Remove HashSet allocation in majority cases

osu.Framework/Bindables/BindableInstanceTracker.cs

@@ -0,0 +1,67 @@
+// Copyright (c) ppy Pty Ltd <[email protected]>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Runtime.CompilerServices;
+
+namespace osu.Framework.Bindables
+{
+    /// <summary>
+    /// Facilitates tracking of bindable instances through recursive invocations on bound bindables.
+    /// Can be provided a small array for fast/naive de-duplication while falling back to <see cref="HashSet{T}"/> when there are too many bindable instances.
+    /// </summary>
+    internal ref struct BindableInstanceTracker
+    {
+        private readonly Span<int> fastStorage;
+        private HashSet<int>? slowStorage;
+        private int count;
+
+        /// <summary>
+        /// Creates a bindable instance tracker.
+        /// </summary>
+        /// <param name="fastStorage">A small array for far de-duplication. Usually allocated on the stack.</param>
+        public BindableInstanceTracker(Span<int> fastStorage)
+        {
+            this.fastStorage = fastStorage;
+        }
+
+        /// <summary>
+        /// Adds an instance to the tracker.
+        /// </summary>
+        /// <param name="id">A unique instance ID.</param>
+        /// <returns>Whether the instance was previously seen.</returns>
+        public bool Add(int id)
+        {
+            Debug.Assert(id != 0);
+
+            bool result = count < fastStorage.Length ? addFast(id) : addSlow(id);
+            count += result ? 1 : 0;
+            return result;
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private bool addSlow(int id)
+        {
+            if (slowStorage == null)
+            {
+                slowStorage = [];
+                for (int i = 0; i < count; i++)
+                    slowStorage.Add(fastStorage[i]);
+            }
+
+            return slowStorage.Add(id);
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private bool addFast(int id)
+        {
+            if (fastStorage[..count].Contains(id))
+                return false;
+
+            fastStorage[count] = id;
+            return true;
+        }
+    }
+}