osu-lazer/osu.Game.Rulesets.Taiko/Difficulty/TaikoDifficultyCalculator.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.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.Scoring;
using osu.Game.Rulesets.Taiko.Difficulty.Preprocessing;
using osu.Game.Rulesets.Taiko.Difficulty.Skills;
using osu.Game.Rulesets.Taiko.Mods;
using osu.Game.Rulesets.Taiko.Objects;
using osu.Game.Rulesets.Taiko.Scoring;

namespace osu.Game.Rulesets.Taiko.Difficulty
{
    public class TaikoDifficultyCalculator : DifficultyCalculator
    {
        private const double rhythm_skill_multiplier = 0.014;
        private const double colour_skill_multiplier = 0.01;
        private const double stamina_skill_multiplier = 0.02;

        public TaikoDifficultyCalculator(Ruleset ruleset, WorkingBeatmap beatmap)
            : base(ruleset, beatmap)
        {
        }

        private double simpleColourPenalty(double staminaDifficulty, double colorDifficulty)
        {
            if (colorDifficulty <= 0) return 0.79 - 0.25;

            return 0.79 - Math.Atan(staminaDifficulty / colorDifficulty - 12) / Math.PI / 2;
        }

        private double norm(double p, params double[] values)
        {
            return Math.Pow(values.Sum(x => Math.Pow(x, p)), 1 / p);
        }

        private double rescale(double sr)
        {
            if (sr < 0) return sr;

            return 10.43 * Math.Log(sr / 8 + 1);
        }

        private double locallyCombinedDifficulty(
            double staminaPenalty, Colour colour, Rhythm rhythm, Stamina staminaRight, Stamina staminaLeft)
        {
            double difficulty = 0;
            double weight = 1;
            List<double> peaks = new List<double>();

            for (int i = 0; i < colour.StrainPeaks.Count; i++)
            {
                double colourPeak = colour.StrainPeaks[i] * colour_skill_multiplier;
                double rhythmPeak = rhythm.StrainPeaks[i] * rhythm_skill_multiplier;
                double staminaPeak = (staminaRight.StrainPeaks[i] + staminaLeft.StrainPeaks[i]) * stamina_skill_multiplier * staminaPenalty;
                peaks.Add(norm(2, colourPeak, rhythmPeak, staminaPeak));
            }

            foreach (double strain in peaks.OrderByDescending(d => d))
            {
                difficulty += strain * weight;
                weight *= 0.9;
            }

            return difficulty;
        }

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

            var colour = (Colour)skills[0];
            var rhythm = (Rhythm)skills[1];
            var staminaRight = (Stamina)skills[2];
            var staminaLeft = (Stamina)skills[3];

            double colourRating = colour.DifficultyValue() * colour_skill_multiplier;
            double rhythmRating = rhythm.DifficultyValue() * rhythm_skill_multiplier;
            double staminaRating = (staminaRight.DifficultyValue() + staminaLeft.DifficultyValue()) * stamina_skill_multiplier;

            double staminaPenalty = simpleColourPenalty(staminaRating, colourRating);
            staminaRating *= staminaPenalty;

            double combinedRating = locallyCombinedDifficulty(staminaPenalty, colour, rhythm, staminaRight, staminaLeft);
            double separatedRating = norm(1.5, colourRating, rhythmRating, staminaRating);
            double starRating = 1.4 * separatedRating + 0.5 * combinedRating;
            starRating = rescale(starRating);

            HitWindows hitWindows = new TaikoHitWindows();
            hitWindows.SetDifficulty(beatmap.BeatmapInfo.BaseDifficulty.OverallDifficulty);

            return new TaikoDifficultyAttributes
            {
                StarRating = starRating,
                Mods = mods,
                StaminaStrain = staminaRating,
                RhythmStrain = rhythmRating,
                ColourStrain = colourRating,
                // Todo: This int cast is temporary to achieve 1:1 results with osu!stable, and should be removed in the future
                GreatHitWindow = (int)hitWindows.WindowFor(HitResult.Great) / clockRate,
                MaxCombo = beatmap.HitObjects.Count(h => h is Hit),
                Skills = skills
            };
        }

        protected override IEnumerable<DifficultyHitObject> CreateDifficultyHitObjects(IBeatmap beatmap, double clockRate)
        {
            List<TaikoDifficultyHitObject> taikoDifficultyHitObjects = new List<TaikoDifficultyHitObject>();

            for (int i = 2; i < beatmap.HitObjects.Count; i++)
            {
                // Check for negative durations
                var currentAfterLast = beatmap.HitObjects[i].StartTime > beatmap.HitObjects[i - 1].StartTime;
                var lastAfterSecondLast = beatmap.HitObjects[i - 1].StartTime > beatmap.HitObjects[i - 2].StartTime;

                if (currentAfterLast && lastAfterSecondLast)
                {
                    taikoDifficultyHitObjects.Add(
                        new TaikoDifficultyHitObject(
                            beatmap.HitObjects[i], beatmap.HitObjects[i - 1], beatmap.HitObjects[i - 2], clockRate, i
                        )
                    );
                }
            }

            new StaminaCheeseDetector().FindCheese(taikoDifficultyHitObjects);
            return taikoDifficultyHitObjects;
        }

        protected override Skill[] CreateSkills(IBeatmap beatmap) => new Skill[]
        {
            new Colour(),
            new Rhythm(),
            new Stamina(true),
            new Stamina(false),
        };

        protected override Mod[] DifficultyAdjustmentMods => new Mod[]
        {
            new TaikoModDoubleTime(),
            new TaikoModHalfTime(),
            new TaikoModEasy(),
            new TaikoModHardRock(),
        };
    }
}