Store hitobject history in the hitobject

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

@@ -49,6 +49,9 @@ namespace osu.Game.Rulesets.Catch.Difficulty
         {
             CatchHitObject lastObject = null;
 
+            List<DifficultyHitObject> objects = new List<DifficultyHitObject>();
+            int index = 0;
+
             // In 2B beatmaps, it is possible that a normal Fruit is placed in the middle of a JuiceStream.
             foreach (var hitObject in beatmap.HitObjects
                                              .SelectMany(obj => obj is JuiceStream stream ? stream.NestedHitObjects.AsEnumerable() : new[] { obj })
@@ -60,10 +63,15 @@ namespace osu.Game.Rulesets.Catch.Difficulty
                     continue;
 
                 if (lastObject != null)
-                    yield return new CatchDifficultyHitObject(hitObject, lastObject, clockRate, halfCatcherWidth);
+                {
+                    objects.Add(new CatchDifficultyHitObject(hitObject, lastObject, clockRate, halfCatcherWidth, objects, index));
+                    index++;
+                }
 
                 lastObject = hitObject;
             }
+
+            return objects;
         }
 
         protected override Skill[] CreateSkills(IBeatmap beatmap, Mod[] mods, double clockRate)

osu.Game.Rulesets.Catch/Difficulty/Preprocessing/CatchDifficultyHitObject.cs

@@ -2,6 +2,7 @@
 // See the LICENCE file in the repository root for full licence text.
 
 using System;
+using System.Collections.Generic;
 using osu.Game.Rulesets.Catch.Objects;
 using osu.Game.Rulesets.Difficulty.Preprocessing;
 using osu.Game.Rulesets.Objects;
@@ -24,8 +25,8 @@ namespace osu.Game.Rulesets.Catch.Difficulty.Preprocessing
         /// </summary>
         public readonly double StrainTime;
 
-        public CatchDifficultyHitObject(HitObject hitObject, HitObject lastObject, double clockRate, float halfCatcherWidth)
+        public CatchDifficultyHitObject(HitObject hitObject, HitObject lastObject, double clockRate, float halfCatcherWidth, List<DifficultyHitObject> objects, int position)
-            : base(hitObject, lastObject, clockRate)
+            : base(hitObject, lastObject, clockRate, objects, position)
         {
             // We will scale everything by this factor, so we can assume a uniform CircleSize among beatmaps.
             float scalingFactor = normalized_hitobject_radius / halfCatcherWidth;

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

@@ -62,8 +62,12 @@ namespace osu.Game.Rulesets.Mania.Difficulty
 
             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>();
+
             for (int i = 1; i < sortedObjects.Length; i++)
-                yield return new ManiaDifficultyHitObject(sortedObjects[i], sortedObjects[i - 1], clockRate);
+                objects.Add(new ManiaDifficultyHitObject(sortedObjects[i], sortedObjects[i - 1], clockRate, objects, i - 1));
+
+            return objects;
         }
 
         // Sorting is done in CreateDifficultyHitObjects, since the full list of hitobjects is required.

osu.Game.Rulesets.Mania/Difficulty/Preprocessing/ManiaDifficultyHitObject.cs

@@ -1,6 +1,7 @@
 // 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 osu.Game.Rulesets.Difficulty.Preprocessing;
 using osu.Game.Rulesets.Mania.Objects;
 using osu.Game.Rulesets.Objects;
@@ -11,8 +12,8 @@ namespace osu.Game.Rulesets.Mania.Difficulty.Preprocessing
     {
         public new ManiaHitObject BaseObject => (ManiaHitObject)base.BaseObject;
 
-        public ManiaDifficultyHitObject(HitObject hitObject, HitObject lastObject, double clockRate)
+        public ManiaDifficultyHitObject(HitObject hitObject, HitObject lastObject, double clockRate, List<DifficultyHitObject> objects, int position)
-            : base(hitObject, lastObject, clockRate)
+            : base(hitObject, lastObject, clockRate, objects, position)
         {
         }
     }

osu.Game.Rulesets.Mania/Difficulty/Skills/Strain.cs

@@ -84,9 +84,9 @@ namespace osu.Game.Rulesets.Mania.Difficulty.Skills
             return individualStrain + overallStrain - CurrentStrain;
         }
 
