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 System.Linq;
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 readonly struct Island : IEquatable<Island>
        {
            public Island()
            {
            }

            public Island(int firstDelta, double epsilon)
            {
                AddDelta(firstDelta, epsilon);
            }

            public List<int> Deltas { get; } = new List<int>();

            public void AddDelta(int delta, double epsilon)
            {
                int existingDelta = Deltas.FirstOrDefault(x => Math.Abs(x - delta) >= epsilon);

                Deltas.Add(existingDelta == default ? delta : existingDelta);
            }

            public double AverageDelta() => Deltas.Count > 0 ? Math.Max(Deltas.Average(), OsuDifficultyHitObject.MIN_DELTA_TIME) : 0;

            public bool IsSimilarPolarity(Island other, double epsilon)
            {
                // 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 Math.Abs(AverageDelta() - other.AverageDelta()) < epsilon &&
                       Deltas.Count % 2 == other.Deltas.Count % 2;
            }

            public override int GetHashCode()
            {
                // we need to compare all deltas and they must be in the exact same order we added them
                string joinedDeltas = string.Join(string.Empty, Deltas);
                return joinedDeltas.GetHashCode();
            }

            public bool Equals(Island other)
            {
                return other.GetHashCode() == GetHashCode();
            }

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

        private const int history_time_max = 5 * 1000; // 5 seconds of calculatingRhythmBonus max.
        private const double rhythm_multiplier = 0.95;

        /// <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;

            var island = new Island();
            var previousIsland = new Island();
            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, 32);

            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);

            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;

                double currRatio = 1.0 + 9.8 * Math.Min(0.5, Math.Pow(Math.Sin(Math.PI / (Math.Min(prevDelta, currDelta) / Math.Max(prevDelta, currDelta))), 2)); // fancy function to calculate rhythmbonuses.

                double windowPenalty = Math.Min(1, Math.Max(0, Math.Abs(prevDelta - currDelta) - currObj.HitWindowGreat * 0.3) / (currObj.HitWindowGreat * 0.3));

                windowPenalty = Math.Min(1, windowPenalty);

                double effectiveRatio = windowPenalty * currRatio;

                double deltaDifferenceEpsilon = currObj.HitWindowGreat * 0.3;

                if (firstDeltaSwitch)
                {
                    if (!(Math.Abs(prevDelta - currDelta) > deltaDifferenceEpsilon))
                    {
                        // island is still progressing
                        island.AddDelta((int)currDelta, deltaDifferenceEpsilon);
                    }
                    else
                    {
                        // 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
                        // therefore we're checking for quick sliders and don't lower the difficulty for them since they don't really make tapping easier (no time to adjust)
                        if (prevObj.BaseObject is Slider && prevObj.TravelTime > prevDelta * 1.5)
                            effectiveRatio *= 0.15;

                        // repeated island polartiy (2 -> 4, 3 -> 5)
                        if (island.IsSimilarPolarity(previousIsland, deltaDifferenceEpsilon))
                            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;

                        if (!islandCounts.TryAdd(island, 1))
                        {
                            islandCounts[island]++;

                            // repeated island (ex: triplet -> triplet)
                            double power = logistic(island.AverageDelta(), 4, 0.165, 10);
                            effectiveRatio *= Math.Min(2.0 / 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 want to be 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_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))));
    }
}