 0.25, 1.0));
         }
 
         private double calculateStarRating(double basePerformance)
         {
-            if (basePerformance <= 0.00001)
-                return 0;
-
-            return Math.Cbrt(OsuPerformanceCalculator.PERFORMANCE_BASE_MULTIPLIER) * star_rating_multiplier * (Math.Cbrt(100000 / Math.Pow(2, 1 / 1.1) * basePerformance) + 4);
+            return Math.Cbrt(basePerformance * OsuPerformanceCalculator.PERFORMANCE_BASE_MULTIPLIER);
         }
 
-        protected override IEnumerable<DifficultyHitObject> CreateDifficultyHitObjects(IBeatmap beatmap, double clockRate)
+        protected override IEnumerable<DifficultyHitObject> CreateDifficultyHitObjects(IBeatmap beatmap, Mod[] mods)
         {
             List<DifficultyHitObject> objects = new List<DifficultyHitObject>();
 
+            double clockRate = ModUtils.CalculateRateWithMods(mods);
+
             // The first jump is formed by the first two hitobjects of the map.
             // If the map has less than two OsuHitObjects, the enumerator will not return anything.
             for (int i = 1; i < beatmap.HitObjects.Count; i++)
@@ -177,13 +178,14 @@ namespace osu.Game.Rulesets.Osu.Difficulty
             return objects;
         }
 
-        protected override Skill[] CreateSkills(IBeatmap beatmap, Mod[] mods, double clockRate)
+        protected override Skill[] CreateSkills(IBeatmap beatmap, Mod[] mods)
         {
             var skills = new List<Skill>
             {
                 new Aim(mods, true),
                 new Aim(mods, false),
-                new Speed(mods)
+                new Speed(mods),
+                new Reading(mods)
             };
 
             if (mods.Any(h => h is OsuModFlashlight))

osu.Game.Rulesets.Osu/Difficulty/OsuLegacyScoreMissCalculator.cs

@@ -115,9 +115,13 @@ namespace osu.Game.Rulesets.Osu.Difficulty
 
             double missCount = 0;
 
+            // If sliders in the map are hard - it's likely for player to drop sliderends
+            // If map has easy sliders - it's more likely for player to sliderbreak
+            double likelyMissedSliderendPortion = 0.04 + 0.06 * Math.Pow(Math.Min(attributes.AimTopWeightedSliderFactor, 1), 2);
+
             // Consider that full combo is maximum combo minus dropped slider tails since they don't contribute to combo but also don't break it
-            // In classic scores we can't know the amount of dropped sliders so we estimate to 10% of all sliders on the map
-            double fullComboThreshold = attributes.MaxCombo - 0.1 * attributes.SliderCount;
+            // In classic scores we can't know the amount of dropped sliders so we estimate it
+            double fullComboThreshold = attributes.MaxCombo - Math.Min(4 + likelyMissedSliderendPortion * attributes.SliderCount, attributes.SliderCount);
 
             if (score.MaxCombo < fullComboThreshold)
                 missCount = Math.Pow(fullComboThreshold / Math.Max(1.0, score.MaxCombo), 2.5);

osu.Game.Rulesets.Osu/Difficulty/OsuPerformanceAttributes.cs

@@ -21,6 +21,9 @@ namespace osu.Game.Rulesets.Osu.Difficulty
         [JsonProperty("flashlight")]
         public double Flashlight { get; set; }
 
+        [JsonProperty("reading")]
+        public double Reading { get; set; }
+
         [JsonProperty("effective_miss_count")]
         public double EffectiveMissCount { get; set; }
 
@@ -48,6 +51,7 @@ namespace osu.Game.Rulesets.Osu.Difficulty
             yield return new PerformanceDisplayAttribute(nameof(Speed), "Speed", Speed);
             yield return new PerformanceDisplayAttribute(nameof(Accuracy), "Accuracy", Accuracy);
             yield return new PerformanceDisplayAttribute(nameof(Flashlight), "Flashlight Bonus", Flashlight);
+            yield return new PerformanceDisplayAttribute(nameof(Reading), "Reading", Reading);
         }
     }
 }

osu.Game.Rulesets.Osu/Difficulty/OsuPerformanceCalculator.cs

@@ -9,6 +9,7 @@ using osu.Game.Rulesets.Difficulty.Utils;
 using osu.Game.Rulesets.Mods;
 using osu.Game.Rulesets.Osu.Scoring;
 using osu.Game.Rulesets.Difficulty;
+using osu.Game.Rulesets.Difficulty.Skills;
 using osu.Game.Rulesets.Osu.Difficulty.Skills;
 using osu.Game.Rulesets.Osu.Mods;
 using osu.Game.Rulesets.Scoring;
@@ -19,7 +20,8 @@ namespace osu.Game.Rulesets.Osu.Difficulty
 {
     public class OsuPerformanceCalculator : PerformanceCalculator
     {
-        public const double PERFORMANCE_BASE_MULTIPLIER = 1.14; // This is being adjusted to keep the final pp value scaled around what it used to be when changing things.
+        public const double PERFORMANCE_BASE_MULTIPLIER = 1.12; // This is being adjusted to keep the final pp value scaled around what it used to be when changing things.
+        public const double PERFORMANCE_NORM_EXPONENT = 1.1;
 
         private bool usingClassicSliderAccuracy;
         private bool usingScoreV2;
@@ -50,14 +52,18 @@ namespace osu.Game.Rulesets.Osu.Difficulty
         private double greatHitWindow;
         private double okHitWindow;
         private double mehHitWindow;
+
         private double overallDifficulty;
         private double approachRate;
+        private double drainRate;
 
         private double? speedDeviation;
 
         private double aimEstimatedSliderBreaks;
         private double speedEstimatedSliderBreaks;
 
+        public static double DifficultyToPerformance(double difficulty) => 4.0 * Math.Pow(difficulty, 3.0);
+
         public OsuPerformanceCalculator()
             : base(new OsuRuleset())
         {
@@ -95,11 +101,12 @@ namespace osu.Game.Rulesets.Osu.Difficulty
 
             approachRate = OsuDifficultyCalculator.CalculateRateAdjustedApproachRate(difficulty.ApproachRate, clockRate);
             overallDifficulty = OsuDifficultyCalculator.CalculateRateAdjustedOverallDifficulty(difficulty.OverallDifficulty, clockRate);
+            drainRate = difficulty.DrainRate;
 
             double comboBasedEstimatedMissCount = calculateComboBasedEstimatedMissCount(osuAttributes);
             double? scoreBasedEstimatedMissCount = null;
 
-            if (usingClassicSliderAccuracy && score.LegacyTotalScore != null)
+            if (usingClassicSliderAccuracy && !usingScoreV2 && score.LegacyTotalScore != null)
             {
                 var legacyScoreMissCalculator = new OsuLegacyScoreMissCalculator(score, osuAttributes);
                 scoreBasedEstimatedMissCount = legacyScoreMissCalculator.Calculate();
@@ -115,6 +122,12 @@ namespace osu.Game.Rulesets.Osu.Difficulty
             effectiveMissCount = Math.Max(countMiss, effectiveMissCount);
             effectiveMissCount = Math.Min(totalHits, effectiveMissCount);
 
+            if (effectiveMissCount > 0)
+            {
+                aimEstimatedSliderBreaks = calculateEstimatedSliderBreaks(osuAttributes.AimTopWeightedSliderFactor, osuAttributes);
+                speedEstimatedSliderBreaks = calculateEstimatedSliderBreaks(osuAttributes.SpeedTopWeightedSliderFactor, osuAttributes);
+            }
+
             double multiplier = PERFORMANCE_BASE_MULTIPLIER;
 
             if (score.Mods.Any(m => m is OsuModNoFail))
@@ -140,15 +153,12 @@ namespace osu.Game.Rulesets.Osu.Difficulty
             double aimValue = computeAimValue(score, osuAttributes);
             double speedValue = computeSpeedValue(score, osuAttributes);
             double accuracyValue = computeAccuracyValue(score, osuAttributes);
-            double flashlightValue = computeFlashlightValue(score, osuAttributes);
 
-            double totalValue =
-                Math.Pow(
-                    Math.Pow(aimValue, 1.1) +
-                    Math.Pow(speedValue, 1.1) +
-                    Math.Pow(accuracyValue, 1.1) +
-                    Math.Pow(flashlightValue, 1.1), 1.0 / 1.1
-                ) * multiplier;
+            double readingValue = computeReadingValue(osuAttributes);
+            double flashlightValue = computeFlashlightValue(score, osuAttributes);
+            double cognitionValue = OsuDifficultyCalculator.SumCognitionDifficulty(readingValue, flashlightValue);
+
+            double totalValue = DifficultyCalculationUtils.Norm(PERFORMANCE_NORM_EXPONENT, aimValue, speedValue, accuracyValue, cognitionValue) * multiplier;
 
             return new OsuPerformanceAttributes
             {
@@ -156,6 +166,7 @@ namespace osu.Game.Rulesets.Osu.Difficulty
                 Speed = speedValue,
                 Accuracy = accuracyValue,
                 Flashlight = flashlightValue,
+                Reading = readingValue,
                 EffectiveMissCount = effectiveMissCount,
                 ComboBasedEstimatedMissCount = comboBasedEstimatedMissCount,
                 ScoreBasedEstimatedMissCount = scoreBasedEstimatedMissCount,
@@ -194,16 +205,14 @@ namespace osu.Game.Rulesets.Osu.Difficulty
                 aimDifficulty *= sliderNerfFactor;
             }
 
-            double aimValue = OsuStrainSkill.DifficultyToPerformance(aimDifficulty);
+            double aimValue = DifficultyToPerformance(aimDifficulty);
 
-            double lengthBonus = 0.95 + 0.4 * Math.Min(1.0, totalHits / 2000.0) +
+            double lengthBonus = 0.95 + 0.35 * Math.Min(1.0, totalHits / 2000.0) +
                                  (totalHits > 2000 ? Math.Log10(totalHits / 2000.0) * 0.5 : 0.0);
             aimValue *= lengthBonus;
 
             if (effectiveMissCount > 0)
             {
-                aimEstimatedSliderBreaks = calculateEstimatedSliderBreaks(attributes.AimTopWeightedSliderFactor, attributes);
-
                 double relevantMissCount = Math.Min(effectiveMissCount + aimEstimatedSliderBreaks, totalImperfectHits + countSliderTickMiss);
 
                 aimValue *= calculateMissPenalty(relevantMissCount, attributes.AimDifficultStrainCount);
@@ -211,10 +220,10 @@ namespace osu.Game.Rulesets.Osu.Difficulty
 
             // TC bonuses are excluded when blinds is present as the increased visual difficulty is unimportant when notes cannot be seen.
             if (score.Mods.Any(m => m is OsuModBlinds))
-                aimValue *= 1.3 + (totalHits * (0.0016 / (1 + 2 * effectiveMissCount)) * Math.Pow(accuracy, 16)) * (1 - 0.003 * attributes.DrainRate * attributes.DrainRate);
+                aimValue *= 1.3 + (totalHits * (0.0016 / (1 + 2 * effectiveMissCount)) * Math.Pow(accuracy, 16)) * (1 - 0.003 * drainRate * drainRate);
             else if (score.Mods.Any(m => m is OsuModTraceable))
             {
-                aimValue *= 1.0 + OsuRatingCalculator.CalculateVisibilityBonus(score.Mods, approachRate, sliderFactor: attributes.SliderFactor);
+                aimValue *= 1.0 + calculateTraceableBonus(attributes.SliderFactor);
             }
 
             aimValue *= accuracy;
@@ -227,44 +236,33 @@ namespace osu.Game.Rulesets.Osu.Difficulty
             if (score.Mods.Any(h => h is OsuModRelax) || speedDeviation == null)
                 return 0.0;
 
-            double speedValue = OsuStrainSkill.DifficultyToPerformance(attributes.SpeedDifficulty);
-
-            double lengthBonus = 0.95 + 0.4 * Math.Min(1.0, totalHits / 2000.0) +
-                                 (totalHits > 2000 ? Math.Log10(totalHits / 2000.0) * 0.5 : 0.0);
-            speedValue *= lengthBonus;
+            double speedValue = HarmonicSkill.DifficultyToPerformance(attributes.SpeedDifficulty);
 
             if (effectiveMissCount > 0)
             {
-                speedEstimatedSliderBreaks = calculateEstimatedSliderBreaks(attributes.SpeedTopWeightedSliderFactor, attributes);
-
                 double relevantMissCount = Math.Min(effectiveMissCount + speedEstimatedSliderBreaks, totalImperfectHits + countSliderTickMiss);
 
                 speedValue *= calculateMissPenalty(relevantMissCount, attributes.SpeedDifficultStrainCount);
             }
 
-            // TC bonuses are excluded when blinds is present as the increased visual difficulty is unimportant when notes cannot be seen.
             if (score.Mods.Any(m => m is OsuModBlinds))
             {
                 // Increasing the speed value by object count for Blinds isn't ideal, so the minimum buff is given.
                 speedValue *= 1.12;
             }
-            else if (score.Mods.Any(m => m is OsuModTraceable))
-            {
-                speedValue *= 1.0 + OsuRatingCalculator.CalculateVisibilityBonus(score.Mods, approachRate);
-            }
 
             double speedHighDeviationMultiplier = calculateSpeedHighDeviationNerf(attributes);
             speedValue *= speedHighDeviationMultiplier;
 
-            // Calculate accuracy assuming the worst case scenario
-            double relevantTotalDiff = Math.Max(0, totalHits - attributes.SpeedNoteCount);
-            double relevantCountGreat = Math.Max(0, countGreat - relevantTotalDiff);
-            double relevantCountOk = Math.Max(0, countOk - Math.Max(0, relevantTotalDiff - countGreat));
-            double relevantCountMeh = Math.Max(0, countMeh - Math.Max(0, relevantTotalDiff - countGreat - countOk));
-            double relevantAccuracy = attributes.SpeedNoteCount == 0 ? 0 : (relevantCountGreat * 6.0 + relevantCountOk * 2.0 + relevantCountMeh) / (attributes.SpeedNoteCount * 6.0);
+            // An effective hit window is created based on the speed SR. The higher the speed difficulty, the shorter the hit window.
+            // For example, a speed SR of 4.0 leads to an effective hit window of 20ms, which is OD 10.
+            double effectiveHitWindow = 20 * Math.Pow(4 / attributes.SpeedDifficulty, 0.35);
 
