Fix and refactor star difficulty calculation boilerplate

Moves star difficulty calculation entry-point to Beatmap, and sets
star difficulty at the correct place for song select to display.

osu.Game.Modes.Catch/CatchDifficultyCalculator.cs

@@ -19,7 +19,7 @@ namespace osu.Game.Modes.Catch
 
         protected override HitObjectConverter<CatchBaseHit> Converter => new CatchConverter();
 
-        protected override double ComputeDifficulty(Dictionary<String, String> categoryDifficulty)
+        protected override double CalculateInternal(Dictionary<String, String> categoryDifficulty)
         {
             return 0;
         }

osu.Game.Modes.Mania/ManiaDifficultyCalculator.cs

@@ -22,7 +22,7 @@ namespace osu.Game.Modes.Mania
 
         protected override HitObjectConverter<ManiaBaseHit> Converter => new ManiaConverter(columns);
 
-        protected override double ComputeDifficulty(Dictionary<String, String> categoryDifficulty)
+        protected override double CalculateInternal(Dictionary<String, String> categoryDifficulty)
         {
             return 0;
         }

osu.Game.Modes.Osu/OsuDifficultyCalculator.cs

@@ -34,7 +34,7 @@ namespace osu.Game.Modes.Osu
                     ((Slider)h).Curve.Calculate();
         }
 
-        protected override double ComputeDifficulty(Dictionary<String, String> categoryDifficulty)
+        protected override double CalculateInternal(Dictionary<String, String> categoryDifficulty)
         {
             // Fill our custom DifficultyHitObject class, that carries additional information
             DifficultyHitObjects.Clear();

osu.Game.Modes.Taiko/TaikoDifficultyCalculator.cs

@@ -19,7 +19,7 @@ namespace osu.Game.Modes.Taiko
 
         protected override HitObjectConverter<TaikoBaseHit> Converter => new TaikoConverter();
 
-        protected override double ComputeDifficulty(Dictionary<String, String> categoryDifficulty)
+        protected override double CalculateInternal(Dictionary<String, String> categoryDifficulty)
         {
             return 0;
         }

osu.Game/Beatmaps/Beatmap.cs

@@ -7,6 +7,7 @@ using OpenTK.Graphics;
 using osu.Game.Beatmaps.Timing;
 using osu.Game.Database;
 using osu.Game.Modes.Objects;
+using osu.Game.Modes;
 
 namespace osu.Game.Beatmaps
 {
@@ -57,5 +58,7 @@ namespace osu.Game.Beatmaps
 
             return timingPoint ?? ControlPoint.Default;
         }
+
+        public double CalculateStarDifficulty() => Ruleset.GetRuleset(BeatmapInfo.Mode).CreateDifficultyCalculator(this).Calculate();
     }
 }

osu.Game/Beatmaps/DifficultyCalculator.cs

@@ -14,7 +14,7 @@ namespace osu.Game.Beatmaps
 
         protected double TimeRate = 1;
 
-        protected abstract double ComputeDifficulty(Dictionary<String, String> categoryDifficulty);
+        protected abstract double CalculateInternal(Dictionary<String, String> categoryDifficulty);
 
         private void loadTiming()
         {
@@ -23,10 +23,10 @@ namespace osu.Game.Beatmaps
             TimeRate = audioRate / 100.0;
         }
 
-        public double GetDifficulty(Dictionary<string, string> categoryDifficulty = null)
+        public double Calculate(Dictionary<string, string> categoryDifficulty = null)
         {
             loadTiming();
-            double difficulty = ComputeDifficulty(categoryDifficulty);
+            double difficulty = CalculateInternal(categoryDifficulty);
             return difficulty;
         }
     }

osu.Game/Beatmaps/Drawables/BeatmapGroup.cs

@@ -59,16 +59,21 @@ namespace osu.Game.Beatmaps.Drawables
             }
         }
 
-        public BeatmapGroup(WorkingBeatmap beatmap)
+        public BeatmapGroup(BeatmapSetInfo beatmapSet, BeatmapDatabase database)
         {
+            BeatmapSet = beatmapSet;
+            WorkingBeatmap beatmap = database.GetWorkingBeatmap(BeatmapSet.Beatmaps.FirstOrDefault());
+            foreach (var b in BeatmapSet.Beatmaps)
+                b.StarDifficulty = (float)database.GetWorkingBeatmap(b).Beatmap.CalculateStarDifficulty();
+
             Header = new BeatmapSetHeader(beatmap)
             {
                 GainedSelection = headerGainedSelection,
                 RelativeSizeAxes = Axes.X,
             };
             
-            BeatmapSet = beatmap.BeatmapSetInfo;
+            BeatmapSet.Beatmaps = BeatmapSet.Beatmaps.OrderBy(b => b.StarDifficulty).ToList();
-            BeatmapPanels = beatmap.BeatmapSetInfo.Beatmaps.Select(b => new BeatmapPanel(b)
+            BeatmapPanels = BeatmapSet.Beatmaps.Select(b => new BeatmapPanel(b)
             {
                 Alpha = 0,
                 GainedSelection = panelGainedSelection,

osu.Game/Database/BeatmapInfo.cs

@@ -73,7 +73,6 @@ namespace osu.Game.Database
         // Metadata
         public string Version { get; set; }
 
-        //todo: background threaded computation of this
         private float starDifficulty = -1;
         public float StarDifficulty
         {
@@ -85,11 +84,6 @@ namespace osu.Game.Database
             set { starDifficulty = value; }
         }
 
-        internal void ComputeDifficulty(BeatmapDatabase database)
-        {
-            StarDifficulty = (float)Ruleset.GetRuleset(Mode).CreateDifficultyCalculator(database.GetWorkingBeatmap(this).Beatmap).GetDifficulty();
-        }
-
         public bool Equals(BeatmapInfo other)
         {
             return ID == other?.ID;

osu.Game/Screens/Select/PlaySongSelect.cs

@@ -325,11 +325,7 @@ namespace osu.Game.Screens.Select
                 if (b.Metadata == null) b.Metadata = beatmapSet.Metadata;
             });
 
-            foreach (var b in beatmapSet.Beatmaps)
+            var group = new BeatmapGroup(beatmapSet, database)
-                b.ComputeDifficulty(database);
-            beatmapSet.Beatmaps = beatmapSet.Beatmaps.OrderBy(b => b.StarDifficulty).ToList();
-
-            var group = new BeatmapGroup(database.GetWorkingBeatmap(beatmapSet.Beatmaps.FirstOrDefault()))
             {
                 SelectionChanged = selectionChanged,
                 StartRequested = b => footer.StartButton.TriggerClick()