OsuDifficultyCalculator.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.Beatmaps;
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.Rulesets.Osu.Difficulty.Preprocessing;
using osu.Game.Rulesets.Osu.Difficulty.Skills;
using osu.Game.Rulesets.Osu.Difficulty.Utils;
using osu.Game.Rulesets.Osu.Mods;
using osu.Game.Rulesets.Osu.Objects;
using osu.Game.Rulesets.Osu.Scoring;
using osu.Game.Rulesets.Scoring;

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)
            : base(ruleset, beatmap)
        {
        }

        public static double CalculateRateAdjustedApproachRate(double approachRate, double clockRate)
        {
            double preempt = IBeatmapDifficultyInfo.DifficultyRange(approachRate, OsuHitObject.PREEMPT_MAX, OsuHitObject.PREEMPT_MID, OsuHitObject.PREEMPT_MIN) / clockRate;
            return IBeatmapDifficultyInfo.InverseDifficultyRange(preempt, OsuHitObject.PREEMPT_MAX, OsuHitObject.PREEMPT_MID, OsuHitObject.PREEMPT_MIN);
        }

        public static double CalculateRateAdjustedOverallDifficulty(double overallDifficulty, double clockRate)
        {
            HitWindows hitWindows = new OsuHitWindows();
            hitWindows.SetDifficulty(overallDifficulty);

            double hitWindowGreat = hitWindows.WindowFor(HitResult.Great) / clockRate;

            return (79.5 - hitWindowGreat) / 6;
        }

        protected override DifficultyAttributes CreateDifficultyAttributes(IBeatmap beatmap, Mod[] mods, Skill[] skills, double clockRate)
        {
            if (beatmap.HitObjects.Count == 0)
                return new OsuDifficultyAttributes { Mods = mods };

            Aim? aim = null;
            Aim? aimWithoutSliders = null;
            Speed? speed = null;
            Flashlight? flashlight = null;

            for (int i = 0; i < skills.Length; i++)
            {
                switch (skills[i])
                {
                    case Aim a when a.IncludeSliders:
                        aim = a;
                        break;

                    case Aim a:
                        aimWithoutSliders = a;
                        break;

                    case Speed s:
                        speed = s;
                        break;

                    case Flashlight f:
                        flashlight = f;
                        break;
                }
            }

            ArgumentNullException.ThrowIfNull(aim);
            ArgumentNullException.ThrowIfNull(aimWithoutSliders);
            ArgumentNullException.ThrowIfNull(speed);

            double speedNotes = speed.RelevantNoteCount();

            double aimDifficultStrainCount = aim.CountTopWeightedStrains();
            double speedDifficultStrainCount = speed.CountTopWeightedStrains();

            double aimNoSlidersTopWeightedSliderCount = aimWithoutSliders.CountTopWeightedSliders();
            double aimNoSlidersDifficultStrainCount = aimWithoutSliders.CountTopWeightedStrains();

            double aimTopWeightedSliderFactor = aimNoSlidersTopWeightedSliderCount / Math.Max(1, aimNoSlidersDifficultStrainCount - aimNoSlidersTopWeightedSliderCount);

            double speedTopWeightedSliderCount = speed.CountTopWeightedSliders();
            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);

            int hitCircleCount = 0, sliderCount = 0, spinnerCount = 0;

            for (int i = 0; i < beatmap.HitObjects.Count; i++)
            {
                switch (beatmap.HitObjects[i])
                {
                    case HitCircle:
                        hitCircleCount++;
                        break;

                    case Slider:
                        sliderCount++;
                        break;

                    case Spinner:
                        spinnerCount++;
                        break;
                }
            }

            int totalHits = beatmap.HitObjects.Count;

            double drainRate = beatmap.Difficulty.DrainRate;