-            // Scale the speed value with accuracy and OD.
-            speedValue *= Math.Pow((accuracy + relevantAccuracy) / 2.0, (14.5 - overallDifficulty) / 2);
+            // Find the proportion of 300s on speed notes assuming the hit window was the effective hit window.
+            double effectiveAccuracy = DifficultyCalculationUtils.Erf(effectiveHitWindow / (double)speedDeviation);
+
+            // Scale speed value by normalized accuracy.
+            speedValue *= Math.Pow(effectiveAccuracy, 2);
 
             return speedValue;
         }
@@ -294,20 +292,19 @@ namespace osu.Game.Rulesets.Osu.Difficulty
             double accuracyValue = Math.Pow(1.52163, overallDifficulty) * Math.Pow(betterAccuracyPercentage, 24) * 2.83;
 
             // Bonus for many hitcircles - it's harder to keep good accuracy up for longer.
-            accuracyValue *= Math.Min(1.15, Math.Pow(amountHitObjectsWithAccuracy / 1000.0, 0.3));
+            accuracyValue *= amountHitObjectsWithAccuracy < 1000
+                ? Math.Pow(amountHitObjectsWithAccuracy / 1000.0, 0.3)
+                : Math.Pow(amountHitObjectsWithAccuracy / 1000.0, 0.1);
 
             // Increasing the accuracy value by object count for Blinds isn't ideal, so the minimum buff is given.
             if (score.Mods.Any(m => m is OsuModBlinds))
                 accuracyValue *= 1.14;
-            else if (score.Mods.Any(m => m is OsuModHidden || m is OsuModTraceable))
+            else if (score.Mods.Any(m => m is OsuModTraceable))
             {
                 // Decrease bonus for AR > 10
                 accuracyValue *= 1 + 0.08 * DifficultyCalculationUtils.ReverseLerp(approachRate, 11.5, 10);
             }
 
-            if (score.Mods.Any(m => m is OsuModFlashlight))
-                accuracyValue *= 1.02;
-
             return accuracyValue;
         }
 
@@ -330,6 +327,19 @@ namespace osu.Game.Rulesets.Osu.Difficulty
             return flashlightValue;
         }
 
+        private double computeReadingValue(OsuDifficultyAttributes attributes)
+        {
+            double readingValue = HarmonicSkill.DifficultyToPerformance(attributes.ReadingDifficulty);
+
+            if (effectiveMissCount > 0)
+                readingValue *= calculateMissPenalty(effectiveMissCount + aimEstimatedSliderBreaks, attributes.ReadingDifficultNoteCount);
+
+            // Scale the reading value with accuracy _harshly_.
+            readingValue *= Math.Pow(accuracy, 3);
+
+            return readingValue;
+        }
+
         private double calculateComboBasedEstimatedMissCount(OsuDifficultyAttributes attributes)
         {
             if (attributes.SliderCount <= 0)
@@ -339,9 +349,13 @@ namespace osu.Game.Rulesets.Osu.Difficulty
 
             if (usingClassicSliderAccuracy)
             {
+                // If sliders in the map are hard - it's likely for player to drop sliderends
+                // If map has easy sliders - it's more likely for player to sliderbreak
+                double likelyMissedSliderendPortion = 0.04 + 0.06 * Math.Pow(Math.Min(attributes.AimTopWeightedSliderFactor, 1), 2);
+
                 // Consider that full combo is maximum combo minus dropped slider tails since they don't contribute to combo but also don't break it
-                // In classic scores we can't know the amount of dropped sliders so we estimate to 10% of all sliders on the map
-                double fullComboThreshold = attributes.MaxCombo - 0.1 * attributes.SliderCount;
+                // In classic scores we can't know the amount of dropped sliders so we estimate it
+                double fullComboThreshold = attributes.MaxCombo - Math.Min(4 + likelyMissedSliderendPortion * attributes.SliderCount, attributes.SliderCount);
 
                 if (scoreMaxCombo < fullComboThreshold)
                     missCount = fullComboThreshold / Math.Max(1.0, scoreMaxCombo);
@@ -376,19 +390,22 @@ namespace osu.Game.Rulesets.Osu.Difficulty
 
         private double calculateEstimatedSliderBreaks(double topWeightedSliderFactor, OsuDifficultyAttributes attributes)
         {
-            if (!usingClassicSliderAccuracy || countOk == 0)
+            int nonMissMistakes = countOk + countMeh;
+
+            if (!usingClassicSliderAccuracy || nonMissMistakes == 0)
                 return 0;
 
             double missedComboPercent = 1.0 - (double)scoreMaxCombo / attributes.MaxCombo;
-            double estimatedSliderBreaks = Math.Min(countOk, effectiveMissCount * topWeightedSliderFactor);
+            double estimatedSliderBreaks = Math.Min(nonMissMistakes, effectiveMissCount * topWeightedSliderFactor);
 
-            // Scores with more Oks are more likely to have slider breaks.
-            double okAdjustment = ((countOk - estimatedSliderBreaks) + 0.5) / countOk;
+            // Scores with more Oks and Mehs are more likely to have slider breaks.
+            // We add an arbitrary value to both sides of the division to make it more stable on extreme ends.
+            double nonMissMistakeAdjustment = (nonMissMistakes - estimatedSliderBreaks + 4.5) / (nonMissMistakes + 4);
 
             // There is a low probability of extra slider breaks on effective miss counts close to 1, as score based calculations are good at indicating if only a single break occurred.
             estimatedSliderBreaks *= DifficultyCalculationUtils.Smoothstep(effectiveMissCount, 1, 2);
 
-            return estimatedSliderBreaks * okAdjustment * DifficultyCalculationUtils.Logistic(missedComboPercent, 0.33, 15);
+            return estimatedSliderBreaks * nonMissMistakeAdjustment * DifficultyCalculationUtils.Logistic(missedComboPercent, 0.33, 15);
         }
 
         /// <summary>
@@ -470,7 +487,7 @@ namespace osu.Game.Rulesets.Osu.Difficulty
             if (speedDeviation == null)
                 return 0;
 
-            double speedValue = OsuStrainSkill.DifficultyToPerformance(attributes.SpeedDifficulty);
+            double speedValue = HarmonicSkill.DifficultyToPerformance(attributes.SpeedDifficulty);
 
             // Decides a point where the PP value achieved compared to the speed deviation is assumed to be tapped improperly. Any PP above this point is considered "excess" speed difficulty.
             // This is used to cause PP above the cutoff to scale logarithmically towards the original speed value thus nerfing the value.
@@ -489,10 +506,34 @@ namespace osu.Game.Rulesets.Osu.Difficulty
             return adjustedSpeedValue / speedValue;
         }
 
+        /// <summary>
+        /// Calculates a visibility bonus that is applicable to Traceable.
+        /// </summary>
+        private double calculateTraceableBonus(double sliderFactor = 1)
+        {
+            // We want to reward slider aim less, more so at lower AR
+            double highApproachRateSliderVisibilityFactor = 0.5 + (Math.Pow(sliderFactor, 6) / 2);
+            double lowApproachRateSliderVisibilityFactor = Math.Pow(sliderFactor, 6);
+
+            // Start from normal curve, rewarding lower AR up to AR7
+            double traceableBonus = 0.0275;
+            traceableBonus += 0.025 * (12.0 - Math.Max(approachRate, 7)) * highApproachRateSliderVisibilityFactor;
+
+            // For AR up to 0 - reduce reward for very low ARs when object is visible
+            if (approachRate < 7)
+                traceableBonus += 0.025 * (7.0 - Math.Max(approachRate, 0)) * lowApproachRateSliderVisibilityFactor;
+
+            // Starting from AR0 - cap values so they won't grow to infinity
+            if (approachRate < 0)
+                traceableBonus += 0.025 * (1 - Math.Pow(1.5, approachRate)) * lowApproachRateSliderVisibilityFactor;
+
+            return traceableBonus;
+        }
+
         // Miss penalty assumes that a player will miss on the hardest parts of a map,
         // so we use the amount of relatively difficult sections to adjust miss penalty
         // to make it more punishing on maps with lower amount of hard sections.
-        private double calculateMissPenalty(double missCount, double difficultStrainCount) => 0.96 / ((missCount / (4 * Math.Pow(Math.Log(difficultStrainCount), 0.94))) + 1);
+        private double calculateMissPenalty(double missCount, double difficultStrainCount) => 0.93 / (missCount / (4 * Math.Log(difficultStrainCount)) + 1);
         private double getComboScalingFactor(OsuDifficultyAttributes attributes) => attributes.MaxCombo <= 0 ? 1.0 : Math.Min(Math.Pow(scoreMaxCombo, 0.8) / Math.Pow(attributes.MaxCombo, 0.8), 1.0);
 
         private int totalHits => countGreat + countOk + countMeh + countMiss;

osu.Game.Rulesets.Osu/Difficulty/OsuRatingCalculator.cs

@@ -2,10 +2,6 @@
 // See the LICENCE file in the repository root for full licence text.
 
 using System;
-using System.Linq;
-using osu.Game.Rulesets.Difficulty.Utils;
-using osu.Game.Rulesets.Mods;
-using osu.Game.Rulesets.Osu.Mods;
 
 namespace osu.Game.Rulesets.Osu.Difficulty
 {
@@ -13,64 +9,21 @@ namespace osu.Game.Rulesets.Osu.Difficulty
     {
         private const double difficulty_multiplier = 0.0675;
 
-        private readonly Mod[] mods;
         private readonly int totalHits;
-        private readonly double approachRate;
         private readonly double overallDifficulty;
-        private readonly double mechanicalDifficultyRating;
-        private readonly double sliderFactor;
 
-        public OsuRatingCalculator(Mod[] mods, int totalHits, double approachRate, double overallDifficulty, double mechanicalDifficultyRating, double sliderFactor)
+        public OsuRatingCalculator(int totalHits, double overallDifficulty)
         {
-            this.mods = mods;
             this.totalHits = totalHits;
-            this.approachRate = approachRate;
             this.overallDifficulty = overallDifficulty;
-            this.mechanicalDifficultyRating = mechanicalDifficultyRating;
-            this.sliderFactor = sliderFactor;
         }
 
         public double ComputeAimRating(double aimDifficultyValue)
         {
-            if (mods.Any(m => m is OsuModAutopilot))
-                return 0;
-
-            double aimRating = CalculateDifficultyRating(aimDifficultyValue);
-
-            if (mods.Any(m => m is OsuModTouchDevice))
-                aimRating = Math.Pow(aimRating, 0.8);
-
-            if (mods.Any(m => m is OsuModRelax))
-                aimRating *= 0.9;
-
-            if (mods.Any(m => m is OsuModMagnetised))
-            {
-                float magnetisedStrength = mods.OfType<OsuModMagnetised>().First().AttractionStrength.Value;
-                aimRating *= 1.0 - magnetisedStrength;
-            }
+            double aimRating = Math.Pow(aimDifficultyValue, 0.63) * 0.02275;
 
             double ratingMultiplier = 1.0;
 
-            double approachRateLengthBonus = 0.95 + 0.4 * Math.Min(1.0, totalHits / 2000.0) +
-                                             (totalHits > 2000 ? Math.Log10(totalHits / 2000.0) * 0.5 : 0.0);
-
-            double approachRateFactor = 0.0;
-            if (approachRate > 10.33)
-                approachRateFactor = 0.3 * (approachRate - 10.33);
-            else if (approachRate < 8.0)
-                approachRateFactor = 0.05 * (8.0 - approachRate);
-
-            if (mods.Any(h => h is OsuModRelax))
-                approachRateFactor = 0.0;
-
-            ratingMultiplier += approachRateFactor * approachRateLengthBonus; // Buff for longer maps with high AR.
-
-            if (mods.Any(m => m is OsuModHidden))
-            {
-                double visibilityFactor = calculateAimVisibilityFactor(approachRate);
-                ratingMultiplier += CalculateVisibilityBonus(mods, approachRate, visibilityFactor, sliderFactor);
-            }
-
             // It is important to consider accuracy difficulty when scaling with accuracy.
             ratingMultiplier *= 0.98 + Math.Pow(Math.Max(0, overallDifficulty), 2) / 2500;
 
@@ -79,73 +32,24 @@ namespace osu.Game.Rulesets.Osu.Difficulty
 
         public double ComputeSpeedRating(double speedDifficultyValue)
         {
-            if (mods.Any(m => m is OsuModRelax))
-                return 0;
+            return CalculateDifficultyRating(speedDifficultyValue);
+        }
 
-            double speedRating = CalculateDifficultyRating(speedDifficultyValue);
-
-            if (mods.Any(m => m is OsuModAutopilot))
-                speedRating *= 0.5;
-
-            if (mods.Any(m => m is OsuModMagnetised))
-            {
-                // reduce speed rating because of the speed distance scaling, with maximum reduction being 0.7x
-                float magnetisedStrength = mods.OfType<OsuModMagnetised>().First().AttractionStrength.Value;
-                speedRating *= 1.0 - magnetisedStrength * 0.3;
-            }
+        public double ComputeReadingRating(double readingDifficultyValue)
+        {
+            double readingRating = CalculateDifficultyRating(readingDifficultyValue);
 
             double ratingMultiplier = 1.0;
 
-            double approachRateLengthBonus = 0.95 + 0.4 * Math.Min(1.0, totalHits / 2000.0) +
-                                             (totalHits > 2000 ? Math.Log10(totalHits / 2000.0) * 0.5 : 0.0);
+            ratingMultiplier *= 0.75 + Math.Pow(Math.Max(0, overallDifficulty), 2.2) / 800;
 
-            double approachRateFactor = 0.0;
-            if (approachRate > 10.33)
-                approachRateFactor = 0.3 * (approachRate - 10.33);
-
-            if (mods.Any(m => m is OsuModAutopilot))
-                approachRateFactor = 0.0;
-
-            ratingMultiplier += approachRateFactor * approachRateLengthBonus; // Buff for longer maps with high AR.
-
-            if (mods.Any(m => m is OsuModHidden))
-            {
-                double visibilityFactor = calculateSpeedVisibilityFactor(approachRate);
-                ratingMultiplier += CalculateVisibilityBonus(mods, approachRate, visibilityFactor);
-            }
-
-            ratingMultiplier *= 0.95 + Math.Pow(Math.Max(0, overallDifficulty), 2) / 750;
-
-            return speedRating * Math.Cbrt(ratingMultiplier);
+            return readingRating * Math.Cbrt(ratingMultiplier);
         }
 
         public double ComputeFlashlightRating(double flashlightDifficultyValue)
         {
-            if (!mods.Any(m => m is OsuModFlashlight))
-                return 0;
-
             double flashlightRating = CalculateDifficultyRating(flashlightDifficultyValue);
 
