osu-lazer/osu.Game.Rulesets.Osu/Difficulty/Evaluators/RhythmEvaluator.cs

// 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.Preprocessing;
using osu.Game.Rulesets.Osu.Difficulty.Preprocessing;
using osu.Game.Rulesets.Osu.Objects;

namespace osu.Game.Rulesets.Osu.Difficulty.Evaluators
{
    public static class RhythmEvaluator
    {
        private struct Island : IEquatable<Island>
        {
            private readonly double deltaDifferenceEpsilon;

            public Island(double epsilon)
            {
                deltaDifferenceEpsilon = epsilon;
            }

            public Island(int delta, double epsilon)
            {
                deltaDifferenceEpsilon = epsilon;
                Delta = Math.Max(delta, OsuDifficultyHitObject.MIN_DELTA_TIME);
            }

            public int Delta { get; private set; }
            public int DeltaCount { get; private set; }

            public void AddDelta(int delta)
            {
                if (Delta == default)
                    Delta = Math.Max(delta, OsuDifficultyHitObject.MIN_DELTA_TIME);

                DeltaCount++;
            }

            public bool IsSimilarPolarity(Island other)
            {
                // 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)
                return DeltaCount % 2 == other.DeltaCount % 2 &&
                       Math.Abs(Delta - other.Delta) < deltaDifferenceEpsilon;
            }

            public override int GetHashCode()
            {
                return HashCode.Combine(Delta, DeltaCount);
            }

            public bool Equals(Island other)
            {
                return Math.Abs(Delta - other.Delta) < deltaDifferenceEpsilon &&
                       DeltaCount == other.DeltaCount;
            }

            public override bool Equals(object? obj)
            {
                return obj?.GetHashCode() == GetHashCode();
            }
        }

        private const int history_time_max = 4 * 1000; // 5 seconds of calculatingRhythmBonus max.
        private const int history_objects_max = 24;
        private const double rhythm_overall_multiplier = 1.25;
        private const double rhythm_ratio_multiplier = 11.0;

        /// <summary>
        /// Calculates a rhythm multiplier for the difficulty of the tap associated with historic data of the current <see cref="OsuDifficultyHitObject"/>.
        /// </summary>
        public static double EvaluateDifficultyOf(DifficultyHitObject current)
        {
            if (current.BaseObject is Spinner)
                return 0;

            double rhythmComplexitySum = 0;

            double deltaDifferenceEpsilon = ((OsuDifficultyHitObject)current).HitWindowGreat * 0.3;

            var island = new Island(deltaDifferenceEpsilon);
            var previousIsland = new Island(deltaDifferenceEpsilon);
            Dictionary<Island, int> islandCounts = new Dictionary<Island, int>();

            double startRatio = 0; // store the ratio of the current start of an island to buff for tighter rhythms

            bool firstDeltaSwitch = false;

            int historicalNoteCount = Math.Min(current.Index, history_objects_max);

            int rhythmStart = 0;

            while (rhythmStart < historicalNoteCount - 2 && current.StartTime - current.Previous(rhythmStart).StartTime < history_time_max)
                rhythmStart++;

            OsuDifficultyHitObject prevObj = (OsuDifficultyHitObject)current.Previous(rhythmStart);
            OsuDifficultyHitObject lastObj = (OsuDifficultyHitObject)current.Previous(rhythmStart + 1);

            // we go from the furthest object back to the current one
            for (int i = rhythmStart; i > 0; i--)
            {
                OsuDifficultyHitObject currObj = (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)(historicalNoteCount - i) / historicalNoteCount, currHistoricalDecay); // either we're limited by time or limited by object count.

                double currDelta = currObj.StrainTime;
                double prevDelta = prevObj.StrainTime;
                double lastDelta = lastObj.StrainTime;

                // 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 deltaDifferenceRatio = Math.Min(prevDelta, currDelta) / Math.Max(prevDelta, currDelta);
                double currRatio = 1.0 + rhythm_ratio_multiplier * Math.Min(0.5, Math.Pow(Math.Sin(Math.PI / deltaDifferenceRatio), 2));

                // reduce ratio bonus if delta difference is too big
                double fraction = Math.Max(prevDelta / currDelta, currDelta / prevDelta);
                double fractionMultiplier = Math.Clamp(2.0 - fraction / 8.0, 0.0, 1.0);

                double windowPenalty = Math.Min(1, Math.Max(0, Math.Abs(prevDelta - currDelta) - deltaDifferenceEpsilon) / deltaDifferenceEpsilon);

                double effectiveRatio = windowPenalty * currRatio * fractionMultiplier;

                // 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 *= 0.2;

                if (firstDeltaSwitch)
                {
                    if (Math.Abs(prevDelta - currDelta) < deltaDifferenceEpsilon)
                    {
                        // island is still progressing
                        island.AddDelta((int)currDelta);
                    }
                    else
                    {
                        // repeated island polarity (2 -> 4, 3 -> 5)
                        if (island.IsSimilarPolarity(previousIsland))
                            effectiveRatio *= 0.3;

                        // previous increase happened a note ago, 1/1->1/2-1/4, dont want to buff this.
                        if (lastDelta > prevDelta + deltaDifferenceEpsilon && prevDelta > currDelta + deltaDifferenceEpsilon)
                            effectiveRatio *= 0.125;

                        // singletaps are easier to control
                        if (island.DeltaCount == 1)
                            effectiveRatio *= 0.7;

                        if (!islandCounts.TryAdd(island, 1))
                        {
                            // only add island to island counts if they're going one after another
                            if (previousIsland.Equals(island))
                                islandCounts[island]++;

                            // repeated island (ex: triplet -> triplet)
                            double power = logistic(island.Delta, 4, 0.165, 10);
                            effectiveRatio *= Math.Min(3.0 / islandCounts[island], Math.Pow(1.0 / islandCounts[island], power));
                        }

                        // scale down the difficulty if the object is doubletappable
                        double doubletapness = prevObj.GetDoubletapness((OsuDifficultyHitObject?)prevObj.Next(0));
                        effectiveRatio *= 1 - doubletapness * 0.75;

                        rhythmComplexitySum += Math.Sqrt(effectiveRatio * startRatio) * currHistoricalDecay;

                        startRatio = effectiveRatio;

                        previousIsland = island;

                        if (prevDelta + deltaDifferenceEpsilon < currDelta) // we're slowing down, stop counting
                            firstDeltaSwitch = false; // if we're speeding up, this stays true and we keep counting island size.

                        island = new Island((int)currDelta, deltaDifferenceEpsilon);
                    }
                }
                else if (prevDelta > currDelta + deltaDifferenceEpsilon) // we're speeding up
                {
                    // Begin counting island until we change speed again.
                    firstDeltaSwitch = true;

                    startRatio = effectiveRatio;

                    island = new Island((int)currDelta, deltaDifferenceEpsilon);
                }

                lastObj = prevObj;
                prevObj = currObj;
            }

            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 logistic(double x, double maxValue, double multiplier, double offset) => (maxValue / (1 + Math.Pow(Math.E, offset - (multiplier * x))));
    }
}