Replace indexed skill access with skills.OfType<...>().Single()

osu.Game.Rulesets.Catch/Difficulty/CatchDifficultyCalculator.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using osu.Game.Beatmaps;
 using osu.Game.Rulesets.Catch.Beatmaps;
 using osu.Game.Rulesets.Catch.Difficulty.Preprocessing;
@@ -40,7 +41,7 @@ protected override DifficultyAttributes CreateDifficultyAttributes(IBeatmap beat
 
             CatchDifficultyAttributes attributes = new CatchDifficultyAttributes
             {
-                StarRating = Math.Sqrt(skills[0].DifficultyValue()) * difficulty_multiplier,
+                StarRating = Math.Sqrt(skills.OfType<Movement>().Single().DifficultyValue()) * difficulty_multiplier,
                 Mods = mods,
                 ApproachRate = preempt > 1200.0 ? -(preempt - 1800.0) / 120.0 : -(preempt - 1200.0) / 150.0 + 5.0,
                 MaxCombo = beatmap.GetMaxCombo(),

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

@@ -48,7 +48,7 @@ protected override DifficultyAttributes CreateDifficultyAttributes(IBeatmap beat
 
             ManiaDifficultyAttributes attributes = new ManiaDifficultyAttributes
             {
-                StarRating = skills[0].DifficultyValue() * difficulty_multiplier,
+                StarRating = skills.OfType<Strain>().Single().DifficultyValue() * difficulty_multiplier,
                 Mods = mods,
                 // In osu-stable mania, rate-adjustment mods don't affect the hit window.
                 // This is done the way it is to introduce fractional differences in order to match osu-stable for the time being.

osu.Game.Rulesets.Osu/Difficulty/OsuDifficultyCalculator.cs

@@ -36,20 +36,22 @@ protected override DifficultyAttributes CreateDifficultyAttributes(IBeatmap beat
             if (beatmap.HitObjects.Count == 0)
                 return new OsuDifficultyAttributes { Mods = mods };
 
-            double aimRating = Math.Sqrt(skills[0].DifficultyValue()) * difficulty_multiplier;
-            double aimRatingNoSliders = Math.Sqrt(skills[1].DifficultyValue()) * difficulty_multiplier;
-            double speedRating = Math.Sqrt(skills[2].DifficultyValue()) * difficulty_multiplier;
-            double speedNotes = ((Speed)skills[2]).RelevantNoteCount();
-            double difficultSliders = ((Aim)skills[0]).GetDifficultSliders();
-            double flashlightRating = 0.0;
-
-            if (mods.Any(h => h is OsuModFlashlight))
-                flashlightRating = Math.Sqrt(skills[3].DifficultyValue()) * difficulty_multiplier;
+            var aim = skills.OfType<Aim>().Single(a => a.IncludeSliders);
+            double aimRating = Math.Sqrt(aim.DifficultyValue()) * difficulty_multiplier;
+            double aimDifficultyStrainCount = aim.CountTopWeightedStrains();
+            double difficultSliders = aim.GetDifficultSliders();
 
+            var aimWithoutSliders = skills.OfType<Aim>().Single(a => !a.IncludeSliders);
+            double aimRatingNoSliders = Math.Sqrt(aimWithoutSliders.DifficultyValue()) * difficulty_multiplier;
             double sliderFactor = aimRating > 0 ? aimRatingNoSliders / aimRating : 1;
 
-            double aimDifficultyStrainCount = ((OsuStrainSkill)skills[0]).CountTopWeightedStrains();
-            double speedDifficultyStrainCount = ((OsuStrainSkill)skills[2]).CountTopWeightedStrains();
+            var speed = skills.OfType<Speed>().Single();
+            double speedRating = Math.Sqrt(speed.DifficultyValue()) * difficulty_multiplier;
+            double speedNotes = speed.RelevantNoteCount();
+            double speedDifficultyStrainCount = speed.CountTopWeightedStrains();
+
+            var flashlight = skills.OfType<Flashlight>().SingleOrDefault();
+            double flashlightRating = flashlight == null ? 0.0 : Math.Sqrt(flashlight.DifficultyValue()) * difficulty_multiplier;
 