-            if (mods.Any(m => m is OsuModTouchDevice))
-                flashlightRating = Math.Pow(flashlightRating, 0.8);
-
-            if (mods.Any(m => m is OsuModRelax))
-                flashlightRating *= 0.7;
-            else if (mods.Any(m => m is OsuModAutopilot))
-                flashlightRating *= 0.4;
-
-            if (mods.Any(m => m is OsuModMagnetised))
-            {
-                float magnetisedStrength = mods.OfType<OsuModMagnetised>().First().AttractionStrength.Value;
-                flashlightRating *= 1.0 - magnetisedStrength;
-            }
-
-            if (mods.Any(m => m is OsuModDeflate))
-            {
-                float deflateInitialScale = mods.OfType<OsuModDeflate>().First().StartScale.Value;
-                flashlightRating *= Math.Clamp(DifficultyCalculationUtils.ReverseLerp(deflateInitialScale, 11, 1), 0.1, 1);
-            }
-
             double ratingMultiplier = 1.0;
 
             // Account for shorter maps having a higher ratio of 0 combo/100 combo flashlight radius.
@@ -158,56 +62,6 @@ namespace osu.Game.Rulesets.Osu.Difficulty
             return flashlightRating * Math.Sqrt(ratingMultiplier);
         }
 
-        private double calculateAimVisibilityFactor(double approachRate)
-        {
-            const double ar_factor_end_point = 11.5;
-
-            double mechanicalDifficultyFactor = DifficultyCalculationUtils.ReverseLerp(mechanicalDifficultyRating, 5, 10);
-            double arFactorStartingPoint = double.Lerp(9, 10.33, mechanicalDifficultyFactor);
-
-            return DifficultyCalculationUtils.ReverseLerp(approachRate, ar_factor_end_point, arFactorStartingPoint);
-        }
-
-        private double calculateSpeedVisibilityFactor(double approachRate)
-        {
-            const double ar_factor_end_point = 11.5;
-
-            double mechanicalDifficultyFactor = DifficultyCalculationUtils.ReverseLerp(mechanicalDifficultyRating, 5, 10);
-            double arFactorStartingPoint = double.Lerp(10, 10.33, mechanicalDifficultyFactor);
-
-            return DifficultyCalculationUtils.ReverseLerp(approachRate, ar_factor_end_point, arFactorStartingPoint);
-        }
-
-        /// <summary>
-        /// Calculates a visibility bonus that is applicable to Hidden and Traceable.
-        /// </summary>
-        public static double CalculateVisibilityBonus(Mod[] mods, double approachRate, double visibilityFactor = 1, double sliderFactor = 1)
-        {
-            // NOTE: TC's effect is only noticeable in performance calculations until lazer mods are accounted for server-side.
-            bool isAlwaysPartiallyVisible = mods.OfType<OsuModHidden>().Any(m => m.OnlyFadeApproachCircles.Value) || mods.OfType<OsuModTraceable>().Any();
-
-            // Start from normal curve, rewarding lower AR up to AR7
-            // TC forcefully requires a lower reading bonus for now as it's post-applied in PP which makes it multiplicative with the regular AR bonuses
-            // This means it has an advantage over HD, so we decrease the multiplier to compensate
-            // This should be removed once we're able to apply TC bonuses in SR (depends on real-time difficulty calculations being possible)
-            double readingBonus = (isAlwaysPartiallyVisible ? 0.025 : 0.04) * (12.0 - Math.Max(approachRate, 7));
-
-            readingBonus *= visibilityFactor;
-
-            // We want to reward slideraim on low AR less
-            double sliderVisibilityFactor = Math.Pow(sliderFactor, 3);
-
-            // For AR up to 0 - reduce reward for very low ARs when object is visible
-            if (approachRate < 7)
-                readingBonus += (isAlwaysPartiallyVisible ? 0.02 : 0.045) * (7.0 - Math.Max(approachRate, 0)) * sliderVisibilityFactor;
-
-            // Starting from AR0 - cap values so they won't grow to infinity
-            if (approachRate < 0)
-                readingBonus += (isAlwaysPartiallyVisible ? 0.01 : 0.1) * (1 - Math.Pow(1.5, approachRate)) * sliderVisibilityFactor;
-
-            return readingBonus;
-        }
-
         public static double CalculateDifficultyRating(double difficultyValue) => Math.Sqrt(difficultyValue) * difficulty_multiplier;
     }
 }

osu.Game.Rulesets.Osu/Difficulty/Preprocessing/OsuDifficultyHitObject.cs

@@ -5,6 +5,7 @@ using System;
 using System.Collections.Generic;
 using System.Linq;
 using osu.Game.Rulesets.Difficulty.Preprocessing;
+using osu.Game.Rulesets.Difficulty.Utils;
 using osu.Game.Rulesets.Objects;
 using osu.Game.Rulesets.Osu.Mods;
 using osu.Game.Rulesets.Osu.Objects;
@@ -35,6 +36,22 @@ namespace osu.Game.Rulesets.Osu.Difficulty.Preprocessing
         /// </summary>
         public readonly double AdjustedDeltaTime;
 
+        /// <summary>
+        /// Amount of time elapsed between lastDifficultyObject's <see cref="DifficultyHitObject.EndTime"/> and <see cref="DifficultyHitObject.StartTime"/> capped to a minimum of <see cref="MIN_DELTA_TIME"/>ms.
+        /// </summary>
+        public double LastObjectEndDeltaTime { get; private set; }
+
+        /// <summary>
+        /// Time (in ms) between the object first appearing and the time it needs to be clicked.
+        /// <see cref="OsuHitObject.TimePreempt"/> adjusted by clock rate.
+        /// </summary>
+        public readonly double Preempt;
+
+        /// <summary>
+        /// Normalised distance from the start position of the previous <see cref="OsuDifficultyHitObject"/> to the start position of this <see cref="OsuDifficultyHitObject"/>.
+        /// </summary>
+        public double JumpDistance { get; private set; }
+
         /// <summary>
         /// Normalised distance from the "lazy" end position of the previous <see cref="OsuDifficultyHitObject"/> to the start position of this <see cref="OsuDifficultyHitObject"/>.
         /// <para>
@@ -101,9 +118,10 @@ namespace osu.Game.Rulesets.Osu.Difficulty.Preprocessing
         public double? Angle { get; private set; }
 
         /// <summary>
-        /// Retrieves the full hit window for a Great <see cref="HitResult"/>.
+        /// Angle of the vector created between current and current-1
+        /// normalised to consider symmetrical vectors in any axis to be the same angle.
         /// </summary>
-        public double HitWindowGreat { get; private set; }
+        public double? NormalisedVectorAngle { get; private set; }
 
         /// <summary>
         /// Selective bonus for maps with higher circle size.
@@ -121,17 +139,11 @@ namespace osu.Game.Rulesets.Osu.Difficulty.Preprocessing
 
             // Capped to 25ms to prevent difficulty calculation breaking from simultaneous objects.
             AdjustedDeltaTime = Math.Max(DeltaTime, MIN_DELTA_TIME);
+            LastObjectEndDeltaTime = lastDifficultyObject != null ? Math.Max(StartTime - lastDifficultyObject.EndTime, MIN_DELTA_TIME) : AdjustedDeltaTime;
 
-            SmallCircleBonus = Math.Max(1.0, 1.0 + (30 - BaseObject.Radius) / 40);
+            SmallCircleBonus = Math.Max(1.0, 1.0 + (30 - BaseObject.Radius) / 70);
 
-            if (BaseObject is Slider sliderObject)
-            {
-                HitWindowGreat = 2 * sliderObject.HeadCircle.HitWindows.WindowFor(HitResult.Great) / clockRate;
-            }
-            else
-            {
-                HitWindowGreat = 2 * BaseObject.HitWindows.WindowFor(HitResult.Great) / clockRate;
-            }
+            Preempt = BaseObject.TimePreempt / clockRate;
 
             computeSliderCursorPosition();
             setDistances(clockRate);
@@ -148,7 +160,9 @@ namespace osu.Game.Rulesets.Osu.Difficulty.Preprocessing
             }
 
             double fadeInStartTime = BaseObject.StartTime - BaseObject.TimePreempt;
-            double fadeInDuration = BaseObject.TimeFadeIn;
+
+            // Equal to `OsuHitObject.TimeFadeIn` minus any adjustments from the HD mod.
+            double fadeInDuration = 400 * Math.Min(1, BaseObject.TimePreempt / OsuHitObject.PREEMPT_MIN);
 
             if (hidden)
             {
@@ -175,10 +189,16 @@ namespace osu.Game.Rulesets.Osu.Difficulty.Preprocessing
             {
                 double currDeltaTime = Math.Max(1, DeltaTime);
                 double nextDeltaTime = Math.Max(1, osuNextObj.DeltaTime);
+
                 double deltaDifference = Math.Abs(nextDeltaTime - currDeltaTime);
+
                 double speedRatio = currDeltaTime / Math.Max(currDeltaTime, deltaDifference);
-                double windowRatio = Math.Pow(Math.Min(1, currDeltaTime / HitWindowGreat), 2);
-                return 1.0 - Math.Pow(speedRatio, 1 - windowRatio);
+                double windowRatio = Math.Pow(Math.Min(1, currDeltaTime / HitWindow(HitResult.Great)), 5);
+
+                // Can't doubletap if circles don't intersect
+                double distanceFactor = Math.Pow(DifficultyCalculationUtils.ReverseLerp(LazyJumpDistance, NORMALISED_DIAMETER, NORMALISED_RADIUS), 2);
+
+                return 1.0 - Math.Pow(speedRatio, distanceFactor * (1 - windowRatio));
             }
 
             return 0;
@@ -189,10 +209,12 @@ namespace osu.Game.Rulesets.Osu.Difficulty.Preprocessing
             if (BaseObject is Slider currentSlider)
             {
                 // Bonus for repeat sliders until a better per nested object strain system can be achieved.
-                TravelDistance = LazyTravelDistance * Math.Pow(1 + currentSlider.RepeatCount / 2.5, 1.0 / 2.5);
+                TravelDistance = LazyTravelDistance * Math.Max(1, Math.Pow(currentSlider.RepeatCount, 0.3));
                 TravelTime = Math.Max(LazyTravelTime / clockRate, MIN_DELTA_TIME);
             }
 
+            MinimumJumpTime = AdjustedDeltaTime;
+
             // We don't need to calculate either angle or distance when one of the last->curr objects is a spinner
             if (BaseObject is Spinner || LastObject is Spinner)
                 return;
@@ -202,8 +224,8 @@ namespace osu.Game.Rulesets.Osu.Difficulty.Preprocessing
 
             Vector2 lastCursorPosition = lastDifficultyObject != null ? getEndCursorPosition(lastDifficultyObject) : LastObject.StackedPosition;
 
-            LazyJumpDistance = (BaseObject.StackedPosition * scalingFactor - lastCursorPosition * scalingFactor).Length;
-            MinimumJumpTime = AdjustedDeltaTime;
+            JumpDistance = (LastObject.StackedPosition - BaseObject.StackedPosition).Length * scalingFactor;
+            LazyJumpDistance = (BaseObject.StackedPosition - lastCursorPosition).Length * scalingFactor;
             MinimumJumpDistance = LazyJumpDistance;
 
             if (LastObject is Slider lastSlider && lastDifficultyObject != null)
@@ -239,15 +261,18 @@ namespace osu.Game.Rulesets.Osu.Difficulty.Preprocessing
 
             if (lastLastDifficultyObject != null && lastLastDifficultyObject.BaseObject is not Spinner)
             {
+                if (lastDifficultyObject!.BaseObject is Slider prevSlider && lastDifficultyObject.TravelDistance > 0)
+                    lastCursorPosition = prevSlider.HeadCircle.StackedPosition;
+
                 Vector2 lastLastCursorPosition = getEndCursorPosition(lastLastDifficultyObject);
 
-                Vector2 v1 = lastLastCursorPosition - LastObject.StackedPosition;
-                Vector2 v2 = BaseObject.StackedPosition - lastCursorPosition;
+                double angle = calculateAngle(BaseObject.StackedPosition, lastCursorPosition, lastLastCursorPosition);
+                double sliderAngle = calculateSliderAngle(lastDifficultyObject!, lastLastCursorPosition);
 
-                float dot = Vector2.Dot(v1, v2);
-                float det = v1.X * v2.Y - v1.Y * v2.X;
+                Vector2 v = BaseObject.StackedPosition - lastCursorPosition;
+                NormalisedVectorAngle = Math.Atan2(Math.Abs(v.Y), Math.Abs(v.X));
 
-                Angle = Math.Abs(Math.Atan2(det, dot));
+                Angle = Math.Min(angle, sliderAngle);
             }
         }
 
@@ -359,6 +384,30 @@ namespace osu.Game.Rulesets.Osu.Difficulty.Preprocessing
             }
         }
 
+        private double calculateSliderAngle(OsuDifficultyHitObject lastDifficultyObject, Vector2 lastLastCursorPosition)
+        {
+            Vector2 lastCursorPosition = getEndCursorPosition(lastDifficultyObject);
+
+            if (lastDifficultyObject.BaseObject is Slider prevSlider && lastDifficultyObject.TravelDistance > 0)
+            {
+                OsuHitObject secondLastNestedObject = (OsuHitObject)prevSlider.NestedHitObjects[^2];
+                lastLastCursorPosition = secondLastNestedObject.StackedPosition;
+            }
+
+            return calculateAngle(BaseObject.StackedPosition, lastCursorPosition, lastLastCursorPosition);
+        }
+
+        private double calculateAngle(Vector2 currentPosition, Vector2 lastPosition, Vector2 lastLastPosition)
+        {
+            Vector2 v1 = lastLastPosition - lastPosition;
+            Vector2 v2 = currentPosition - lastPosition;
+
+            float dot = Vector2.Dot(v1, v2);
+            float det = v1.X * v2.Y - v1.Y * v2.X;
+
+            return Math.Abs(Math.Atan2(det, dot));
+        }
+
         private Vector2 getEndCursorPosition(OsuDifficultyHitObject difficultyHitObject)
         {
             return difficultyHitObject.LazyEndPosition ?? difficultyHitObject.BaseObject.StackedPosition;

osu.Game.Rulesets.Osu/Difficulty/Skills/Aim.cs

@@ -4,10 +4,14 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using osu.Framework.Utils;
 using osu.Game.Rulesets.Difficulty.Preprocessing;
+using osu.Game.Rulesets.Difficulty.Skills;
+using osu.Game.Rulesets.Difficulty.Utils;
 using osu.Game.Rulesets.Mods;
-using osu.Game.Rulesets.Osu.Difficulty.Evaluators;
-using osu.Game.Rulesets.Osu.Difficulty.Utils;
+using osu.Game.Rulesets.Osu.Difficulty.Evaluators.Aim;
+using osu.Game.Rulesets.Osu.Difficulty.Preprocessing;
+using osu.Game.Rulesets.Osu.Mods;
 using osu.Game.Rulesets.Osu.Objects;
 
 namespace osu.Game.Rulesets.Osu.Difficulty.Skills
@@ -15,7 +19,7 @@ namespace osu.Game.Rulesets.Osu.Difficulty.Skills
     /// <summary>
     /// Represents the skill required to correctly aim at every object in the map with a uniform CircleSize and normalized distances.
     /// </summary>
-    public class Aim : OsuStrainSkill
+    public class Aim : VariableLengthStrainSkill
     {
         public readonly bool IncludeSliders;
 
