From 16e60eed56d8586ef69df2eeeb1e5b2fd6b2836f Mon Sep 17 00:00:00 2001
From: Dean Herbert <pe@ppy.sh>
Date: Mon, 20 Sep 2021 01:48:33 +0900
Subject: [PATCH] Reduce `NestedHitObject` enumerator overhead

This was especially bad due to it allocating on any and every start time
change, even the first (see usage in `HitObject.ctor`).
---
 .../Difficulty/CatchDifficultyCalculator.cs    |  2 +-
 osu.Game/Rulesets/Objects/HitObject.cs         | 18 +++++++++++-------
 2 files changed, 12 insertions(+), 8 deletions(-)

diff --git a/osu.Game.Rulesets.Catch/Difficulty/CatchDifficultyCalculator.cs b/osu.Game.Rulesets.Catch/Difficulty/CatchDifficultyCalculator.cs
index 9feaa55051..82d76252d2 100644
--- a/osu.Game.Rulesets.Catch/Difficulty/CatchDifficultyCalculator.cs
+++ b/osu.Game.Rulesets.Catch/Difficulty/CatchDifficultyCalculator.cs
@@ -52,7 +52,7 @@ namespace osu.Game.Rulesets.Catch.Difficulty
 
             // In 2B beatmaps, it is possible that a normal Fruit is placed in the middle of a JuiceStream.
             foreach (var hitObject in beatmap.HitObjects
-                                             .SelectMany(obj => obj is JuiceStream stream ? stream.NestedHitObjects : new[] { obj })
+                                             .SelectMany(obj => obj is JuiceStream stream ? stream.NestedHitObjects.AsEnumerable() : new[] { obj })
                                              .Cast<CatchHitObject>()
                                              .OrderBy(x => x.StartTime))
             {
diff --git a/osu.Game/Rulesets/Objects/HitObject.cs b/osu.Game/Rulesets/Objects/HitObject.cs
index c4b9e6e1ad..ae0cb895bc 100644
--- a/osu.Game/Rulesets/Objects/HitObject.cs
+++ b/osu.Game/Rulesets/Objects/HitObject.cs
@@ -3,11 +3,12 @@
 
 using System;
 using System.Collections.Generic;
-using System.Linq;
 using System.Threading;
 using JetBrains.Annotations;
 using Newtonsoft.Json;
 using osu.Framework.Bindables;
+using osu.Framework.Extensions.ListExtensions;
+using osu.Framework.Lists;
 using osu.Game.Audio;
 using osu.Game.Beatmaps;
 using osu.Game.Beatmaps.ControlPoints;
@@ -83,7 +84,7 @@ namespace osu.Game.Rulesets.Objects
         private readonly List<HitObject> nestedHitObjects = new List<HitObject>();
 
         [JsonIgnore]
-        public IReadOnlyList<HitObject> NestedHitObjects => nestedHitObjects;
+        public SlimReadOnlyListWrapper<HitObject> NestedHitObjects => nestedHitObjects.AsSlimReadOnly();
 
         public HitObject()
         {
@@ -91,7 +92,7 @@ namespace osu.Game.Rulesets.Objects
             {
                 double offset = time.NewValue - time.OldValue;
 
-                foreach (var nested in NestedHitObjects)
+                foreach (var nested in nestedHitObjects)
                     nested.StartTime += offset;
             };
         }
@@ -122,11 +123,14 @@ namespace osu.Game.Rulesets.Objects
 
             if (this is IHasComboInformation hasCombo)
             {
-                foreach (var n in NestedHitObjects.OfType<IHasComboInformation>())
+                foreach (HitObject hitObject in nestedHitObjects)
                 {
-                    n.ComboIndexBindable.BindTo(hasCombo.ComboIndexBindable);
-                    n.ComboIndexWithOffsetsBindable.BindTo(hasCombo.ComboIndexWithOffsetsBindable);
-                    n.IndexInCurrentComboBindable.BindTo(hasCombo.IndexInCurrentComboBindable);
+                    if (hitObject is IHasComboInformation n)
+                    {
+                        n.ComboIndexBindable.BindTo(hasCombo.ComboIndexBindable);
+                        n.ComboIndexWithOffsetsBindable.BindTo(hasCombo.ComboIndexWithOffsetsBindable);
+                        n.IndexInCurrentComboBindable.BindTo(hasCombo.IndexInCurrentComboBindable);
+                    }
                 }
             }