Add ByteMemoryManager<T> type

src/ImageSharp/Memory/ByteMemoryManager{T}.cs

@@ -0,0 +1,67 @@
+// Copyright (c) Six Labors.
+// Licensed under the Apache License, Version 2.0.
+using System;
+using System.Buffers;
+using System.Runtime.InteropServices;
+
+namespace SixLabors.ImageSharp.Memory
+{
+    /// <summary>
+    /// A custom <see cref="MemoryManager{T}"/> that can wrap <see cref="Memory{T}"/> of <see cref="byte"/> instances
+    /// and cast them to be <see cref="Memory{T}"/> for any arbitrary unmanaged <typeparamref name="T"/> value type.
+    /// </summary>
+    /// <typeparam name="T">The value type to use when casting the wrapped <see cref="Memory{T}"/> instance.</typeparam>
+    internal sealed class ByteMemoryManager<T> : MemoryManager<T>
+        where T : unmanaged
+    {
+        /// <summary>
+        /// The wrapped <see cref="Memory{T}"/> of <see cref="byte"/> instance.
+        /// </summary>
+        private readonly Memory<byte> memory;
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="ByteMemoryManager{T}"/> class.
+        /// </summary>
+        /// <param name="memory">The <see cref="Memory{T}"/> of <see cref="byte"/> instance to wrap.</param>
+        public ByteMemoryManager(Memory<byte> memory)
+        {
+            this.memory = memory;
+        }
+
+        /// <inheritdoc/>
+        protected override void Dispose(bool disposing)
+        {
+        }
+
+        /// <inheritdoc/>
+        public override Span<T> GetSpan()
+        {
+            if (MemoryMarshal.TryGetArray(this.memory, out ArraySegment<byte> arraySegment))
+            {
+                return MemoryMarshal.Cast<byte, T>(arraySegment.AsSpan());
+            }
+
+            if (MemoryMarshal.TryGetMemoryManager<byte, MemoryManager<byte>>(this.memory, out MemoryManager<byte> memoryManager))
+            {
+                return MemoryMarshal.Cast<byte, T>(memoryManager.GetSpan());
+            }
+
+            // This should never be reached, as Memory<T> can currently only be wrapping
+            // either a byte[] array or a MemoryManager<byte> instance in this case.
+            ThrowHelper.ThrowArgumentException("The input Memory<byte> instance was not valid.", nameof(this.memory));
+
+            return default;
+        }
+
+        /// <inheritdoc/>
+        public override MemoryHandle Pin(int elementIndex = 0)
+        {
+            return this.memory.Pin();
+        }
+
+        /// <inheritdoc/>
+        public override void Unpin()
+        {
+        }
+    }
+}