@@ -27,19 +31,39 @@ namespace osu.Game.Rulesets.Osu.Difficulty.Skills
 
         private double currentStrain;
 
-        private double skillMultiplier => 26;
-        private double strainDecayBase => 0.15;
+        private double skillMultiplierSnap => 70.9;
+        private double skillMultiplierAgility => 2.35;
+        private double skillMultiplierFlow => 242.0;
+        private double skillMultiplierTotal => 1.12;
+        private double combinedSnapNormExponent => 1.2;
+
+        /// <summary>
+        /// The number of sections with the highest strains, which the peak strain reductions will apply to.
+        /// This is done in order to decrease their impact on the overall difficulty of the map for this skill.
+        /// </summary>
+        private int reducedSectionTime => 4000;
+
+        /// <summary>
+        /// The baseline multiplier applied to the section with the biggest strain.
+        /// </summary>
+        private double reducedStrainBaseline => 0.727;
 
         private readonly List<double> sliderStrains = new List<double>();
 
-        private double strainDecay(double ms) => Math.Pow(strainDecayBase, ms / 1000);
+        private double strainDecay(double ms) => Math.Pow(0.2, ms / 1000);
 
-        protected override double CalculateInitialStrain(double time, DifficultyHitObject current) => currentStrain * strainDecay(time - current.Previous(0).StartTime);
+        protected override double CalculateInitialStrain(double time, DifficultyHitObject current) =>
+            currentStrain * strainDecay(time - current.Previous(0).StartTime);
 
         protected override double StrainValueAt(DifficultyHitObject current)
         {
-            currentStrain *= strainDecay(current.DeltaTime);
-            currentStrain += AimEvaluator.EvaluateDifficultyOf(current, IncludeSliders) * skillMultiplier;
+            if (Mods.Any(m => m is OsuModAutopilot))
+                return 0;
+
+            double decay = strainDecay(((OsuDifficultyHitObject)current).AdjustedDeltaTime);
+
+            currentStrain *= decay;
+            currentStrain += calculateModAdjustedDifficulty(current) * (1 - decay);
 
             if (current.BaseObject is Slider)
                 sliderStrains.Add(currentStrain);
@@ -47,6 +71,73 @@ namespace osu.Game.Rulesets.Osu.Difficulty.Skills
             return currentStrain;
         }
 
+        private double calculateModAdjustedDifficulty(DifficultyHitObject current)
+        {
+            double snapDifficulty = SnapAimEvaluator.EvaluateDifficultyOf(current, IncludeSliders) * skillMultiplierSnap;
+            double agilityDifficulty = AgilityEvaluator.EvaluateDifficultyOf(current) * skillMultiplierAgility;
+            double flowDifficulty = FlowAimEvaluator.EvaluateDifficultyOf(current, IncludeSliders) * skillMultiplierFlow;
+
+            double totalDifficulty = calculateTotalValue(snapDifficulty, agilityDifficulty, flowDifficulty);
+
+            if (Mods.Any(m => m is OsuModMagnetised))
+            {
+                float magnetisedStrength = Mods.OfType<OsuModMagnetised>().First().AttractionStrength.Value;
+                totalDifficulty *= 1.0 - magnetisedStrength;
+            }
+
+            return totalDifficulty;
+        }
+
+        private double calculateTotalValue(double snapDifficulty, double agilityDifficulty, double flowDifficulty)
+        {
+            // We compare flow to combined snap and agility because snap by itself doesn't have enough difficulty to be above flow on streams
+            // Agility on the other hand is supposed to measure the rate of cursor velocity changes while snapping
+            // So snapping every circle on a stream requires an enormous amount of agility at which point it's easier to flow
+            double combinedSnapDifficulty = DifficultyCalculationUtils.Norm(combinedSnapNormExponent, snapDifficulty, agilityDifficulty);
+
+            double pSnap = calculateSnapFlowProbability(flowDifficulty / combinedSnapDifficulty);
+            double pFlow = 1 - pSnap;
+
+            if (Mods.Any(m => m is OsuModTouchDevice))
+            {
+                // we don't adjust agility here since agility represents TD difficulty in a decent enough way
+                snapDifficulty = Math.Pow(snapDifficulty, 0.89);
+                combinedSnapDifficulty = DifficultyCalculationUtils.Norm(combinedSnapNormExponent, snapDifficulty, agilityDifficulty);
+            }
+
+            if (Mods.Any(m => m is OsuModRelax))
+            {
+                combinedSnapDifficulty *= 0.75;
+                flowDifficulty *= 0.6;
+            }
+
+            double totalDifficulty = combinedSnapDifficulty * pSnap + flowDifficulty * pFlow;
+
+            double totalStrain = totalDifficulty * skillMultiplierTotal;
+
+            return totalStrain;
+        }
+
+        // A function that turns the ratio of snap : flow into the probability of snapping/flowing
+        // It has the constraints:
+        // P(snap) + P(flow) = 1 (the object is always either snapped or flowed)
+        // P(snap) = f(snap/flow), P(flow) = f(flow/snap) (ie snap and flow are symmetric and reversible)
+        // Therefore: f(x) + f(1/x) = 1
+        // 0 <= f(x) <= 1 (cannot have negative or greater than 100% probability of snapping or flowing)
+        // This logistic function is a solution, which fits nicely with the general idea of interpolation and provides a tuneable constant
+        private static double calculateSnapFlowProbability(double ratio)
+        {
+            const double k = 7.27;
+
+            if (ratio == 0)
+                return 0;
+
+            if (double.IsNaN(ratio))
+                return 1;
+
+            return DifficultyCalculationUtils.Logistic(-k * Math.Log(ratio));
+        }
+
         public double GetDifficultSliders()
         {
             if (sliderStrains.Count == 0)
@@ -60,6 +151,97 @@ namespace osu.Game.Rulesets.Osu.Difficulty.Skills
             return sliderStrains.Sum(strain => 1.0 / (1.0 + Math.Exp(-(strain / maxSliderStrain * 12.0 - 6.0))));
         }
 
-        public double CountTopWeightedSliders() => OsuStrainUtils.CountTopWeightedSliders(sliderStrains, DifficultyValue());
+        public double CountTopWeightedSliders(double difficultyValue)
+        {
+            if (sliderStrains.Count == 0)
+                return 0;
+
+            double consistentTopStrain = difficultyValue * (1 - DecayWeight); // What would the top strain be if all strain values were identical
+
+            if (consistentTopStrain == 0)
+                return 0;
+
+            // Use a weighted sum of all strains. Constants are arbitrary and give nice values
+            return sliderStrains.Sum(s => DifficultyCalculationUtils.Logistic(s / consistentTopStrain, 0.88, 10, 1.1));
+        }
+
+        public override double DifficultyValue()
+        {
+            double difficulty = 0;
+            double time = 0;
+
+            var strains = getReducedStrainPeaks();
+
+            // Difficulty is a continuous weighted sum of the sorted strains
+            foreach (StrainPeak strain in strains)
+            {
+                /* Weighting function can be thought of as:
+                        b
+                        ∫ DecayWeight^x dx
+                        a
+                    where a = startTime and b = endTime
+
+                    Technically, the function below has been slightly modified from the equation above.
+                    The real function would be
+                        double weight = Math.Pow(DecayWeight, startTime) - Math.Pow(DecayWeight, endTime);
+                        ...
+                        return difficulty / Math.Log(1 / DecayWeight);
+                    E.g. for a DecayWeight of 0.9, we're multiplying by 10 instead of 9.49122...
+
+                    This change makes it so that a map composed solely of MaxSectionLength chunks will have the exact same value when summed in this class and StrainSkill.
+                    Doing this ensures the relationship between strain values and difficulty values remains the same between the two classes.
+                */
+                double startTime = time;
+                double endTime = time + strain.SectionLength / MaxSectionLength;
+
+                double weight = Math.Pow(DecayWeight, startTime) - Math.Pow(DecayWeight, endTime);
+
+                difficulty += strain.Value * weight;
+                time = endTime;
+            }
+
+            return difficulty / (1 - DecayWeight);
+        }
+
+        /// <summary>
+        /// Returns a sorted enumerable of strain peaks with the highest values reduced.
+        /// </summary>
+        /// <returns></returns>
+        private IEnumerable<StrainPeak> getReducedStrainPeaks()
+        {
+            // Sections with 0 strain are excluded to avoid worst-case time complexity of the following sort (e.g. /b/2351871).
+            // These sections will not contribute to the difficulty.
+            var peaks = GetCurrentStrainPeaks().Where(p => p.Value > 0);
+
+            List<StrainPeak> strains = peaks.OrderByDescending(p => p.Value).ToList();
+
+            const int chunk_size = 20;
+            double time = 0;
+            int strainsToRemove = 0; // All strains are removed at the end for optimization purposes
+
+            // We are reducing the highest strains first to account for extreme difficulty spikes
+            // Strains are split into 20ms chunks to try to mitigate inconsistencies caused by reducing strains
+            while (strains.Count > strainsToRemove && time < reducedSectionTime)
+            {
+                StrainPeak strain = strains[strainsToRemove];
+
+                for (double addedTime = 0; addedTime < strain.SectionLength; addedTime += chunk_size)
+                {
+                    double scale = Math.Log10(Interpolation.Lerp(1, 10, Math.Clamp((time + addedTime) / reducedSectionTime, 0, 1)));
+
+                    strains.Add(new StrainPeak(
+                        strain.Value * Interpolation.Lerp(reducedStrainBaseline, 1.0, scale),
+                        Math.Min(chunk_size, strain.SectionLength - addedTime)
+                    ));
+                }
+
+                time += strain.SectionLength;
+                strainsToRemove++;
+            }
+
+            strains.RemoveRange(0, strainsToRemove);
+
+            return strains.OrderByDescending(p => p.Value);
+        }
     }
 }

osu.Game.Rulesets.Osu/Difficulty/Skills/Flashlight.cs

@@ -5,6 +5,7 @@ using System;
 using System.Linq;
 using osu.Game.Rulesets.Difficulty.Preprocessing;
 using osu.Game.Rulesets.Difficulty.Skills;
+using osu.Game.Rulesets.Difficulty.Utils;
 using osu.Game.Rulesets.Mods;
 using osu.Game.Rulesets.Osu.Difficulty.Evaluators;
 using osu.Game.Rulesets.Osu.Mods;
@@ -16,15 +17,12 @@ namespace osu.Game.Rulesets.Osu.Difficulty.Skills
     /// </summary>
     public class Flashlight : StrainSkill
     {
-        private readonly bool hasHiddenMod;
-
         public Flashlight(Mod[] mods)
             : base(mods)
         {
-            hasHiddenMod = mods.Any(m => m is OsuModHidden);
         }
 
-        private double skillMultiplier => 0.05512;
+        private double skillMultiplier => 0.058;
         private double strainDecayBase => 0.15;
 
         private double currentStrain;
@@ -35,12 +33,43 @@ namespace osu.Game.Rulesets.Osu.Difficulty.Skills
 
         protected override double StrainValueAt(DifficultyHitObject current)
         {
+            if (!Mods.Any(m => m is OsuModFlashlight))
+                return 0;
+
             currentStrain *= strainDecay(current.DeltaTime);
-            currentStrain += FlashlightEvaluator.EvaluateDifficultyOf(current, hasHiddenMod) * skillMultiplier;
+            currentStrain += calculateModAdjustedDifficulty(current) * skillMultiplier;
 
             return currentStrain;
         }
 
+        private double calculateModAdjustedDifficulty(DifficultyHitObject current)
+        {
+            double difficulty = FlashlightEvaluator.EvaluateDifficultyOf(current, Mods);
+
+            if (Mods.Any(m => m is OsuModTouchDevice))
+                difficulty = Math.Pow(difficulty, 0.9);
+
+            if (Mods.Any(m => m is OsuModMagnetised))
+            {
+                float magnetisedStrength = Mods.OfType<OsuModMagnetised>().First().AttractionStrength.Value;
+                difficulty *= 1.0 - magnetisedStrength;
+            }
+
+            if (Mods.Any(m => m is OsuModDeflate))
+            {
+                float deflateInitialScale = Mods.OfType<OsuModDeflate>().First().StartScale.Value;
+                difficulty *= Math.Clamp(DifficultyCalculationUtils.ReverseLerp(deflateInitialScale, 11, 1), 0.1, 1);
+            }
+
+            if (Mods.Any(m => m is OsuModRelax))
+                difficulty *= 0.7;
+
+            if (Mods.Any(m => m is OsuModAutopilot))
+                difficulty *= 0.4;
+
+            return difficulty;
+        }
+
         public override double DifficultyValue() => GetCurrentStrainPeaks().Sum();
 
         public static double DifficultyToPerformance(double difficulty) => 25 * Math.Pow(difficulty, 2);

osu.Game.Rulesets.Osu/Difficulty/Skills/OsuStrainSkill.cs

@@ -1,62 +0,0 @@
-// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
-// See the LICENCE file in the repository root for full licence text.
-
-using System;
-using System.Collections.Generic;
-using osu.Game.Rulesets.Difficulty.Skills;
-using osu.Game.Rulesets.Mods;
-using System.Linq;
-using osu.Framework.Utils;
-
-namespace osu.Game.Rulesets.Osu.Difficulty.Skills
-{
-    public abstract class OsuStrainSkill : StrainSkill
-    {
-        /// <summary>
-        /// The number of sections with the highest strains, which the peak strain reductions will apply to.
-        /// This is done in order to decrease their impact on the overall difficulty of the map for this skill.
-        /// </summary>
-        protected virtual int ReducedSectionCount => 10;
-
-        /// <summary>
-        /// The baseline multiplier applied to the section with the biggest strain.
-        /// </summary>
-        protected virtual double ReducedStrainBaseline => 0.75;
-
-        protected OsuStrainSkill(Mod[] mods)
-            : base(mods)
-        {
-        }
-
-        public override double DifficultyValue()
-        {
-            double difficulty = 0;
-            double weight = 1;
-
-            // Sections with 0 strain are excluded to avoid worst-case time complexity of the following sort (e.g. /b/2351871).
-            // These sections will not contribute to the difficulty.
-            var peaks = GetCurrentStrainPeaks().Where(p => p > 0);
-
-            List<double> strains = peaks.OrderDescending().ToList();
-
-            // We are reducing the highest strains first to account for extreme difficulty spikes
-            for (int i = 0; i < Math.Min(strains.Count, ReducedSectionCount); i++)
-            {
-                double scale = Math.Log10(Interpolation.Lerp(1, 10, Math.Clamp((float)i / ReducedSectionCount, 0, 1)));
-                strains[i] *= Interpolation.Lerp(ReducedStrainBaseline, 1.0, scale);
-            }
-
-            // Difficulty is the weighted sum of the highest strains from every section.
-            // We're sorting from highest to lowest strain.
-            foreach (double strain in strains.OrderDescending())
-            {
-                difficulty += strain * weight;
-                weight *= DecayWeight;
-            }
-
-            return difficulty;
-        }
-
-        public static double DifficultyToPerformance(double difficulty) => Math.Pow(5.0 * Math.Max(1.0, difficulty / 0.0675) - 4.0, 3.0) / 100000.0;
-    }
-}