-        protected override double CalculateInitialStrain(double offset)
+        protected override double CalculateInitialStrain(double offset, DifficultyHitObject current)
-            => applyDecay(individualStrain, offset - Previous[0].StartTime, individual_decay_base)
+            => applyDecay(individualStrain, offset - current.Previous(0).StartTime, individual_decay_base)
-               + applyDecay(overallStrain, offset - Previous[0].StartTime, overall_decay_base);
+               + applyDecay(overallStrain, offset - current.Previous(0).StartTime, overall_decay_base);
 
         private double applyDecay(double value, double deltaTime, double decayBase)
             => value * Math.Pow(decayBase, deltaTime / 1000);

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

@@ -87,16 +87,14 @@ namespace osu.Game.Rulesets.Osu.Difficulty
 
         protected override IEnumerable<DifficultyHitObject> CreateDifficultyHitObjects(IBeatmap beatmap, double clockRate)
         {
+            List<DifficultyHitObject> objects = new List<DifficultyHitObject>();
+
             // 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++)
-            {
+                objects.Add(new OsuDifficultyHitObject(beatmap.HitObjects[i], beatmap.HitObjects[i - 1], clockRate, objects, i - 1));
-                var lastLast = i > 1 ? beatmap.HitObjects[i - 2] : null;
-                var last = beatmap.HitObjects[i - 1];
-                var current = beatmap.HitObjects[i];
 
-                yield return new OsuDifficultyHitObject(current, lastLast, last, clockRate);
+            return objects;
-            }
         }
 
         protected override Skill[] CreateSkills(IBeatmap beatmap, Mod[] mods, double clockRate)

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

@@ -2,6 +2,7 @@
 // See the LICENCE file in the repository root for full licence text.
 
 using System;
+using System.Collections.Generic;
 using osu.Game.Rulesets.Difficulty.Preprocessing;
 using osu.Game.Rulesets.Objects;
 using osu.Game.Rulesets.Osu.Mods;
@@ -12,7 +13,7 @@ namespace osu.Game.Rulesets.Osu.Difficulty.Preprocessing
 {
     public class OsuDifficultyHitObject : DifficultyHitObject
     {
-        private const int normalised_radius = 50; // Change radius to 50 to make 100 the diameter. Easier for mental maths.
+        private const int normalised_radius = 50;
         private const int min_delta_time = 25;
         private const float maximum_slider_radius = normalised_radius * 2.4f;
         private const float assumed_slider_radius = normalised_radius * 1.8f;
@@ -74,10 +75,10 @@ namespace osu.Game.Rulesets.Osu.Difficulty.Preprocessing
         private readonly OsuHitObject lastLastObject;
         private readonly OsuHitObject lastObject;
 
-        public OsuDifficultyHitObject(HitObject hitObject, HitObject lastLastObject, HitObject lastObject, double clockRate)
+        public OsuDifficultyHitObject(HitObject hitObject, HitObject lastObject, double clockRate, List<DifficultyHitObject> objects, int position)
-            : base(hitObject, lastObject, clockRate)
+            : base(hitObject, lastObject, clockRate, objects, position)
         {
-            this.lastLastObject = (OsuHitObject)lastLastObject;
+            lastLastObject = position > 1 ? (OsuHitObject)Previous(1).BaseObject : null;
             this.lastObject = (OsuHitObject)lastObject;
 
             // Capped to 25ms to prevent difficulty calculation breaking from simultaneous objects.

