Implement osu!mania performance calculation

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

@@ -60,7 +60,8 @@ public override double Calculate(Dictionary<string, double> categoryDifficulty =
 
             double starRating = calculateDifficulty() * star_scaling_factor;
 
-            categoryDifficulty?.Add("Strain", starRating);
+            if (categoryDifficulty != null)
+                categoryDifficulty["Strain"] = starRating;
 
             return starRating;
         }

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

@@ -0,0 +1,122 @@
+// Copyright (c) 2007-2018 ppy Pty Ltd <contact@ppy.sh>.
+// Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using osu.Game.Beatmaps;
+using osu.Game.Rulesets.Difficulty;
+using osu.Game.Rulesets.Mods;
+using osu.Game.Rulesets.Scoring;
+
+namespace osu.Game.Rulesets.Mania.Difficulty
+{
+    public class ManiaPerformanceCalculator : PerformanceCalculator
+    {
+        private Mod[] mods;
+        private int countGeki;
+        private int countKatu;
+        private int count300;
+        private int count100;
+        private int count50;
+        private int countMiss;
+
+        public ManiaPerformanceCalculator(Ruleset ruleset, IBeatmap beatmap, Score score)
+            : base(ruleset, beatmap, score)
+        {
+        }
+
+        public override double Calculate(Dictionary<string, double> categoryDifficulty = null)
+        {
+            mods = Score.Mods;
+            countGeki = Convert.ToInt32(Score.Statistics[HitResult.Perfect]);
+            countKatu = Convert.ToInt32(Score.Statistics[HitResult.Ok]);
+            count300 = Convert.ToInt32(Score.Statistics[HitResult.Great]);
+            count100 = Convert.ToInt32(Score.Statistics[HitResult.Good]);
+            count50 = Convert.ToInt32(Score.Statistics[HitResult.Meh]);
+            countMiss = Convert.ToInt32(Score.Statistics[HitResult.Miss]);
+
+            if (mods.Any(m => !m.Ranked))
+                return 0;
+
+            // Custom multipliers for NoFail
+            double multiplier = 1.1; // This is being adjusted to keep the final pp value scaled around what it used to be when changing things
+
+            if (mods.Any(m => m is ModNoFail))
+                multiplier *= 0.9;
+            if (mods.Any(m => m is ModEasy))
+                multiplier *= 0.5;
+
+            double strainValue = computeStrainValue();
+            double accValue = computeAccuracyValue();
+            double totalValue =
+                Math.Pow(
+                    Math.Pow(strainValue, 1.1) +
+                    Math.Pow(accValue, 1.1), 1.0 / 1.1
+                ) * multiplier;
+
+            if (categoryDifficulty != null)
+            {
+                categoryDifficulty["Strain"] = strainValue;
+                categoryDifficulty["Accuracy"] = accValue;
+
+            }
+
+            return totalValue;
+        }
+
+        private double computeStrainValue()
+        {
+            IEnumerable<Mod> scoreIncreaseMods = new ManiaRuleset().GetModsFor(ModType.DifficultyIncrease);
+
+            double scoreMultiplier = 1.0;
+            foreach (var m in mods.Where(m => !scoreIncreaseMods.Contains(m)))
+                scoreMultiplier *= m.ScoreMultiplier;
+
+            // Score after being scaled by non-difficulty-increasing mods
+            double scaledScore = Score.TotalScore;
+
+            // Scale score up, so it's comparable to other keymods
+            scaledScore *= 1.0 / scoreMultiplier;
+
+            // Obtain strain difficulty
+            double strainValue = Math.Pow(5 * Math.Max(1, Attributes["Strain"] / 0.0825) - 4.0, 3.0) / 110000.0;
+
+            // Longer maps are worth more
+            strainValue *= 1.0 + 0.1 * Math.Min(1.0, totalHits / 1500.0);
+
+            if (scaledScore <= 500000)
+                strainValue = 0;
+            else if (scaledScore <= 600000)
+                strainValue *= (scaledScore - 500000) / 100000 * 0.3;
+            else if (scaledScore <= 700000)
+                strainValue *= 0.3 + (scaledScore - 600000) / 100000 * 0.35;
+            else if (scaledScore <= 800000)
+                strainValue *= 0.65 + (scaledScore - 700000) / 100000 * 0.20;
+            else if (scaledScore <= 900000)
+                strainValue *= 0.85 + (scaledScore - 800000) / 100000 * 0.1;
+            else
+                strainValue *= 0.95 + (scaledScore - 900000) / 100000 * 0.05;
+
+            return strainValue;
+        }
+
+        private double computeAccuracyValue()
+        {
+            double hitWindow300 = (Beatmap.HitObjects.First().HitWindows.Great / 2 - 0.5) / TimeRate;
+            if (hitWindow300 <= 0)
+                return 0;
+
+            // Lots of arbitrary values from testing.
+            // Considering to use derivation from perfect accuracy in a probabilistic manner - assume normal distribution
+            double accuracyValue = Math.Pow(150.0 / hitWindow300 * Math.Pow(Score.Accuracy, 16), 1.8) * 2.5;
+
+            // Bonus for many hitcircles - it's harder to keep good accuracy up for longer
+            accuracyValue *= Math.Min(1.15, Math.Pow(totalHits / 1500.0, 0.3));
+
+            return accuracyValue;
+        }
+
+        private double totalHits => countGeki + countKatu + count300 + count100 + count50 + countMiss;
+    }
+}

osu.Game.Rulesets.Mania/ManiaRuleset.cs

@@ -18,6 +18,7 @@
 using osu.Game.Rulesets.Difficulty;
 using osu.Game.Rulesets.Mania.Beatmaps;
 using osu.Game.Rulesets.Mania.Difficulty;
+using osu.Game.Rulesets.Scoring;
 
 namespace osu.Game.Rulesets.Mania
 {
@@ -25,6 +26,7 @@ public class ManiaRuleset : Ruleset
     {
         public override RulesetContainer CreateRulesetContainerWith(WorkingBeatmap beatmap) => new ManiaRulesetContainer(this, beatmap);
         public override IBeatmapConverter CreateBeatmapConverter(IBeatmap beatmap) => new ManiaBeatmapConverter(beatmap);
+        public override PerformanceCalculator CreatePerformanceCalculator(IBeatmap beatmap, Score score) => new ManiaPerformanceCalculator(this, beatmap, score);
 
         public override IEnumerable<Mod> ConvertLegacyMods(LegacyMods mods)
         {