Add "Synesthesia" mod for osu!catch ruleset

osu.Game.Rulesets.Catch/CatchRuleset.cs

@@ -155,6 +155,7 @@ public override IEnumerable<Mod> GetModsFor(ModType type)
                         new CatchModMuted(),
                         new CatchModNoScope(),
                         new CatchModMovingFast(),
+                        new CatchModSynesthesia(),
                     };
 
                 case ModType.System:

osu.Game.Rulesets.Catch/Mods/CatchModSynesthesia.cs

@@ -0,0 +1,54 @@
+// 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 osu.Game.Beatmaps;
+using osu.Game.Graphics;
+using osu.Game.Rulesets.Catch.Objects;
+using osu.Game.Rulesets.Mods;
+using osu.Game.Rulesets.Objects;
+using osu.Game.Rulesets.Objects.Drawables;
+using osu.Game.Screens.Edit;
+using osuTK.Graphics;
+
+namespace osu.Game.Rulesets.Catch.Mods
+{
+    /// <summary>
+    /// Mod that colours <see cref="HitObject"/>s based on the musical division they are on
+    /// </summary>
+    public class CatchModSynesthesia : ModSynesthesia, IApplicableToBeatmap, IApplicableToDrawableHitObject
+    {
+        private readonly OsuColour colours = new OsuColour();
+
+        private IBeatmap? currentBeatmap { get; set; }
+
+        public void ApplyToBeatmap(IBeatmap beatmap)
+        {
+            //Store a reference to the current beatmap to look up the beat divisor when notes are drawn
+            if (currentBeatmap != beatmap)
+                currentBeatmap = beatmap;
+        }
+
+        public void ApplyToDrawableHitObject(DrawableHitObject d)
+        {
+            if (currentBeatmap == null) return;
+
+            Color4? timingBasedColour = null;
+
+            d.HitObjectApplied += _ =>
+                {
+                    // Block bananas from getting coloured.
+                    if (d.HitObject is not Banana)
+                    {
+                        timingBasedColour = BindableBeatDivisor.GetColourFor(currentBeatmap.ControlPointInfo.GetClosestBeatDivisor(d.HitObject.StartTime), colours);
+                    }
+                };
+
+            // Need to set this every update to ensure it doesn't get overwritten by DrawableHitObject.OnApply() -> UpdateComboColour().
+            d.OnUpdate += _ =>
+            {
+                if (timingBasedColour != null)
+                    d.AccentColour.Value = timingBasedColour.Value;
+            };
+        }
+    }
+}