osu.Game.Rulesets.Osu/Difficulty/Skills/Reading.cs

@@ -0,0 +1,121 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using osu.Framework.Utils;
+using osu.Game.Rulesets.Difficulty.Preprocessing;
+using osu.Game.Rulesets.Difficulty.Skills;
+using osu.Game.Rulesets.Difficulty.Utils;
+using osu.Game.Rulesets.Mods;
+using osu.Game.Rulesets.Osu.Difficulty.Evaluators;
+using osu.Game.Rulesets.Osu.Mods;
+
+namespace osu.Game.Rulesets.Osu.Difficulty.Skills
+{
+    public class Reading : HarmonicSkill
+    {
+        private readonly List<DifficultyHitObject> objectList = new List<DifficultyHitObject>();
+
+        private readonly bool hasHiddenMod;
+
+        public Reading(Mod[] mods)
+            : base(mods)
+        {
+            hasHiddenMod = mods.OfType<OsuModHidden>().Any(m => !m.OnlyFadeApproachCircles.Value);
+        }
+
+        private double currentStrain;
+
+        private double skillMultiplier => 2.5;
+        private double strainDecayBase => 0.8;
+
+        private double strainDecay(double ms) => Math.Pow(strainDecayBase, ms / 1000);
+
+        protected override double ObjectDifficultyOf(DifficultyHitObject current)
+        {
+            objectList.Add(current);
+
+            double decay = strainDecay(current.DeltaTime);
+
+            currentStrain *= decay;
+            currentStrain += calculateModAdjustedDifficulty(current) * (1 - decay) * skillMultiplier;
+
+            return currentStrain;
+        }
+
+        private double calculateModAdjustedDifficulty(DifficultyHitObject current)
+        {
+            double difficulty = ReadingEvaluator.EvaluateDifficultyOf(current, hasHiddenMod);
+
+            if (Mods.Any(m => m is OsuModTouchDevice))
+                difficulty = Math.Pow(difficulty, 0.89);
+
+            if (Mods.Any(m => m is OsuModMagnetised))
+            {
+                float magnetisedStrength = Mods.OfType<OsuModMagnetised>().First().AttractionStrength.Value;
+                difficulty *= 1.0 - magnetisedStrength;
+            }
+
+            if (Mods.Any(m => m is OsuModRelax))
+                difficulty *= 0.4;
+
+            if (Mods.Any(m => m is OsuModAutopilot))
+                difficulty *= 0.1;
+
+            return difficulty;
+        }
+
+        protected override void ApplyDifficultyTransformation(double[] difficulties)
+        {
+            const double reduced_difficulty_base_line = 0.0; // Assume the first seconds are completely memorised
+
+            int reducedNoteCount = calculateReducedNoteCount();
+
+            for (int i = 0; i < Math.Min(difficulties.Length, reducedNoteCount); i++)
+            {
+                double scale = Math.Log10(Interpolation.Lerp(1, 10, Math.Clamp((double)i / reducedNoteCount, 0, 1)));
+                difficulties[i] *= Interpolation.Lerp(reduced_difficulty_base_line, 1.0, scale);
+            }
+        }
+
+        private int calculateReducedNoteCount()
+        {
+            const double reduced_difficulty_duration = 60 * 1000;
+
+            if (objectList.Count == 0)
+                return 0;
+
+            double reducedDuration = objectList.First().StartTime + reduced_difficulty_duration;
+
+            int reducedNoteCount = 0;
+
+            foreach (var hitObject in objectList)
+            {
+                if (hitObject.StartTime > reducedDuration)
+                    break;
+
+                reducedNoteCount++;
+            }
+
+            return reducedNoteCount;
+        }
+
+        public override double CountTopWeightedObjectDifficulties(double difficultyValue)
+        {
+            if (ObjectDifficulties.Count == 0)
+                return 0.0;
+
+            if (NoteWeightSum == 0)
+                return 0.0;
+
+            double consistentTopNote = difficultyValue / NoteWeightSum; // What would the top difficulty be if all object difficulties were identical
+
+            if (consistentTopNote == 0)
+                return 0;
+
+            return ObjectDifficulties.Sum(d => DifficultyCalculationUtils.Logistic(d / consistentTopNote, 1.15, 5, 1.1));
+        }
+    }
+}

osu.Game.Rulesets.Osu/Difficulty/Skills/Speed.cs

@@ -3,30 +3,33 @@
 
 using System;
 using System.Collections.Generic;
-using osu.Game.Rulesets.Difficulty.Preprocessing;
-using osu.Game.Rulesets.Mods;
-using osu.Game.Rulesets.Osu.Difficulty.Evaluators;
-using osu.Game.Rulesets.Osu.Difficulty.Preprocessing;
-using osu.Game.Rulesets.Osu.Objects;
 using System.Linq;
-using osu.Game.Rulesets.Osu.Difficulty.Utils;
+using osu.Game.Rulesets.Difficulty.Preprocessing;
+using osu.Game.Rulesets.Difficulty.Skills;
+using osu.Game.Rulesets.Difficulty.Utils;
+using osu.Game.Rulesets.Mods;
+using osu.Game.Rulesets.Osu.Difficulty.Evaluators.Speed;
+using osu.Game.Rulesets.Osu.Difficulty.Preprocessing;
+using osu.Game.Rulesets.Osu.Mods;
+using osu.Game.Rulesets.Osu.Objects;
 
 namespace osu.Game.Rulesets.Osu.Difficulty.Skills
 {
     /// <summary>
     /// Represents the skill required to press keys with regards to keeping up with the speed at which objects need to be hit.
     /// </summary>
-    public class Speed : OsuStrainSkill
+    public class Speed : HarmonicSkill
     {
-        private double skillMultiplier => 1.47;
-        private double strainDecayBase => 0.3;
-
-        private double currentStrain;
-        private double currentRhythm;
+        private double skillMultiplier => 1.16;
 
         private readonly List<double> sliderStrains = new List<double>();
 
-        protected override int ReducedSectionCount => 5;
+        private double currentStrain;
+
+        private double strainDecayBase => 0.3;
+
+        protected override double HarmonicScale => 20;
+        protected override double DecayExponent => 0.9;
 
         public Speed(Mod[] mods)
             : base(mods)
@@ -35,14 +38,17 @@ namespace osu.Game.Rulesets.Osu.Difficulty.Skills
 
         private double strainDecay(double ms) => Math.Pow(strainDecayBase, ms / 1000);
 
-        protected override double CalculateInitialStrain(double time, DifficultyHitObject current) => (currentStrain * currentRhythm) * strainDecay(time - current.Previous(0).StartTime);
-
-        protected override double StrainValueAt(DifficultyHitObject current)
+        protected override double ObjectDifficultyOf(DifficultyHitObject current)
         {
-            currentStrain *= strainDecay(((OsuDifficultyHitObject)current).AdjustedDeltaTime);
-            currentStrain += SpeedEvaluator.EvaluateDifficultyOf(current, Mods) * skillMultiplier;
+            if (Mods.Any(m => m is OsuModRelax))
+                return 0;
 
-            currentRhythm = RhythmEvaluator.EvaluateDifficultyOf(current);
+            double decay = strainDecay(((OsuDifficultyHitObject)current).AdjustedDeltaTime);
+
+            currentStrain *= decay;
+            currentStrain += calculateModAdjustedDifficulty(current) * (1 - decay) * skillMultiplier;
+
+            double currentRhythm = RhythmEvaluator.EvaluateDifficultyOf(current);
 
             double totalStrain = currentStrain * currentRhythm;
 
@@ -52,18 +58,44 @@ namespace osu.Game.Rulesets.Osu.Difficulty.Skills
             return totalStrain;
         }
 
+        private double calculateModAdjustedDifficulty(DifficultyHitObject current)
+        {
+            double difficulty = SpeedEvaluator.EvaluateDifficultyOf(current);
+
+            if (Mods.Any(m => m is OsuModAutopilot))
+                difficulty *= 0.5;
+
+            return difficulty;
+        }
+
         public double RelevantNoteCount()
         {
-            if (ObjectStrains.Count == 0)
+            if (ObjectDifficulties.Count == 0)
                 return 0;
 
-            double maxStrain = ObjectStrains.Max();
+            double maxStrain = ObjectDifficulties.Max();
+
             if (maxStrain == 0)
                 return 0;
 
-            return ObjectStrains.Sum(strain => 1.0 / (1.0 + Math.Exp(-(strain / maxStrain * 12.0 - 6.0))));
+            return ObjectDifficulties.Sum(strain => 1.0 / (1.0 + Math.Exp(-(strain / maxStrain * 12.0 - 6.0))));
         }
 
-        public double CountTopWeightedSliders() => OsuStrainUtils.CountTopWeightedSliders(sliderStrains, DifficultyValue());
+        public double CountTopWeightedSliders(double difficultyValue)
+        {
+            if (sliderStrains.Count == 0)
+                return 0;
+
+            if (NoteWeightSum == 0)
+                return 0.0;
+
+            double consistentTopNote = difficultyValue / NoteWeightSum; // What would the top note be if all note values were identical
+
+            if (consistentTopNote == 0)
+                return 0;
+
+            // Use a weighted sum of all notes. Constants are arbitrary and give nice values
+            return sliderStrains.Sum(s => DifficultyCalculationUtils.Logistic(s / consistentTopNote, 0.88, 10, 1.1));
+        }
     }
 }

osu.Game.Rulesets.Osu/Difficulty/Utils/OsuStrainUtils.cs

@@ -1,26 +0,0 @@
-// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
-// See the LICENCE file in the repository root for full licence text.
-
-using System.Collections.Generic;
-using System.Linq;
-using osu.Game.Rulesets.Difficulty.Utils;
-
-namespace osu.Game.Rulesets.Osu.Difficulty.Utils
-{
-    public static class OsuStrainUtils
-    {
-        public static double CountTopWeightedSliders(IReadOnlyCollection<double> sliderStrains, double difficultyValue)
-        {
-            if (sliderStrains.Count == 0)
-                return 0;
-
-            double consistentTopStrain = difficultyValue / 10; // What would the top strain be if all strain values were identical
-
-            if (consistentTopStrain == 0)
-                return 0;
-
-            // Use a weighted sum of all strains. Constants are arbitrary and give nice values
-            return sliderStrains.Sum(s => DifficultyCalculationUtils.Logistic(s / consistentTopStrain, 0.88, 10, 1.1));
-        }
-    }
-}

osu.Game.Rulesets.Taiko/Difficulty/Evaluators/RhythmEvaluator.cs

@@ -5,9 +5,11 @@ using System;
 using System.Collections.Generic;
 using osu.Game.Rulesets.Difficulty.Preprocessing;
 using osu.Game.Rulesets.Difficulty.Utils;
+using osu.Game.Rulesets.Scoring;
 using osu.Game.Rulesets.Taiko.Difficulty.Preprocessing;
 using osu.Game.Rulesets.Taiko.Difficulty.Preprocessing.Rhythm;
 using osu.Game.Rulesets.Taiko.Difficulty.Preprocessing.Rhythm.Data;
+using osu.Game.Rulesets.Taiko.Objects;
 
 namespace osu.Game.Rulesets.Taiko.Difficulty.Evaluators
 {
@@ -16,25 +18,32 @@ namespace osu.Game.Rulesets.Taiko.Difficulty.Evaluators
         /// <summary>
         /// Evaluate the difficulty of a hitobject considering its interval change.
         /// </summary>
-        public static double EvaluateDifficultyOf(DifficultyHitObject hitObject, double hitWindow)
+        public static double EvaluateDifficultyOf(DifficultyHitObject hitObject)
         {
+            if (hitObject.BaseObject is not Hit)
+                return 0;
+
             TaikoRhythmData rhythmData = ((TaikoDifficultyHitObject)hitObject).RhythmData;
             double difficulty = 0.0d;
 
             double sameRhythm = 0;
             double samePattern = 0;
             double intervalPenalty = 0;
+            double gapPenalty = 0;
+
+            double hitWindow = hitObject.HitWindow(HitResult.Great);
 
             if (rhythmData.SameRhythmGroupedHitObjects?.FirstHitObject == hitObject) // Difficulty for SameRhythmGroupedHitObjects
             {
                 sameRhythm += 10.0 * evaluateDifficultyOf(rhythmData.SameRhythmGroupedHitObjects, hitWindow);
                 intervalPenalty = repeatedIntervalPenalty(rhythmData.SameRhythmGroupedHitObjects, hitWindow);
+                gapPenalty = longGapPenalty(rhythmData.SameRhythmGroupedHitObjects.Previous);
             }
 
             if (rhythmData.SamePatternsGroupedHitObjects?.FirstHitObject == hitObject) // Difficulty for SamePatternsGroupedHitObjects
                 samePattern += 1.15 * ratioDifficulty(rhythmData.SamePatternsGroupedHitObjects.IntervalRatio);
 
-            difficulty += Math.Max(sameRhythm, samePattern) * intervalPenalty;
+            difficulty += Math.Max(sameRhythm, samePattern) * intervalPenalty * gapPenalty;
 
             return difficulty;
         }
@@ -56,8 +65,8 @@ namespace osu.Game.Rulesets.Taiko.Difficulty.Evaluators
                 {
                     intervalDifficulty *= DifficultyCalculationUtils.Logistic(
                         durationDifference / hitWindow,
-                        midpointOffset: 0.7,
-                        multiplier: 1.0,
+                        midpointOffset: 0.35,
+                        multiplier: 2,
                         maxValue: 1);
                 }
             }
@@ -65,8 +74,8 @@ namespace osu.Game.Rulesets.Taiko.Difficulty.Evaluators
             // Penalise patterns that can be hit within a single hit window.
             intervalDifficulty *= DifficultyCalculationUtils.Logistic(
                 sameRhythmGroupedHitObjects.Duration / hitWindow,
-                midpointOffset: 0.6,
-                multiplier: 1,
+                midpointOffset: 0.3,
+                multiplier: 2,
                 maxValue: 1);
 
             return Math.Pow(intervalDifficulty, 0.75);
@@ -118,6 +127,32 @@ namespace osu.Game.Rulesets.Taiko.Difficulty.Evaluators
             }
         }
 