            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);

            double aimRating = osuRatingCalculator.ComputeAimRating(aimDifficultyValue);
            double speedRating = osuRatingCalculator.ComputeSpeedRating(speedDifficultyValue);

            double flashlightRating = 0.0;

            if (flashlight is not null)
                flashlightRating = osuRatingCalculator.ComputeFlashlightRating(flashlight.DifficultyValue());

            double sliderNestedScorePerObject = LegacyScoreUtils.CalculateNestedScorePerObject(beatmap, totalHits);
            double legacyScoreBaseMultiplier = LegacyScoreUtils.CalculateDifficultyPeppyStars(beatmap);

            var simulator = new OsuLegacyScoreSimulator();
            var scoreAttributes = simulator.Simulate(WorkingBeatmap, beatmap);

            double baseAimPerformance = OsuStrainSkill.DifficultyToPerformance(aimRating);
            double baseSpeedPerformance = OsuStrainSkill.DifficultyToPerformance(speedRating);
            double baseFlashlightPerformance = Flashlight.DifficultyToPerformance(flashlightRating);

            double basePerformance =
                Math.Pow(
                    Math.Pow(baseAimPerformance, 1.1) +
                    Math.Pow(baseSpeedPerformance, 1.1) +
                    Math.Pow(baseFlashlightPerformance, 1.1), 1.0 / 1.1
                );

            double starRating = calculateStarRating(basePerformance);

            OsuDifficultyAttributes attributes = new OsuDifficultyAttributes
            {
                StarRating = starRating,
                Mods = mods,
                AimDifficulty = aimRating,
                AimDifficultSliderCount = difficultSliders,
                SpeedDifficulty = speedRating,
                SpeedNoteCount = speedNotes,
                FlashlightDifficulty = flashlightRating,
                SliderFactor = sliderFactor,
                AimDifficultStrainCount = aimDifficultStrainCount,
                SpeedDifficultStrainCount = speedDifficultStrainCount,
                AimTopWeightedSliderFactor = aimTopWeightedSliderFactor,
                SpeedTopWeightedSliderFactor = speedTopWeightedSliderFactor,
                DrainRate = drainRate,
                MaxCombo = beatmap.GetMaxCombo(),
                HitCircleCount = hitCircleCount,
                SliderCount = sliderCount,
                SpinnerCount = spinnerCount,
                NestedScorePerObject = sliderNestedScorePerObject,
                LegacyScoreBaseMultiplier = legacyScoreBaseMultiplier,
                MaximumLegacyComboScore = scoreAttributes.ComboScore
            };

            return attributes;
        }

        private double calculateMechanicalDifficultyRating(double aimDifficultyValue, double speedDifficultyValue)
        {
            double aimValue = OsuStrainSkill.DifficultyToPerformance(OsuRatingCalculator.CalculateDifficultyRating(aimDifficultyValue));
            double speedValue = OsuStrainSkill.DifficultyToPerformance(OsuRatingCalculator.CalculateDifficultyRating(speedDifficultyValue));

            double totalValue = Math.Pow(Math.Pow(aimValue, 1.1) + Math.Pow(speedValue, 1.1), 1 / 1.1);

            return calculateStarRating(totalValue);
        }

        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);
        }

        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, objects.Count));
            }

            return objects;
        }

        protected override Skill[] CreateSkills(IBeatmap beatmap, Mod[] mods, double clockRate)
        {
            var skills = new List<Skill>
            {
                new Aim(mods, true),
                new Aim(mods, false),
                new Speed(mods)
            };

            bool hasFlashlight = false;

            for (int i = 0; i < mods.Length; i++)
            {
                if (mods[i] is OsuModFlashlight)
                {
                    hasFlashlight = true;
                    break;
                }
            }

            if (hasFlashlight)
                skills.Add(new Flashlight(mods));

            return skills.ToArray();
        }

        protected override Mod[] DifficultyAdjustmentMods => new Mod[]
        {
            new OsuModTouchDevice(),
            new OsuModDoubleTime(),
            new OsuModHalfTime(),
            new OsuModEasy(),
            new OsuModHardRock(),
            new OsuModFlashlight(),
            new OsuModHidden(),
        };
    }
}