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

@@ -22,8 +22,6 @@ namespace osu.Game.Rulesets.Osu.Difficulty.Skills
 
         private readonly bool withSliders;
 
-        protected override int HistoryLength => 2;
-
         private const double wide_angle_multiplier = 1.5;
         private const double acute_angle_multiplier = 2.0;
         private const double slider_multiplier = 1.5;
@@ -36,12 +34,12 @@ namespace osu.Game.Rulesets.Osu.Difficulty.Skills
 
         private double strainValueOf(DifficultyHitObject current)
         {
-            if (current.BaseObject is Spinner || Previous.Count <= 1 || Previous[0].BaseObject is Spinner)
+            if (current.BaseObject is Spinner || current.Position <= 1 || current.Previous(0).BaseObject is Spinner)
                 return 0;
 
             var osuCurrObj = (OsuDifficultyHitObject)current;
-            var osuLastObj = (OsuDifficultyHitObject)Previous[0];
+            var osuLastObj = (OsuDifficultyHitObject)current.Previous(0);
-            var osuLastLastObj = (OsuDifficultyHitObject)Previous[1];
+            var osuLastLastObj = (OsuDifficultyHitObject)current.Previous(1);
 
             // 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.StrainTime;
@@ -152,7 +150,7 @@ namespace osu.Game.Rulesets.Osu.Difficulty.Skills
 
         private double strainDecay(double ms) => Math.Pow(strainDecayBase, ms / 1000);
 
-        protected override double CalculateInitialStrain(double time) => currentStrain * strainDecay(time - Previous[0].StartTime);
+        protected override double CalculateInitialStrain(double time, DifficultyHitObject current) => currentStrain * strainDecay(time - current.Previous(0).StartTime);
 
         protected override double StrainValueAt(DifficultyHitObject current)
         {

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

@@ -25,7 +25,6 @@ namespace osu.Game.Rulesets.Osu.Difficulty.Skills
         private double skillMultiplier => 0.05;
         private double strainDecayBase => 0.15;
         protected override double DecayWeight => 1.0;
-        protected override int HistoryLength => 10; // Look back for 10 notes is added for the sake of flashlight calculations.
 
         private readonly bool hidden;
 
@@ -51,9 +50,9 @@ namespace osu.Game.Rulesets.Osu.Difficulty.Skills
             OsuDifficultyHitObject lastObj = osuCurrent;
 
             // This is iterating backwards in time from the current object.
-            for (int i = 0; i < Previous.Count; i++)
+            for (int i = 0; i < Math.Min(current.Position, 10); i++)
             {
-                var currentObj = (OsuDifficultyHitObject)Previous[i];
+                var currentObj = (OsuDifficultyHitObject)current.Previous(i);
                 var currentHitObject = (OsuHitObject)(currentObj.BaseObject);
 
                 if (!(currentObj.BaseObject is Spinner))
@@ -89,7 +88,7 @@ namespace osu.Game.Rulesets.Osu.Difficulty.Skills
 
         private double strainDecay(double ms) => Math.Pow(strainDecayBase, ms / 1000);
 
-        protected override double CalculateInitialStrain(double time) => currentStrain * strainDecay(time - Previous[0].StartTime);
+        protected override double CalculateInitialStrain(double time, DifficultyHitObject current) => currentStrain * strainDecay(time - current.Previous(0).StartTime);
 
         protected override double StrainValueAt(DifficultyHitObject current)
         {

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

@@ -29,8 +29,6 @@ namespace osu.Game.Rulesets.Osu.Difficulty.Skills
 
         protected override int ReducedSectionCount => 5;
         protected override double DifficultyMultiplier => 1.04;
-        protected override int HistoryLength => 32;
-
         private readonly double greatWindow;
 
         public Speed(Mod[] mods, double hitWindowGreat)
@@ -55,20 +53,22 @@ namespace osu.Game.Rulesets.Osu.Difficulty.Skills
 
             bool firstDeltaSwitch = false;
 
+            int historicalNoteCount = Math.Min(current.Position, 32);
+
             int rhythmStart = 0;
 