             if (mods.Any(m => m is OsuModTouchDevice))
             {

osu.Game.Rulesets.Osu/Difficulty/Skills/Aim.cs

@@ -16,14 +16,14 @@ namespace osu.Game.Rulesets.Osu.Difficulty.Skills
     /// </summary>
     public class Aim : OsuStrainSkill
     {
-        public Aim(Mod[] mods, bool withSliders)
+        public readonly bool IncludeSliders;
+
+        public Aim(Mod[] mods, bool includeSliders)
             : base(mods)
         {
-            this.withSliders = withSliders;
+            IncludeSliders = includeSliders;
         }
 
-        private readonly bool withSliders;
-
         private double currentStrain;
 
         private double skillMultiplier => 25.6;
@@ -38,7 +38,7 @@ public Aim(Mod[] mods, bool withSliders)
         protected override double StrainValueAt(DifficultyHitObject current)
         {
             currentStrain *= strainDecay(current.DeltaTime);
-            currentStrain += AimEvaluator.EvaluateDifficultyOf(current, withSliders) * skillMultiplier;
+            currentStrain += AimEvaluator.EvaluateDifficultyOf(current, IncludeSliders) * skillMultiplier;
 
             if (current.BaseObject is Slider)
             {

osu.Game.Rulesets.Taiko/Difficulty/Skills/Stamina.cs

@@ -19,7 +19,7 @@ public class Stamina : StrainSkill
         private double skillMultiplier => 1.1;
         private double strainDecayBase => 0.4;
 
-        private readonly bool singleColourStamina;
+        public readonly bool SingleColourStamina;
         private readonly bool isConvert;
 
         private double currentStrain;
@@ -33,7 +33,7 @@ public class Stamina : StrainSkill
         public Stamina(Mod[] mods, bool singleColourStamina, bool isConvert)
             : base(mods)
         {
-            this.singleColourStamina = singleColourStamina;
+            SingleColourStamina = singleColourStamina;
             this.isConvert = isConvert;
         }
 
@@ -50,12 +50,12 @@ protected override double StrainValueAt(DifficultyHitObject current)
 
             double monolengthBonus = isConvert ? 1 : 1 + Math.Min(Math.Max((index - 5) / 50.0, 0), 0.30);
 
-            if (singleColourStamina)
+            if (SingleColourStamina)
                 return DifficultyCalculationUtils.Logistic(-(index - 10) / 2.0, currentStrain);
 
             return currentStrain * monolengthBonus;
         }
 
-        protected override double CalculateInitialStrain(double time, DifficultyHitObject current) => singleColourStamina ? 0 : currentStrain * strainDecay(time - current.Previous(0).StartTime);
+        protected override double CalculateInitialStrain(double time, DifficultyHitObject current) => SingleColourStamina ? 0 : currentStrain * strainDecay(time - current.Previous(0).StartTime);
     }
 }

osu.Game.Rulesets.Taiko/Difficulty/TaikoDifficultyCalculator.cs

@@ -109,11 +109,11 @@ protected override DifficultyAttributes CreateDifficultyAttributes(IBeatmap beat
 
             bool isRelax = mods.Any(h => h is TaikoModRelax);
 
-            Rhythm rhythm = (Rhythm)skills.First(x => x is Rhythm);
-            Reading reading = (Reading)skills.First(x => x is Reading);
-            Colour colour = (Colour)skills.First(x => x is Colour);
-            Stamina stamina = (Stamina)skills.First(x => x is Stamina);
-            Stamina singleColourStamina = (Stamina)skills.Last(x => x is Stamina);
+            var rhythm = skills.OfType<Rhythm>().Single();
+            var reading = skills.OfType<Reading>().Single();
+            var colour = skills.OfType<Colour>().Single();
+            var stamina = skills.OfType<Stamina>().Single(s => !s.SingleColourStamina);
+            var singleColourStamina = skills.OfType<Stamina>().Single(s => s.SingleColourStamina);
 
             double rhythmRating = rhythm.DifficultyValue() * rhythm_skill_multiplier;
             double readingRating = reading.DifficultyValue() * reading_skill_multiplier;