+        /// <summary>
+        /// Frequent rhythm changes containing long gaps (i.e. 1/4 + 1/6 with 1/2 gaps) award more difficulty than expected.
+        /// Due to limitations of the current rhythm evaluation, these cases are targeted and penalised.
+        /// The previous hit object grouping is used as often the rhythm change *two* rhythms after a long gap awards the unexpected difficulty.
+        /// </summary>
+        private static double longGapPenalty(SameRhythmHitObjectGrouping? previous)
+        {
+            if (previous == null)
+                return 1.0;
+
+            double gapInterval = previous.FirstHitObject.DeltaTime;
+            double rhythmInterval = previous.HitObjectInterval ?? gapInterval;
+            double rhythmLength = previous.HitObjects.Count;
+
+            // The ratio of the gap before this rhythm to the rhythm itself.
+            double gapRatio = gapInterval / Math.Max(rhythmInterval, 1);
+
+            // The gap ratio normalised to represent if the gap is long.
+            double gapFactor = DifficultyCalculationUtils.Logistic(gapRatio, 1.75, 20);
+
+            // The length in objects of this rhythm normalised to represent if the rhythm change is frequent enough to be penalised.
+            double lengthFactor = DifficultyCalculationUtils.ReverseLerp(rhythmLength, 8, 2);
+
+            return 1.0 - 0.75 * gapFactor * lengthFactor;
+        }
+
         /// <summary>
         /// Calculates the difficulty of a given ratio using a combination of periodic penalties and bonuses.
         /// </summary>

osu.Game.Rulesets.Taiko/Difficulty/Preprocessing/TaikoDifficultyHitObject.cs

@@ -7,10 +7,10 @@ using osu.Game.Beatmaps.ControlPoints;
 using osu.Game.Rulesets.Difficulty.Preprocessing;
 using osu.Game.Rulesets.Objects;
 using osu.Game.Rulesets.Taiko.Difficulty.Evaluators;
-using osu.Game.Rulesets.Taiko.Objects;
 using osu.Game.Rulesets.Taiko.Difficulty.Preprocessing.Colour;
 using osu.Game.Rulesets.Taiko.Difficulty.Preprocessing.Rhythm;
 using osu.Game.Rulesets.Taiko.Difficulty.Utils;
+using osu.Game.Rulesets.Taiko.Objects;
 
 namespace osu.Game.Rulesets.Taiko.Difficulty.Preprocessing
 {

osu.Game.Rulesets.Taiko/Difficulty/Skills/Rhythm.cs

@@ -17,17 +17,14 @@ namespace osu.Game.Rulesets.Taiko.Difficulty.Skills
         protected override double SkillMultiplier => 1.0;
         protected override double StrainDecayBase => 0.4;
 
-        private readonly double greatHitWindow;
-
-        public Rhythm(Mod[] mods, double greatHitWindow)
+        public Rhythm(Mod[] mods)
             : base(mods)
         {
-            this.greatHitWindow = greatHitWindow;
         }
 
         protected override double StrainValueOf(DifficultyHitObject current)
         {
-            double difficulty = RhythmEvaluator.EvaluateDifficultyOf(current, greatHitWindow);
+            double difficulty = RhythmEvaluator.EvaluateDifficultyOf(current);
 
             // To prevent abuse of exceedingly long intervals between awkward rhythms, we penalise its difficulty.
             double staminaDifficulty = StaminaEvaluator.EvaluateDifficultyOf(current) - 0.5; // Remove base strain

osu.Game.Rulesets.Taiko/Difficulty/TaikoDifficultyCalculator.cs

@@ -10,20 +10,19 @@ using osu.Game.Rulesets.Difficulty.Preprocessing;
 using osu.Game.Rulesets.Difficulty.Skills;
 using osu.Game.Rulesets.Difficulty.Utils;
 using osu.Game.Rulesets.Mods;
-using osu.Game.Rulesets.Scoring;
 using osu.Game.Rulesets.Taiko.Difficulty.Preprocessing;
 using osu.Game.Rulesets.Taiko.Difficulty.Preprocessing.Colour;
 using osu.Game.Rulesets.Taiko.Difficulty.Preprocessing.Rhythm;
 using osu.Game.Rulesets.Taiko.Difficulty.Skills;
 using osu.Game.Rulesets.Taiko.Mods;
-using osu.Game.Rulesets.Taiko.Scoring;
+using osu.Game.Utils;
 
 namespace osu.Game.Rulesets.Taiko.Difficulty
 {
     public class TaikoDifficultyCalculator : DifficultyCalculator
     {
         private const double difficulty_multiplier = 0.084375;
-        private const double rhythm_skill_multiplier = 0.750 * difficulty_multiplier;
+        private const double rhythm_skill_multiplier = 0.770 * difficulty_multiplier;
         private const double reading_skill_multiplier = 0.100 * difficulty_multiplier;
         private const double colour_skill_multiplier = 0.375 * difficulty_multiplier;
         private const double stamina_skill_multiplier = 0.445 * difficulty_multiplier;
@@ -41,17 +40,14 @@ namespace osu.Game.Rulesets.Taiko.Difficulty
         {
         }
 
-        protected override Skill[] CreateSkills(IBeatmap beatmap, Mod[] mods, double clockRate)
+        protected override Skill[] CreateSkills(IBeatmap beatmap, Mod[] mods)
         {
-            HitWindows hitWindows = new TaikoHitWindows();
-            hitWindows.SetDifficulty(beatmap.Difficulty.OverallDifficulty);
-
             isConvert = beatmap.BeatmapInfo.Ruleset.OnlineID == 0;
             isRelax = mods.Any(h => h is TaikoModRelax);
 
             return new Skill[]
             {
-                new Rhythm(mods, hitWindows.WindowFor(HitResult.Great) / clockRate),
+                new Rhythm(mods),
                 new Reading(mods),
                 new Colour(mods),
                 new Stamina(mods, false, isConvert),
@@ -67,13 +63,15 @@ namespace osu.Game.Rulesets.Taiko.Difficulty
             new TaikoModHardRock(),
         };
 
-        protected override IEnumerable<DifficultyHitObject> CreateDifficultyHitObjects(IBeatmap beatmap, double clockRate)
+        protected override IEnumerable<DifficultyHitObject> CreateDifficultyHitObjects(IBeatmap beatmap, Mod[] mods)
         {
             var difficultyHitObjects = new List<DifficultyHitObject>();
             var centreObjects = new List<TaikoDifficultyHitObject>();
             var rimObjects = new List<TaikoDifficultyHitObject>();
             var noteObjects = new List<TaikoDifficultyHitObject>();
 
+            double clockRate = ModUtils.CalculateRateWithMods(mods);
+
             // Generate TaikoDifficultyHitObjects from the beatmap's hit objects.
             for (int i = 2; i < beatmap.HitObjects.Count; i++)
             {
@@ -97,7 +95,7 @@ namespace osu.Game.Rulesets.Taiko.Difficulty
             return difficultyHitObjects;
         }
 
-        protected override DifficultyAttributes CreateDifficultyAttributes(IBeatmap beatmap, Mod[] mods, Skill[] skills, double clockRate)
+        protected override DifficultyAttributes CreateDifficultyAttributes(IBeatmap beatmap, Mod[] mods, Skill[] skills)
         {
             if (beatmap.HitObjects.Count == 0)
                 return new TaikoDifficultyAttributes { Mods = mods };
@@ -108,14 +106,16 @@ namespace osu.Game.Rulesets.Taiko.Difficulty
             var stamina = skills.OfType<Stamina>().Single(s => !s.SingleColourStamina);
             var singleColourStamina = skills.OfType<Stamina>().Single(s => s.SingleColourStamina);
 
+            double staminaDifficultyValue = stamina.DifficultyValue();
+
             double rhythmSkill = rhythm.DifficultyValue() * rhythm_skill_multiplier;
             double readingSkill = reading.DifficultyValue() * reading_skill_multiplier;
             double colourSkill = colour.DifficultyValue() * colour_skill_multiplier;
-            double staminaSkill = stamina.DifficultyValue() * stamina_skill_multiplier;
+            double staminaSkill = staminaDifficultyValue * stamina_skill_multiplier;
             double monoStaminaSkill = singleColourStamina.DifficultyValue() * stamina_skill_multiplier;
             double monoStaminaFactor = staminaSkill == 0 ? 1 : Math.Pow(monoStaminaSkill / staminaSkill, 5);
 
-            double staminaDifficultStrains = stamina.CountTopWeightedStrains();
+            double staminaDifficultStrains = stamina.CountTopWeightedStrains(staminaDifficultyValue);
 
             // As we don't have pattern integration in osu!taiko, we apply the other two skills relative to rhythm.
             patternMultiplier = Math.Pow(staminaSkill * colourSkill, 0.10);
@@ -184,10 +184,10 @@ namespace osu.Game.Rulesets.Taiko.Difficulty
             }
 
             List<double> hitObjectStrainPeaks = combinePeaks(
-                rhythm.GetObjectStrains().ToList(),
-                reading.GetObjectStrains().ToList(),
-                colour.GetObjectStrains().ToList(),
-                stamina.GetObjectStrains().ToList()
+                rhythm.GetObjectDifficulties(),
+                reading.GetObjectDifficulties(),
+                colour.GetObjectDifficulties(),
+                stamina.GetObjectDifficulties()
             );
 
             if (hitObjectStrainPeaks.Count == 0)
@@ -209,7 +209,7 @@ namespace osu.Game.Rulesets.Taiko.Difficulty
         /// <summary>
         /// Combines lists of peak strains from multiple skills into a list of single peak strains for each section.
         /// </summary>
-        private List<double> combinePeaks(List<double> rhythmPeaks, List<double> readingPeaks, List<double> colourPeaks, List<double> staminaPeaks)
+        private List<double> combinePeaks(IReadOnlyList<double> rhythmPeaks, IReadOnlyList<double> readingPeaks, IReadOnlyList<double> colourPeaks, IReadOnlyList<double> staminaPeaks)
         {
             var combinedPeaks = new List<double>();
 

osu.Game.Tests/NonVisual/DifficultyAdjustmentModCombinationsTest.cs

@@ -223,17 +223,17 @@ namespace osu.Game.Tests.NonVisual
 
             protected override Mod[] DifficultyAdjustmentMods { get; }
 
-            protected override DifficultyAttributes CreateDifficultyAttributes(IBeatmap beatmap, Mod[] mods, Skill[] skills, double clockRate)
+            protected override DifficultyAttributes CreateDifficultyAttributes(IBeatmap beatmap, Mod[] mods, Skill[] skills)
             {
                 throw new NotImplementedException();
             }
 
-            protected override IEnumerable<DifficultyHitObject> CreateDifficultyHitObjects(IBeatmap beatmap, double clockRate)
+            protected override IEnumerable<DifficultyHitObject> CreateDifficultyHitObjects(IBeatmap beatmap, Mod[] mods)
             {
                 throw new NotImplementedException();
             }
 
-            protected override Skill[] CreateSkills(IBeatmap beatmap, Mod[] mods, double clockRate)
+            protected override Skill[] CreateSkills(IBeatmap beatmap, Mod[] mods)
             {
                 throw new NotImplementedException();
             }

osu.Game.Tests/NonVisual/TestSceneTimedDifficultyCalculation.cs

@@ -15,6 +15,7 @@ using osu.Game.Rulesets.Mods;
 using osu.Game.Rulesets.Objects;
 using osu.Game.Rulesets.UI;
 using osu.Game.Tests.Beatmaps;
+using osu.Game.Utils;
 
 namespace osu.Game.Tests.NonVisual
 {
@@ -172,13 +173,15 @@ namespace osu.Game.Tests.NonVisual
             {
             }
 
-            protected override DifficultyAttributes CreateDifficultyAttributes(IBeatmap beatmap, Mod[] mods, Skill[] skills, double clockRate)
+            protected override DifficultyAttributes CreateDifficultyAttributes(IBeatmap beatmap, Mod[] mods, Skill[] skills)
                 => new TestDifficultyAttributes { Objects = beatmap.HitObjects.ToArray() };
 
-            protected override IEnumerable<DifficultyHitObject> CreateDifficultyHitObjects(IBeatmap beatmap, double clockRate)
+            protected override IEnumerable<DifficultyHitObject> CreateDifficultyHitObjects(IBeatmap beatmap, Mod[] mods)
             {
                 List<DifficultyHitObject> objects = new List<DifficultyHitObject>();
 
+                double clockRate = ModUtils.CalculateRateWithMods(mods);
+
                 foreach (var obj in beatmap.HitObjects.OfType<TestHitObject>())
                 {
                     if (!obj.Skip)
@@ -191,7 +194,7 @@ namespace osu.Game.Tests.NonVisual
                 return objects;
             }
 
-            protected override Skill[] CreateSkills(IBeatmap beatmap, Mod[] mods, double clockRate) => new Skill[] { new PassThroughSkill(mods) };
+            protected override Skill[] CreateSkills(IBeatmap beatmap, Mod[] mods) => new Skill[] { new PassThroughSkill(mods) };
 
             private class PassThroughSkill : Skill
             {
@@ -200,8 +203,9 @@ namespace osu.Game.Tests.NonVisual
                 {
                 }
 
-                public override void Process(DifficultyHitObject current)
+                protected override double ProcessInternal(DifficultyHitObject current)
                 {
+                    return 0;
                 }
 
                 public override double DifficultyValue() => 1;

osu.Game.Tests/Visual/UserInterface/TestSceneBeatmapAttributeText.cs

@@ -208,13 +208,13 @@ namespace osu.Game.Tests.Visual.UserInterface
             {
             }
 
-            protected override DifficultyAttributes CreateDifficultyAttributes(IBeatmap beatmap, Mod[] mods, Skill[] skills, double clockRate)
+            protected override DifficultyAttributes CreateDifficultyAttributes(IBeatmap beatmap, Mod[] mods, Skill[] skills)
                 => new DifficultyAttributes(mods, mods.OfType<TestMod>().SingleOrDefault()?.Difficulty.Value ?? 0);
 
-            protected override IEnumerable<DifficultyHitObject> CreateDifficultyHitObjects(IBeatmap beatmap, double clockRate)
+            protected override IEnumerable<DifficultyHitObject> CreateDifficultyHitObjects(IBeatmap beatmap, Mod[] mods)
                 => Array.Empty<DifficultyHitObject>();
 