-            while (rhythmStart < Previous.Count - 2 && current.StartTime - Previous[rhythmStart].StartTime < history_time_max)
+            while (rhythmStart < historicalNoteCount - 2 && current.StartTime - current.Previous(rhythmStart).StartTime < history_time_max)
                 rhythmStart++;
 
             for (int i = rhythmStart; i > 0; i--)
             {
-                OsuDifficultyHitObject currObj = (OsuDifficultyHitObject)Previous[i - 1];
+                OsuDifficultyHitObject currObj = (OsuDifficultyHitObject)current.Previous(i - 1);
-                OsuDifficultyHitObject prevObj = (OsuDifficultyHitObject)Previous[i];
+                OsuDifficultyHitObject prevObj = (OsuDifficultyHitObject)current.Previous(i);
-                OsuDifficultyHitObject lastObj = (OsuDifficultyHitObject)Previous[i + 1];
+                OsuDifficultyHitObject lastObj = (OsuDifficultyHitObject)current.Previous(i + 1);
 
                 double currHistoricalDecay = (history_time_max - (current.StartTime - currObj.StartTime)) / history_time_max; // scales note 0 to 1 from history to now
 
-                currHistoricalDecay = Math.Min((double)(Previous.Count - i) / Previous.Count, currHistoricalDecay); // either we're limited by time or limited by object count.
+                currHistoricalDecay = Math.Min((double)(historicalNoteCount - i) / historicalNoteCount, currHistoricalDecay); // either we're limited by time or limited by object count.
 
                 double currDelta = currObj.StrainTime;
                 double prevDelta = prevObj.StrainTime;
@@ -90,10 +90,10 @@ namespace osu.Game.Rulesets.Osu.Difficulty.Skills
                     }
                     else
                     {
-                        if (Previous[i - 1].BaseObject is Slider) // bpm change is into slider, this is easy acc window
+                        if (current.Previous(i - 1).BaseObject is Slider) // bpm change is into slider, this is easy acc window
                             effectiveRatio *= 0.125;
 
-                        if (Previous[i].BaseObject is Slider) // bpm change was from a slider, this is easier typically than circle -> circle
+                        if (current.Previous(i).BaseObject is Slider) // bpm change was from a slider, this is easier typically than circle -> circle
                             effectiveRatio *= 0.25;
 
                         if (previousIslandSize == islandSize) // repeated island size (ex: triplet -> triplet)
@@ -136,7 +136,7 @@ namespace osu.Game.Rulesets.Osu.Difficulty.Skills
 
             // derive strainTime for calculation
             var osuCurrObj = (OsuDifficultyHitObject)current;
-            var osuPrevObj = Previous.Count > 0 ? (OsuDifficultyHitObject)Previous[0] : null;
+            var osuPrevObj = current.Position > 0 ? (OsuDifficultyHitObject)current.Previous(0) : null;
 
             double strainTime = osuCurrObj.StrainTime;
             double greatWindowFull = greatWindow * 2;
@@ -164,7 +164,7 @@ namespace osu.Game.Rulesets.Osu.Difficulty.Skills
 
         private double strainDecay(double ms) => Math.Pow(strainDecayBase, ms / 1000);
 
-        protected override double CalculateInitialStrain(double time) => (currentStrain * currentRhythm) * strainDecay(time - Previous[0].StartTime);
+        protected override double CalculateInitialStrain(double time, DifficultyHitObject current) => (currentStrain * currentRhythm) * strainDecay(time - current.Previous(0).StartTime);
 
         protected override double StrainValueAt(DifficultyHitObject current)
         {

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

@@ -3,6 +3,7 @@
 
 using System;
 using System.Collections.Generic;
+using osu.Game.Rulesets.Difficulty.Preprocessing;
 using osu.Game.Rulesets.Difficulty.Utils;
 using osu.Game.Rulesets.Taiko.Objects;
 
@@ -34,9 +35,9 @@ namespace osu.Game.Rulesets.Taiko.Difficulty.Preprocessing
         /// <summary>
         /// The list of all <see cref="TaikoDifficultyHitObject"/>s in the map.
         /// </summary>
-        private readonly List<TaikoDifficultyHitObject> hitObjects;
+        private readonly List<DifficultyHitObject> hitObjects;
 
