Added MemoryGroupEnumerator<T> struct

src/ImageSharp/Memory/DiscontiguousBuffers/MemoryGroupEnumerator{T}.cs

@@ -0,0 +1,59 @@
+// Copyright (c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+using System;
+using System.Runtime.CompilerServices;
+
+namespace SixLabors.ImageSharp.Memory.DiscontiguousBuffers
+{
+    /// <summary>
+    /// A value-type enumerator for <see cref="MemoryGroup{T}"/> instances.
+    /// </summary>
+    /// <typeparam name="T">The element type.</typeparam>
+    public ref struct MemoryGroupEnumerator<T>
+        where T : struct
+    {
+        private readonly MemoryGroup<T> memoryGroup;
+        private readonly int count;
+        private int index;
+
+        [MethodImpl(InliningOptions.ShortMethod)]
+        internal MemoryGroupEnumerator(MemoryGroup<T>.Owned memoryGroup)
+        {
+            this.memoryGroup = memoryGroup;
+            this.count = memoryGroup.Count;
+            this.index = -1;
+        }
+
+        [MethodImpl(InliningOptions.ShortMethod)]
+        internal MemoryGroupEnumerator(MemoryGroup<T>.Consumed memoryGroup)
+        {
+            this.memoryGroup = memoryGroup;
+            this.count = memoryGroup.Count;
+            this.index = -1;
+        }
+
+        /// <inheritdoc cref="System.Collections.Generic.IEnumerator{T}.Current"/>
+        public Memory<T> Current
+        {
+            [MethodImpl(InliningOptions.ShortMethod)]
+            get => this.memoryGroup[this.index];
+        }
+
+        /// <inheritdoc cref="System.Collections.IEnumerator.MoveNext"/>
+        [MethodImpl(InliningOptions.ShortMethod)]
+        public bool MoveNext()
+        {
+            int index = this.index + 1;
+
+            if (index < this.count)
+            {
+                this.index = index;
+
+                return true;
+            }
+
+            return false;
+        }
+    }
+}