-            protected override Skill[] CreateSkills(IBeatmap beatmap, Mod[] mods, double clockRate)
+            protected override Skill[] CreateSkills(IBeatmap beatmap, Mod[] mods)
                 => Array.Empty<Skill>();
         }
 

osu.Game/Rulesets/Difficulty/DifficultyAttributes.cs

@@ -33,6 +33,8 @@ namespace osu.Game.Rulesets.Difficulty
         protected const int ATTRIB_ID_MAXIMUM_LEGACY_COMBO_SCORE = 41;
         protected const int ATTRIB_ID_RHYTHM_DIFFICULTY = 43;
         protected const int ATTRIB_ID_CONSISTENCY_FACTOR = 45;
+        protected const int ATTRIB_ID_READING = 47;
+        protected const int ATTRIB_ID_READING_DIFFICULT_NOTE_COUNT = 49;
 
         /// <summary>
         /// The mods which were applied to the beatmap.

osu.Game/Rulesets/Difficulty/DifficultyCalculator.cs

@@ -34,7 +34,6 @@ namespace osu.Game.Rulesets.Difficulty
         protected readonly IWorkingBeatmap WorkingBeatmap;
 
         private Mod[] playableMods;
-        private double clockRate;
 
         private readonly IRulesetInfo ruleset;
 
@@ -74,10 +73,10 @@ namespace osu.Game.Rulesets.Difficulty
             // ReSharper disable once PossiblyMistakenUseOfCancellationToken
             preProcess(mods, cancellationToken);
 
-            var skills = CreateSkills(Beatmap, playableMods, clockRate);
+            var skills = CreateSkills(Beatmap, playableMods);
 
             if (!Beatmap.HitObjects.Any())
-                return CreateDifficultyAttributes(Beatmap, playableMods, skills, clockRate);
+                return CreateDifficultyAttributes(Beatmap, playableMods, skills);
 
             foreach (var hitObject in getDifficultyHitObjects())
             {
@@ -88,7 +87,7 @@ namespace osu.Game.Rulesets.Difficulty
                 }
             }
 
-            return CreateDifficultyAttributes(Beatmap, playableMods, skills, clockRate);
+            return CreateDifficultyAttributes(Beatmap, playableMods, skills);
         }
 
         /// <summary>
@@ -121,7 +120,7 @@ namespace osu.Game.Rulesets.Difficulty
             if (!Beatmap.HitObjects.Any())
                 return attribs;
 
-            var skills = CreateSkills(Beatmap, playableMods, clockRate);
+            var skills = CreateSkills(Beatmap, playableMods);
             var progressiveBeatmap = new ProgressiveCalculationBeatmap(Beatmap);
             var difficultyObjects = getDifficultyHitObjects().ToArray();
 
@@ -142,7 +141,7 @@ namespace osu.Game.Rulesets.Difficulty
                     currentIndex++;
                 }
 
-                attribs.Add(new TimedDifficultyAttributes(obj.GetEndTime(), CreateDifficultyAttributes(progressiveBeatmap, playableMods, skills, clockRate)));
+                attribs.Add(new TimedDifficultyAttributes(obj.GetEndTime(), CreateDifficultyAttributes(progressiveBeatmap, playableMods, skills)));
             }
 
             return attribs;
@@ -174,7 +173,7 @@ namespace osu.Game.Rulesets.Difficulty
         /// <summary>
         /// Retrieves the <see cref="DifficultyHitObject"/>s to calculate against.
         /// </summary>
-        private IEnumerable<DifficultyHitObject> getDifficultyHitObjects() => SortObjects(CreateDifficultyHitObjects(Beatmap, clockRate));
+        private IEnumerable<DifficultyHitObject> getDifficultyHitObjects() => SortObjects(CreateDifficultyHitObjects(Beatmap, playableMods));
 
         /// <summary>
         /// Performs required tasks before every calculation.
@@ -185,8 +184,6 @@ namespace osu.Game.Rulesets.Difficulty
         {
             playableMods = mods.Select(m => m.DeepClone()).ToArray();
             Beatmap = WorkingBeatmap.GetPlayableBeatmap(ruleset, playableMods, cancellationToken);
-
-            clockRate = ModUtils.CalculateRateWithMods(playableMods);
         }
 
         /// <summary>
@@ -277,16 +274,15 @@ namespace osu.Game.Rulesets.Difficulty
         /// This may differ from <see cref="Beatmap"/> in the case of timed calculation.</param>
         /// <param name="mods">The <see cref="Mod"/>s that difficulty was calculated with.</param>
         /// <param name="skills">The skills which processed the beatmap.</param>
-        /// <param name="clockRate">The rate at which the gameplay clock is run at.</param>
-        protected abstract DifficultyAttributes CreateDifficultyAttributes(IBeatmap beatmap, Mod[] mods, Skill[] skills, double clockRate);
+        protected abstract DifficultyAttributes CreateDifficultyAttributes(IBeatmap beatmap, Mod[] mods, Skill[] skills);
 
         /// <summary>
         /// Enumerates <see cref="DifficultyHitObject"/>s to be processed from <see cref="HitObject"/>s in the <see cref="IBeatmap"/>.
         /// </summary>
         /// <param name="beatmap">The <see cref="IBeatmap"/> providing the <see cref="HitObject"/>s to enumerate.</param>
-        /// <param name="clockRate">The rate at which the gameplay clock is run at.</param>
+        /// <param name="mods">Mods to create difficulty objects with.</param>
         /// <returns>The enumerated <see cref="DifficultyHitObject"/>s.</returns>
-        protected abstract IEnumerable<DifficultyHitObject> CreateDifficultyHitObjects(IBeatmap beatmap, double clockRate);
+        protected abstract IEnumerable<DifficultyHitObject> CreateDifficultyHitObjects(IBeatmap beatmap, Mod[] mods);
 
         /// <summary>
         /// Creates the <see cref="Skill"/>s to calculate the difficulty of an <see cref="IBeatmap"/>.
@@ -294,9 +290,8 @@ namespace osu.Game.Rulesets.Difficulty
         /// <param name="beatmap">The <see cref="IBeatmap"/> whose difficulty will be calculated.
         /// This may differ from <see cref="Beatmap"/> in the case of timed calculation.</param>
         /// <param name="mods">Mods to calculate difficulty with.</param>
-        /// <param name="clockRate">Clockrate to calculate difficulty with.</param>
         /// <returns>The <see cref="Skill"/>s.</returns>
-        protected abstract Skill[] CreateSkills(IBeatmap beatmap, Mod[] mods, double clockRate);
+        protected abstract Skill[] CreateSkills(IBeatmap beatmap, Mod[] mods);
 
         /// <summary>
         /// Used to calculate timed difficulty attributes, where only a subset of hitobjects should be visible at any point in time.

osu.Game/Rulesets/Difficulty/Preprocessing/DifficultyHitObject.cs

@@ -5,6 +5,7 @@
 
 using System.Collections.Generic;
 using osu.Game.Rulesets.Objects;
+using osu.Game.Rulesets.Scoring;
 
 namespace osu.Game.Rulesets.Difficulty.Preprocessing
 {
@@ -45,6 +46,11 @@ namespace osu.Game.Rulesets.Difficulty.Preprocessing
         /// </summary>
         public readonly double EndTime;
 
+        /// <summary>
+        /// Beatmap playback rate.
+        /// </summary>
+        public readonly double ClockRate;
+
         /// <summary>
         /// Creates a new <see cref="DifficultyHitObject"/>.
         /// </summary>
@@ -62,6 +68,7 @@ namespace osu.Game.Rulesets.Difficulty.Preprocessing
             DeltaTime = (hitObject.StartTime - lastObject.StartTime) / clockRate;
             StartTime = hitObject.StartTime / clockRate;
             EndTime = hitObject.GetEndTime() / clockRate;
+            ClockRate = clockRate;
         }
 
         public DifficultyHitObject Previous(int backwardsIndex)
@@ -75,5 +82,26 @@ namespace osu.Game.Rulesets.Difficulty.Preprocessing
             int index = Index + (forwardsIndex + 1);
             return index >= 0 && index < difficultyHitObjects.Count ? difficultyHitObjects[index] : default;
         }
+
+        /// <summary>
+        /// Retrieves the full hit window for a <see cref="HitResult"/>.
+        /// </summary>
+        public virtual double HitWindow(HitResult hitResult)
+        {
+            // Try to get HitWindows from nested hit objects
+            // This is important for objects such as Slider in osu! where the object itself has HitWindows set to Empty, but the nested SliderHead has proper hit windows
+            if (BaseObject.HitWindows == HitWindows.Empty)
+            {
+                foreach (var nestedHitObject in BaseObject.NestedHitObjects)
+                {
+                    if (nestedHitObject.HitWindows == HitWindows.Empty)
+                        continue;
+
+                    return 2 * nestedHitObject.HitWindows.WindowFor(hitResult) / ClockRate;
+                }
+            }
+
+            return 2 * BaseObject.HitWindows.WindowFor(hitResult) / ClockRate;
+        }
     }
 }

osu.Game/Rulesets/Difficulty/Skills/HarmonicSkill.cs

@@ -0,0 +1,105 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System;
+using System.Linq;
+using osu.Game.Rulesets.Difficulty.Preprocessing;
+using osu.Game.Rulesets.Difficulty.Utils;
+using osu.Game.Rulesets.Mods;
+
+namespace osu.Game.Rulesets.Difficulty.Skills
+{
+    public abstract class HarmonicSkill : Skill
+    {
+        /// <summary>
+        /// The sum of note weights, calculated during summation.
+        /// Required for any calculations which need to normalise difficulty value.
+        /// </summary>
+        protected double NoteWeightSum;
+
+        /// <summary>
+        /// Scaling factor applied as HarmonicScale / (1 + index) during weight calculations.
+        /// A higher value will increase the influence of the hardest object difficulties during summation.
+        /// </summary>
+        protected virtual double HarmonicScale => 1.0;
+
+        /// <summary>
+        /// Exponent that controls the rate of which decay increases as the index increases.
+        /// Values closer to 1 decay faster whilst lower values give more weight to lower object difficulties.
+        /// </summary>
+        protected virtual double DecayExponent => 0.9;
+
+        protected HarmonicSkill(Mod[] mods)
+            : base(mods)
+        {
+        }
+
+        /// <summary>
+        /// Returns the difficulty value of the current <see cref="DifficultyHitObject"/>. This value is calculated with or without respect to previous objects.
+        /// </summary>
+        protected abstract double ObjectDifficultyOf(DifficultyHitObject current);
+
+        protected sealed override double ProcessInternal(DifficultyHitObject current)
+            => ObjectDifficultyOf(current);
+
+        /// <summary>
+        /// Transforms the object difficulties specifically for final difficulty summation.
+        /// This can be used to decrease weight of certain notes based on a skill-specific criteria.
+        /// </summary>
+        protected virtual void ApplyDifficultyTransformation(double[] difficulties)
+        {
+        }
+
+        public override double DifficultyValue()
+        {
+            if (ObjectDifficulties.Count == 0)
+                return 0;
+
+            // Notes with 0 difficulty are excluded to avoid worst-case time complexity of the following sort (e.g. /b/2351871).
+            // These notes will not contribute to the difficulty.
+            double[] difficulties = ObjectDifficulties.Where(p => p > 0).ToArray();
+
+            if (difficulties.Length == 0)
+                return 0;
+
+            ApplyDifficultyTransformation(difficulties);
+
+            double difficulty = 0;
+            int index = 0;
+
+            foreach (double note in difficulties.OrderDescending())
+            {
+                // Use a harmonic sum that considers each note of the map according to a predefined weight.
+                double weight = (1 + (HarmonicScale / (1 + index))) / (Math.Pow(index, DecayExponent) + 1 + (HarmonicScale / (1 + index)));
+
+                NoteWeightSum += weight;
+
+                difficulty += note * weight;
+                index += 1;
+            }
+
+            return difficulty;
+        }
+
+        /// <summary>
+        /// Calculates the number of object difficulties weighted against the top object difficulty.
+        /// </summary>
+        public virtual double CountTopWeightedObjectDifficulties(double difficultyValue)
+        {
+            if (ObjectDifficulties.Count == 0)
+                return 0.0;
+
+            if (NoteWeightSum == 0)
+                return 0.0;
+
+            double consistentTopNote = difficultyValue / NoteWeightSum; // What would the top difficulty be if all object difficulties were identical
+
+            if (consistentTopNote == 0)
+                return 0;
+
+            return ObjectDifficulties.Sum(d => DifficultyCalculationUtils.Logistic(d / consistentTopNote, 0.88, 10, 1.1));
+        }
+
+        public static double DifficultyToPerformance(double difficulty) => 4.0 * Math.Pow(difficulty, 3.0);
+    }
+}

osu.Game/Rulesets/Difficulty/Skills/Skill.cs

@@ -20,6 +20,11 @@ namespace osu.Game.Rulesets.Difficulty.Skills
         /// </summary>
         protected IReadOnlyList<Mod> Mods => mods;
 
+        /// <summary>
+        /// List of calculated per-object difficulties, populated by Process
+        /// </summary>
+        protected readonly List<double> ObjectDifficulties = new List<double>();
+
         private readonly Mod[] mods;
 
         protected Skill(Mod[] mods)
@@ -31,11 +36,19 @@ namespace osu.Game.Rulesets.Difficulty.Skills
         /// Process a <see cref="DifficultyHitObject"/>.
         /// </summary>
         /// <param name="current">The <see cref="DifficultyHitObject"/> to process.</param>
-        public abstract void Process(DifficultyHitObject current);
+        public void Process(DifficultyHitObject current)
+        {
+            double difficultyValue = ProcessInternal(current);
+            ObjectDifficulties.Add(difficultyValue);
+        }
+
+        protected abstract double ProcessInternal(DifficultyHitObject current);
 
         /// <summary>
         /// Returns the calculated difficulty value representing all <see cref="DifficultyHitObject"/>s that have been processed up to this point.
         /// </summary>
         public abstract double DifficultyValue();
+
+        public IReadOnlyList<double> GetObjectDifficulties() => ObjectDifficulties;
     }
 }

osu.Game/Rulesets/Difficulty/Skills/StrainSkill.cs

@@ -29,7 +29,6 @@ namespace osu.Game.Rulesets.Difficulty.Skills
         private double currentSectionEnd;
 
         private readonly List<double> strainPeaks = new List<double>();
-        protected readonly List<double> ObjectStrains = new List<double>(); // Store individual strains
 
         protected StrainSkill(Mod[] mods)
             : base(mods)
@@ -44,7 +43,7 @@ namespace osu.Game.Rulesets.Difficulty.Skills
         /// <summary>
         /// Process a <see cref="DifficultyHitObject"/> and update current strain values accordingly.
         /// </summary>
-        public sealed override void Process(DifficultyHitObject current)
+        protected sealed override double ProcessInternal(DifficultyHitObject current)
         {
             // The first object doesn't generate a strain, so we begin with an incremented section end
             if (current.Index == 0)
@@ -60,26 +59,25 @@ namespace osu.Game.Rulesets.Difficulty.Skills
             double strain = StrainValueAt(current);
             currentSectionPeak = Math.Max(strain, currentSectionPeak);
 