-        public StaminaCheeseDetector(List<TaikoDifficultyHitObject> hitObjects)
+        public StaminaCheeseDetector(List<DifficultyHitObject> hitObjects)
         {
             this.hitObjects = hitObjects;
         }
@@ -62,7 +63,7 @@ namespace osu.Game.Rulesets.Taiko.Difficulty.Preprocessing
         /// <param name="patternLength">The length of a single repeating pattern to consider (triplets/quadruplets).</param>
         private void findRolls(int patternLength)
         {
-            var history = new LimitedCapacityQueue<TaikoDifficultyHitObject>(2 * patternLength);
+            var history = new LimitedCapacityQueue<DifficultyHitObject>(2 * patternLength);
 
             // for convenience, we're tracking the index of the item *before* our suspected repeat's start,
             // as that index can be simply subtracted from the current index to get the number of elements in between
@@ -97,11 +98,11 @@ namespace osu.Game.Rulesets.Taiko.Difficulty.Preprocessing
         /// <summary>
         /// Determines whether the objects stored in <paramref name="history"/> contain a repetition of a pattern of length <paramref name="patternLength"/>.
         /// </summary>
-        private static bool containsPatternRepeat(LimitedCapacityQueue<TaikoDifficultyHitObject> history, int patternLength)
+        private static bool containsPatternRepeat(LimitedCapacityQueue<DifficultyHitObject> history, int patternLength)
         {
             for (int j = 0; j < patternLength; j++)
             {
-                if (history[j].HitType != history[j + patternLength].HitType)
+                if (((TaikoDifficultyHitObject)history[j]).HitType != ((TaikoDifficultyHitObject)history[j + patternLength]).HitType)
                     return false;
             }
 
@@ -120,7 +121,7 @@ namespace osu.Game.Rulesets.Taiko.Difficulty.Preprocessing
 
             for (int i = parity; i < hitObjects.Count; i += 2)
             {
-                if (hitObjects[i].HitType == type)
+                if (((TaikoDifficultyHitObject)hitObjects[i]).HitType == type)
                     tlLength += 2;
                 else
                     tlLength = -2;
@@ -139,7 +140,7 @@ namespace osu.Game.Rulesets.Taiko.Difficulty.Preprocessing
         private void markObjectsAsCheese(int start, int end)
         {
             for (int i = start; i <= end; i++)
-                hitObjects[i].StaminaCheese = true;
+                ((TaikoDifficultyHitObject)hitObjects[i]).StaminaCheese = true;
         }
     }
 }

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

@@ -2,6 +2,7 @@
 // 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.Objects;
@@ -24,11 +25,6 @@ namespace osu.Game.Rulesets.Taiko.Difficulty.Preprocessing
         /// </summary>
         public readonly HitType? HitType;
 
-        /// <summary>
-        /// The index of the object in the beatmap.
-        /// </summary>
-        public readonly int ObjectIndex;
-
         /// <summary>
         /// Whether the object should carry a penalty due to being hittable using special techniques
         /// making it easier to do so.
@@ -42,16 +38,15 @@ namespace osu.Game.Rulesets.Taiko.Difficulty.Preprocessing
         /// <param name="lastObject">The gameplay <see cref="HitObject"/> preceding <paramref name="hitObject"/>.</param>
         /// <param name="lastLastObject">The gameplay <see cref="HitObject"/> preceding <paramref name="lastObject"/>.</param>
         /// <param name="clockRate">The rate of the gameplay clock. Modified by speed-changing mods.</param>
-        /// <param name="objectIndex">The index of the object in the beatmap.</param>
+        /// <param name="objects">The list of <see cref="DifficultyHitObject"/>s in the current beatmap.</param>
-        public TaikoDifficultyHitObject(HitObject hitObject, HitObject lastObject, HitObject lastLastObject, double clockRate, int objectIndex)
+        /// <param name="position">The index of the object in the beatmap.</param>
-            : base(hitObject, lastObject, clockRate)
+        public TaikoDifficultyHitObject(HitObject hitObject, HitObject lastObject, HitObject lastLastObject, double clockRate, List<DifficultyHitObject> objects, int position)
+            : base(hitObject, lastObject, clockRate, objects, position)
         {
             var currentHit = hitObject as Hit;
 
