value type enumerator on IndexableSet

src/IndexableSet.cs

@@ -1,15 +1,15 @@
 using System;
-using System.Collections;
 using System.Collections.Generic;
-using System.Runtime.InteropServices;
+using System.Runtime.CompilerServices;
 
 namespace MoonTools.ECS
 {
-	internal class IndexableSet<T> : IEnumerable<T> where T : unmanaged
+	internal class IndexableSet<T> where T : unmanaged
 	{
 		private Dictionary<T, int> indices;
 		private T[] array;
 		public int Count { get; private set; }
+        public Enumerator GetEnumerator() => new Enumerator(this);
 
 		public IndexableSet(int size = 32)
 		{
@@ -64,25 +64,47 @@ namespace MoonTools.ECS
 			return true;
 		}
 
-		public IEnumerator<T> GetEnumerator()
-		{
-			for (var i = Count - 1; i >= 0; i -= 1)
-			{
-				yield return array[i];
-			}
-		}
-
-		IEnumerator IEnumerable.GetEnumerator()
-		{
-			for (var i = Count - 1; i >= 0; i -= 1)
-			{
-				yield return array[i];
-			}
-		}
-
 		public void Clear()
 		{
 			Count = 0;
 		}
+
+		public struct Enumerator
+        {
+            /// <summary>The set being enumerated.</summary>
+            private readonly IndexableSet<T> _set;
+            /// <summary>The next index to yield.</summary>
+            private int _index;
+
+            /// <summary>Initialize the enumerator.</summary>
+            /// <param name="set">The set to enumerate.</param>
+            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+            internal Enumerator(IndexableSet<T> set)
+            {
+				_set = set;
+				_index = _set.Count;
+            }
+
+            /// <summary>Advances the enumerator to the next element of the span.</summary>
+            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+            public bool MoveNext()
+            {
+                int index = _index - 1;
+                if (index >= 0)
+                {
+                    _index = index;
+                    return true;
+                }
+
+                return false;
+            }
+
+            /// <summary>Gets the element at the current position of the enumerator.</summary>
+            public T Current
+            {
+                [MethodImpl(MethodImplOptions.AggressiveInlining)]
+                get => _set[_index];
+            }
+        }
 	}
 }