Updated stream extensions.

Removed superfluous CopyTo(Stream) extension.
Added dual buffer extension.

Author: electricessence

benchmarking/Open.Collections.Benchmarking.csproj

@@ -22,7 +22,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="BenchmarkDotNet" Version="0.13.0" />
+    <PackageReference Include="BenchmarkDotNet" Version="0.13.1" />
     <PackageReference Include="Open.Diagnostics" Version="1.4.1" />
   </ItemGroup>
 

source/Extensions.Stream.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Buffers;
 using System.IO;
+using System.Threading.Tasks;
 
 namespace Open.Collections
 {
@@ -9,24 +10,39 @@ public static partial class Extensions
 		/// <summary>
 		/// Copies the source stream to the target.
 		/// </summary>
-		public static void CopyTo(this Stream source, Stream target, int bufferSize = 4096, bool clearBufferAfter = false)
+		public static async ValueTask DualBufferCopyToAsync(this Stream source, Stream target, int bufferSize = 4096, bool clearBufferAfter = false)
 		{
 			if (source is null)
 				throw new NullReferenceException();
 			if (target is null)
 				throw new ArgumentNullException(nameof(target));
 
-			var pool = bufferSize > 128 ? ArrayPool<byte>.Shared : null;
-			var bytes = pool?.Rent(bufferSize) ?? new byte[bufferSize];
+			var pool = ArrayPool<byte>.Shared;
+			var cNext = pool.Rent(bufferSize);
+			var cCurrent = pool.Rent(bufferSize);
+
 			try
 			{
-				int cnt;
-				while ((cnt = source.Read(bytes, 0, bufferSize)) != 0)
-					target.Write(bytes, 0, cnt);
+				var next = source.ReadAsync(cNext, 0, bufferSize);
+				while (true)
+				{
+					var n = await next.ConfigureAwait(false);
+					if (n == 0) break;
+
+					// Preemptive request before yielding.
+					var current = source.ReadAsync(cCurrent, 0, bufferSize);
+					await target.WriteAsync(cNext, 0, n);
+
+					var swap = cNext;
+					cNext = cCurrent;
+					cCurrent = swap;
+					next = current;
+				}
 			}
 			finally
 			{
-				pool?.Return(bytes, clearBufferAfter);
+				pool.Return(cNext, clearBufferAfter);
+				pool.Return(cCurrent, clearBufferAfter);
 			}
 		}
 	}

source/Open.Collections.csproj

@@ -17,7 +17,7 @@
 		<PackageProjectUrl>https://github.com/Open-NET-Libraries/Open.Collections/</PackageProjectUrl>
 		<RepositoryUrl>https://github.com/Open-NET-Libraries/Open.Collections/</RepositoryUrl>
 		<RepositoryType>git</RepositoryType>
-		<Version>2.12.3</Version>
+		<Version>2.12.4</Version>
 		<PackageReleaseNotes></PackageReleaseNotes>
 		<PackageLicenseExpression>MIT</PackageLicenseExpression>
 		<PublishRepositoryUrl>true</PublishRepositoryUrl>