             Rhythm = getClosestRhythm(lastObject, lastLastObject, clockRate);
             HitType = currentHit?.Type;
-
-            ObjectIndex = objectIndex;
         }
 
         /// <summary>

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

@@ -106,7 +106,7 @@ namespace osu.Game.Rulesets.Taiko.Difficulty.Skills
                     if (!samePattern(start, mostRecentPatternsToCompare))
                         continue;
 
-                    int notesSince = hitObject.ObjectIndex - rhythmHistory[start].ObjectIndex;
+                    int notesSince = hitObject.Position - rhythmHistory[start].Position;
                     penalty *= repetitionPenalty(notesSince);
                     break;
                 }

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

@@ -66,11 +66,11 @@ namespace osu.Game.Rulesets.Taiko.Difficulty.Skills
 
             TaikoDifficultyHitObject hitObject = (TaikoDifficultyHitObject)current;
 
-            if (hitObject.ObjectIndex % 2 == hand)
+            if (hitObject.Position % 2 == hand)
             {
                 double objectStrain = 1;
 
-                if (hitObject.ObjectIndex == 1)
+                if (hitObject.Position == 1)
                     return 1;
 
                 notePairDurationHistory.Enqueue(hitObject.DeltaTime + offhandObjectDuration);

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

@@ -47,13 +47,13 @@ namespace osu.Game.Rulesets.Taiko.Difficulty
 
         protected override IEnumerable<DifficultyHitObject> CreateDifficultyHitObjects(IBeatmap beatmap, double clockRate)
         {
-            List<TaikoDifficultyHitObject> taikoDifficultyHitObjects = new List<TaikoDifficultyHitObject>();
+            List<DifficultyHitObject> taikoDifficultyHitObjects = new List<DifficultyHitObject>();
 
             for (int i = 2; i < beatmap.HitObjects.Count; i++)
             {
                 taikoDifficultyHitObjects.Add(
                     new TaikoDifficultyHitObject(
-                        beatmap.HitObjects[i], beatmap.HitObjects[i - 1], beatmap.HitObjects[i - 2], clockRate, i
+                        beatmap.HitObjects[i], beatmap.HitObjects[i - 1], beatmap.HitObjects[i - 2], clockRate, taikoDifficultyHitObjects, i - 2
                     )
                 );
             }

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

@@ -1,6 +1,7 @@
 // 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 osu.Game.Rulesets.Objects;
 
 namespace osu.Game.Rulesets.Difficulty.Preprocessing
@@ -10,6 +11,13 @@ namespace osu.Game.Rulesets.Difficulty.Preprocessing
     /// </summary>
     public class DifficultyHitObject
     {
+        private readonly List<DifficultyHitObject> difficultyHitObjects;
+
+        /// <summary>
+        /// The position of this <see cref="DifficultyHitObject"/> in the <see cref="difficultyHitObjects"/> list.
+        /// </summary>
+        public int Position;
+
         /// <summary>
         /// The <see cref="HitObject"/> this <see cref="DifficultyHitObject"/> wraps.
         /// </summary>
@@ -41,13 +49,21 @@ namespace osu.Game.Rulesets.Difficulty.Preprocessing
         /// <param name="hitObject">The <see cref="HitObject"/> which this <see cref="DifficultyHitObject"/> wraps.</param>
         /// <param name="lastObject">The last <see cref="HitObject"/> which occurs before <paramref name="hitObject"/> in the beatmap.</param>
         /// <param name="clockRate">The rate at which the gameplay clock is run at.</param>
-        public DifficultyHitObject(HitObject hitObject, HitObject lastObject, double clockRate)
+        /// <param name="objects">The list of <see cref="DifficultyHitObject"/>s in the current beatmap.</param>
+        /// <param name="position">The position of this <see cref="DifficultyHitObject"/> in the <see cref="difficultyHitObjects"/> list.</param>
+        public DifficultyHitObject(HitObject hitObject, HitObject lastObject, double clockRate, List<DifficultyHitObject> objects, int position)
         {
+            difficultyHitObjects = objects;
+            Position = position;
             BaseObject = hitObject;
             LastObject = lastObject;
             DeltaTime = (hitObject.StartTime - lastObject.StartTime) / clockRate;
             StartTime = hitObject.StartTime / clockRate;
             EndTime = hitObject.GetEndTime() / clockRate;
         }
+
+        public DifficultyHitObject Previous(int backwardsIndex) => difficultyHitObjects[Position - (backwardsIndex + 1)];
+
+        public DifficultyHitObject Next(int forwardsIndex) => difficultyHitObjects[Position + (forwardsIndex + 1)];
     }
 }

