Added ArrayPoolBufferWriter<T> type

Modified port from the MVVM Toolkit type

Author: Sergio0694

Microsoft.Toolkit.Uwp.UI/Helpers/Internals/ArrayPoolBufferWriter{T}.cs

@@ -0,0 +1,129 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using System.Buffers;
+using System.Diagnostics.Contracts;
+using System.Runtime.CompilerServices;
+
+namespace Microsoft.Toolkit.Uwp.UI.Helpers.Internals
+{
+    /// <summary>
+    /// A simple buffer writer implementation using pooled arrays.
+    /// </summary>
+    /// <typeparam name="T">The type of items to store in the list.</typeparam>
+    /// <remarks>
+    /// This type is a <see langword="ref"/> <see langword="struct"/> to avoid the object allocation and to
+    /// enable the pattern-based <see cref="IDisposable"/> support. We aren't worried with consumers not
+    /// using this type correctly since it's private and only accessible within the parent type.
+    /// </remarks>
+    internal ref struct ArrayPoolBufferWriter<T>
+    {
+        /// <summary>
+        /// The default buffer size to use to expand empty arrays.
+        /// </summary>
+        private const int DefaultInitialBufferSize = 128;
+
+        /// <summary>
+        /// The underlying <typeparamref name="T"/> array.
+        /// </summary>
+        private T[] array;
+
+        /// <summary>
+        /// The starting offset within <see cref="array"/>.
+        /// </summary>
+        private int index;
+
+        /// <summary>
+        /// Creates a new instance of the <see cref="ArrayPoolBufferWriter{T}"/> struct.
+        /// </summary>
+        /// <returns>A new <see cref="ArrayPoolBufferWriter{T}"/> instance.</returns>
+        [Pure]
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static ArrayPoolBufferWriter<T> Create()
+        {
+            return new() { array = ArrayPool<T>.Shared.Rent(DefaultInitialBufferSize) };
+        }
+
+        /// <summary>
+        /// Gets the total number of items stored in the current instance.
+        /// </summary>
+        public int Count
+        {
+            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+            get => this.array.Length;
+        }
+
+        /// <summary>
+        /// Gets the item at the specified offset into the current buffer in use.
+        /// </summary>
+        /// <param name="index">The index of the element to retrieve.</param>
+        /// <returns>The item at the specified offset into the current buffer in use.</returns>
+        /// <exception cref="ArgumentOutOfRangeException">Thrown when <paramref name="index"/> is out of range.</exception>
+        public T this[int index]
+        {
+            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+            get
+            {
+                if ((uint)index >= this.index)
+                {
+                    static void Throw() => throw new ArgumentOutOfRangeException(nameof(index));
+
+                    Throw();
+                }
+
+                return this.array[index];
+            }
+        }
+
+        /// <summary>
+        /// Adds a new item to the current collection.
+        /// </summary>
+        /// <param name="item">The item to add.</param>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public void Add(T item)
+        {
+            if (this.index == this.array.Length)
+            {
+                ResizeBuffer();
+            }
+
+            this.array[this.index++] = item;
+        }
+
+        /// <summary>
+        /// Resets the underlying array and the stored items.
+        /// </summary>
+        public void Reset()
+        {
+            Array.Clear(this.array, 0, this.index);
+
+            this.index = 0;
+        }
+
+        /// <summary>
+        /// Resizes <see cref="array"/> when there is no space left for new items.
+        /// </summary>
+        [MethodImpl(MethodImplOptions.NoInlining)]
+        private void ResizeBuffer()
+        {
+            T[] rent = ArrayPool<T>.Shared.Rent(this.index << 2);
+
+            Array.Copy(this.array, 0, rent, 0, this.index);
+            Array.Clear(this.array, 0, this.index);
+
+            ArrayPool<T>.Shared.Return(this.array);
+
+            this.array = rent;
+        }
+
+        /// <inheritdoc cref="IDisposable.Dispose"/>
+        public void Dispose()
+        {
+            Array.Clear(this.array, 0, this.index);
+
+            ArrayPool<T>.Shared.Return(this.array);
+        }
+    }
+}