-            // Store the strain value for the object
-            ObjectStrains.Add(strain);
+            return strain;
         }
 
         /// <summary>
         /// Calculates the number of strains weighted against the top strain.
         /// The result is scaled by clock rate as it affects the total number of strains.
         /// </summary>
-        public virtual double CountTopWeightedStrains()
+        public virtual double CountTopWeightedStrains(double difficultyValue)
         {
-            if (ObjectStrains.Count == 0)
+            if (ObjectDifficulties.Count == 0)
                 return 0.0;
 
-            double consistentTopStrain = DifficultyValue() / 10; // What would the top strain be if all strain values were identical
+            double consistentTopStrain = difficultyValue * (1 - DecayWeight); // What would the top strain be if all strain values were identical
 
             if (consistentTopStrain == 0)
-                return ObjectStrains.Count;
+                return ObjectDifficulties.Count;
 
             // Use a weighted sum of all strains. Constants are arbitrary and give nice values
-            return ObjectStrains.Sum(s => 1.1 / (1 + Math.Exp(-10 * (s / consistentTopStrain - 0.88))));
+            return ObjectDifficulties.Sum(s => 1.1 / (1 + Math.Exp(-10 * (s / consistentTopStrain - 0.88))));
         }
 
         /// <summary>
@@ -116,8 +114,6 @@ namespace osu.Game.Rulesets.Difficulty.Skills
         /// </summary>
         public IEnumerable<double> GetCurrentStrainPeaks() => strainPeaks.Append(currentSectionPeak);
 
-        public IEnumerable<double> GetObjectStrains() => ObjectStrains;
-
         /// <summary>
         /// Returns the calculated difficulty value representing all <see cref="DifficultyHitObject"/>s that have been processed up to this point.
         /// </summary>

osu.Game/Rulesets/Difficulty/Skills/VariableLengthStrainSkill.cs

@@ -0,0 +1,277 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using osu.Framework.Extensions;
+using osu.Game.Rulesets.Difficulty.Preprocessing;
+using osu.Game.Rulesets.Mods;
+
+namespace osu.Game.Rulesets.Difficulty.Skills
+{
+    /// <summary>
+    /// Similar to <see cref="StrainSkill"/>, but instead of strains having a fixed length, strains can be any length.
+    /// A new <see cref="StrainPeak"/> is created for each <see cref="DifficultyHitObject"/>.
+    /// </summary>
+    public abstract class VariableLengthStrainSkill : Skill
+    {
+        /// <summary>
+        /// The weight by which each strain value decays.
+        /// </summary>
+        protected virtual double DecayWeight => 0.9;
+
+        /// <summary>
+        /// The maximum length of each strain section.
+        /// </summary>
+        protected virtual int MaxSectionLength => 400;
+
+        private double currentSectionPeak; // We also keep track of the peak strain in the current section.
+        private double currentSectionBegin;
+        private double currentSectionEnd;
+
+        /// <summary>
+        /// The number of `MaxSectionLength` sections calculated such that enough of the difficulty value is preserved.
+        /// WARNING: This should be overridden if strains are ever used outside of <see cref="DifficultyValue"/>,
+        /// or if <see cref="DifficultyValue"/> is overridden to not use the default geometric sum. This should be removed
+        /// in the future when a better memory-saving technique is implemented.
+        /// </summary>
+        private double maxStoredSections => 11 / (1 - DecayWeight);
+
+        private readonly List<StrainPeak> strainPeaks = new List<StrainPeak>();
+
+        private double totalLength;
+
+        /// <summary>
+        /// Stores previous strains so that, if a high difficulty hit object is followed by a lower
+        /// difficulty hit object, the high difficulty hit object gets a full strain instead of being cut short.
+        /// </summary>
+        private readonly List<(double StrainValue, double StartTime)> queuedStrains = new List<(double, double)>();
+
+        protected VariableLengthStrainSkill(Mod[] mods)
+            : base(mods)
+        {
+        }
+
+        /// <summary>
+        /// Returns the strain value at <see cref="DifficultyHitObject"/>. This value is calculated with or without respect to previous objects.
+        /// </summary>
+        protected abstract double StrainValueAt(DifficultyHitObject current);
+
+        /// <summary>
+        /// Process a <see cref="DifficultyHitObject"/> and update current strain values accordingly.
+        /// </summary>
+        protected sealed override double ProcessInternal(DifficultyHitObject current)
+        {
+            // If we're on the first object, set up the first section to end `MaxSectionLength` after it.
+            if (current.Index == 0)
+            {
+                currentSectionBegin = current.StartTime;
+                currentSectionEnd = currentSectionBegin + MaxSectionLength;
+
+                // No work is required for first object after calculating difficulty
+                currentSectionPeak = StrainValueAt(current);
+                return currentSectionPeak;
+            }
+
+            backfillPeaks(current);
+
+            double currentStrain = StrainValueAt(current);
+
+            // If the current strain is larger than the current peak, begin a new peak
+            // Otherwise, add the current strain to the queue
+            if (currentStrain > currentSectionPeak)
+            {
+                // Clear the queue since none of the strains inside of it will be contributing to the difficulty.
+                queuedStrains.Clear();
+
+                // End the current section with the new peak
+                saveCurrentPeak(current.StartTime - currentSectionBegin);
+
+                // Set up the new section to start at the current object with the current strain
+                currentSectionBegin = current.StartTime;
+                currentSectionEnd = currentSectionBegin + MaxSectionLength;
+                currentSectionPeak = currentStrain;
+            }
+            else
+            {
+                // Empty the queue of smaller elements as they won't be relevant to difficulty
+                while (queuedStrains.Count > 0 && queuedStrains[^1].StrainValue < currentStrain)
+                    queuedStrains.RemoveAt(queuedStrains.Count - 1);
+
+                queuedStrains.Add((currentStrain, current.StartTime));
+            }
+
+            return currentStrain;
+        }
+
+        /// <summary>
+        /// Fills the space between the end of the current section and the current object, if there is any.
+        /// </summary>
+        /// <param name="current">The object who's <see cref="DifficultyHitObject.StartTime"/> is backfilled to.</param>
+        private void backfillPeaks(DifficultyHitObject current)
+        {
+            // If the current object starts after the current section ends
+            // then we want to start a new section without any harsh drop-off.
+            // If we have previous strains that influence the current difficulty we will prioritise those first.
+            // Otherwise, start with the current object's initial strain.
+            while (current.StartTime > currentSectionEnd)
+            {
+                // Save the current peak, marking the end of the section.
+                saveCurrentPeak(currentSectionEnd - currentSectionBegin);
+                currentSectionBegin = currentSectionEnd;
+
+                // If we have any strains queued, then we will use those until the object falls into the new section.
+                if (queuedStrains.Count > 0)
+                {
+                    (double strain, double startTime) = queuedStrains[0];
+                    queuedStrains.RemoveAt(0);
+
+                    // We want the section to end `MaxSectionLength` after the strain we're using as an influence.
+                    // This effectively means the queued strain will exist in its own section if the gap between the queued strain and current object is large enough.
+                    // This is required to make sure there's no harsh difficulty difference between 2 sections if there was a large gap.
+                    currentSectionEnd = startTime + MaxSectionLength;
+                    startNewSectionFrom(currentSectionBegin, current);
+
+                    // If the current object's peak was higher, we don't want to override it with a lower strain.
+                    // Only use the queued strain if it contributes more difficulty.
+                    currentSectionPeak = Math.Max(currentSectionPeak, strain);
+                }
+                // If the queue is empty then we should start the section from the current object instead.
+                // The queue can be empty if we're starting off of the back of a new peak, or if we drained through all the queue
+                // and the current object is still later than the section end.
+                else
+                {
+                    // We don't have any prior strains to take as a reference, so end the new section `MaxSectionLength` after it starts.
+                    currentSectionEnd = currentSectionBegin + MaxSectionLength;
+                    startNewSectionFrom(currentSectionBegin, current);
+                }
+            }
+        }
+
+        /// <summary>
+        /// Saves the current peak strain level to the list of strain peaks, which will be used to calculate an overall difficulty.
+        /// </summary>
+        private void saveCurrentPeak(double sectionLength)
+        {
+            strainPeaks.AddInPlace(new StrainPeak(currentSectionPeak, sectionLength));
+            totalLength += sectionLength;
+
+            // Remove from the back of our strain peaks if there's any which are too deep to contribute to difficulty.
+            // `maxStoredSections` dictates for us how many sections will preserve at least 99.999% of the difficulty value.
+            while (totalLength > maxStoredSections * MaxSectionLength)
+            {
+                totalLength -= strainPeaks[0].SectionLength;
+                strainPeaks.RemoveAt(0);
+            }
+        }
+
+        /// <summary>
+        /// Sets the initial strain level for a new section.
+        /// </summary>
+        /// <param name="time">The beginning of the new section in milliseconds.</param>
+        /// <param name="current">The current hit object.</param>
+        private void startNewSectionFrom(double time, DifficultyHitObject current)
+        {
+            // The maximum strain of the new section is not zero by default
+            // This means we need to capture the strain level at the beginning of the new section, and use that as the initial peak level.
+            currentSectionPeak = CalculateInitialStrain(time, current);
+        }
+
+        /// <summary>
+        /// Retrieves the peak strain at a point in time.
+        /// </summary>
+        /// <param name="time">The time to retrieve the peak strain at.</param>
+        /// <param name="current">The current hit object.</param>
+        /// <returns>The peak strain.</returns>
+        protected abstract double CalculateInitialStrain(double time, DifficultyHitObject current);
+
+        /// <summary>
+        /// Returns a live enumerable of the peak strains for each <see cref="MaxSectionLength"/> section of the beatmap,
+        /// including the peak of the current section.
+        /// </summary>
+        public IEnumerable<StrainPeak> GetCurrentStrainPeaks() => strainPeaks.Append(new StrainPeak(currentSectionPeak, currentSectionEnd - currentSectionBegin));
+
+        /// <summary>
+        /// Returns the calculated difficulty value representing all <see cref="DifficultyHitObject"/>s that have been processed up to this point.
+        /// </summary>
+        public override double DifficultyValue()
+        {
+            double difficulty = 0;
+
+            // Sections with 0 strain are excluded to avoid worst-case time complexity of the following sort (e.g. /b/2351871).
+            // These sections will not contribute to the difficulty.
+            var peaks = GetCurrentStrainPeaks().Where(p => p.Value > 0);
+
+            List<StrainPeak> strains = peaks.OrderByDescending(p => (p.Value, p.SectionLength)).ToList();
+
+            // Time is measured in units of strains
+            double time = 0;
+
+            // Difficulty is a continuous weighted sum of the sorted strains
+            for (int i = 0; i < strains.Count; i++)
+            {
+                /* Weighting function can be thought of as:
+                        b
+                        ∫ DecayWeight^x dx
+                        a
+                    where a = startTime and b = endTime
+
+                    Technically, the function below has been slightly modified from the equation above.
+                    The real function would be
+                        double weight = Math.Pow(DecayWeight, startTime) - Math.Pow(DecayWeight, endTime))
+                        ...
+                        return difficulty / Math.Log(1 / DecayWeight)
+                    E.g. for a DecayWeight of 0.9, we're multiplying by 10 instead of 9.49122...
+
+                    This change makes it so that a map composed solely of MaxSectionLength chunks will have the exact same value when summed in this class and StrainSkill.
+                    Doing this ensures the relationship between strain values and difficulty values remains the same between the two classes.
+                */
+                double startTime = time;
+                double endTime = time + strains[i].SectionLength;
+
+                double weight = Math.Pow(DecayWeight, startTime) - Math.Pow(DecayWeight, endTime);
+
+                difficulty += strains[i].Value * weight;
+                time = endTime;
+            }
+
+            return difficulty / (1 - DecayWeight);
+        }
+
+        /// <summary>
+        /// Calculates the number of strains weighted against the top strain.
+        /// The result is scaled by clock rate as it affects the total number of strains.
+        /// </summary>
+        public virtual double CountTopWeightedStrains(double difficultyValue)
+        {
+            if (ObjectDifficulties.Count == 0)
+                return 0.0;
+
+            double consistentTopStrain = difficultyValue * (1 - DecayWeight); // What would the top strain be if all strain values were identical
+
+            if (consistentTopStrain == 0)
+                return ObjectDifficulties.Count;
+
+            // Use a weighted sum of all strains. Constants are arbitrary and give nice values
+            return ObjectDifficulties.Sum(s => 1.1 / (1 + Math.Exp(-10 * (s / consistentTopStrain - 0.88))));
+        }
+
+        /// <summary>
+        /// Used to store the difficulty of a section of a map.
+        /// </summary>
+        public readonly struct StrainPeak : IComparable<StrainPeak>
+        {
+            public StrainPeak(double value, double sectionLength)
+            {
+                Value = value;
+                SectionLength = Math.Round(sectionLength);
+            }
+
+            public double Value { get; }
+            public double SectionLength { get; }
+
+            public int CompareTo(StrainPeak other) => Value.CompareTo(other.Value);
+        }
+    }
+}

osu.Game/Tests/Beatmaps/DifficultyCalculatorTest.cs

@@ -23,16 +23,16 @@ namespace osu.Game.Tests.Beatmaps
 
         protected abstract string ResourceAssembly { get; }
 
-        protected void Test(double expectedStarRating, int expectedMaxCombo, string name, params Mod[] mods)
+        protected void Test(double? expectedStarRating, int expectedMaxCombo, string name, params Mod[] mods)
         {
-            var attributes = CreateDifficultyCalculator(getBeatmap(name)).Calculate(mods);
+            var attributes = CreateDifficultyCalculator(GetBeatmap(name)).Calculate(mods);
 
             // Platform-dependent math functions (Pow, Cbrt, Exp, etc) may result in minute differences.
             Assert.That(attributes.StarRating, Is.EqualTo(expectedStarRating).Within(0.00001));
             Assert.That(attributes.MaxCombo, Is.EqualTo(expectedMaxCombo));
         }
 
-        private IWorkingBeatmap getBeatmap(string name)
+        protected IWorkingBeatmap GetBeatmap(string name)
         {
             using (var resStream = openResource($"{resource_namespace}.{name}.osu"))
             using (var stream = new LineBufferedReader(resStream))