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

@@ -3,7 +3,6 @@
 
 using System.Collections.Generic;
 using osu.Game.Rulesets.Difficulty.Preprocessing;
-using osu.Game.Rulesets.Difficulty.Utils;
 using osu.Game.Rulesets.Mods;
 
 namespace osu.Game.Rulesets.Difficulty.Skills
@@ -12,21 +11,10 @@ namespace osu.Game.Rulesets.Difficulty.Skills
     /// A bare minimal abstract skill for fully custom skill implementations.
     /// </summary>
     /// <remarks>
-    /// This class should be considered a "processing" class and not persisted, as it keeps references to
+    /// This class should be considered a "processing" class and not persisted.
-    /// gameplay objects after processing is run (see <see cref="Previous"/>).
     /// </remarks>
     public abstract class Skill
     {
-        /// <summary>
-        /// <see cref="DifficultyHitObject"/>s that were processed previously. They can affect the strain values of the following objects.
-        /// </summary>
-        protected readonly ReverseQueue<DifficultyHitObject> Previous;
-
-        /// <summary>
-        /// Number of previous <see cref="DifficultyHitObject"/>s to keep inside the <see cref="Previous"/> queue.
-        /// </summary>
-        protected virtual int HistoryLength => 1;
-
         /// <summary>
         /// Mods for use in skill calculations.
         /// </summary>
@@ -37,17 +25,11 @@ namespace osu.Game.Rulesets.Difficulty.Skills
         protected Skill(Mod[] mods)
         {
             this.mods = mods;
-            Previous = new ReverseQueue<DifficultyHitObject>(HistoryLength + 1);
         }
 
         internal void ProcessInternal(DifficultyHitObject current)
         {
-            while (Previous.Count > HistoryLength)
-                Previous.Dequeue();
-
             Process(current);
-
-            Previous.Enqueue(current);
         }
 
         /// <summary>

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

@@ -34,7 +34,7 @@ namespace osu.Game.Rulesets.Difficulty.Skills
         {
         }
 
-        protected override double CalculateInitialStrain(double time) => CurrentStrain * strainDecay(time - Previous[0].StartTime);
+        protected override double CalculateInitialStrain(double time, DifficultyHitObject current) => CurrentStrain * strainDecay(time - current.Previous(0).StartTime);
 
         protected override double StrainValueAt(DifficultyHitObject current)
         {

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

@@ -47,13 +47,13 @@ namespace osu.Game.Rulesets.Difficulty.Skills
         protected sealed override void Process(DifficultyHitObject current)
         {
             // The first object doesn't generate a strain, so we begin with an incremented section end
-            if (Previous.Count == 0)
+            if (current.Position == 0)
                 currentSectionEnd = Math.Ceiling(current.StartTime / SectionLength) * SectionLength;
 
             while (current.StartTime > currentSectionEnd)
             {
                 saveCurrentPeak();
-                startNewSectionFrom(currentSectionEnd);
+                startNewSectionFrom(currentSectionEnd, current);
                 currentSectionEnd += SectionLength;
             }
 
@@ -72,19 +72,21 @@ namespace osu.Game.Rulesets.Difficulty.Skills
         /// Sets the initial strain level for a new section.
         /// </summary>
         /// <param name="time">The beginning of the new section in milliseconds.</param>
-        private void startNewSectionFrom(double time)
+        /// <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);
+            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);
+        protected abstract double CalculateInitialStrain(double time, DifficultyHitObject current);
 
         /// <summary>
         /// Returns a live enumerable of the peak strains for each <see cref="SectionLength"/> section of the beatmap,