Reuse new copy method for better performance

Author: sakno

src/DotNext.IO/IO/Pipelines/PipeExtensions.Readers.cs

@@ -253,27 +253,33 @@ private static async ValueTask<TResult> ParseAsync<TResult, TArg>(this PipeReade
 
         static TResult Parse(PipeReader reader, TArg arg, ReadOnlySpanFunc<byte, TArg, TResult> parser, int length, ReadOnlySequence<byte> source)
         {
+            var consumed = source.Start;
             try
             {
                 if (source.Length < length)
                 {
-                    length = (int)source.Length;
+                    consumed = source.End;
                     throw new EndOfStreamException();
                 }
 
                 if (source.TryGetBlock(length, out var block))
+                {
+                    consumed = source.GetPosition(block.Length, consumed);
                     return parser(block.Span, arg);
+                }
+                else
+                {
+                    using var destination = (uint)length <= (uint)SpanOwner<byte>.StackallocThreshold
+                        ? stackalloc byte[length]
+                        : new SpanOwner<byte>(length);
 
-                using var destination = (uint)length <= (uint)SpanOwner<byte>.StackallocThreshold
-                    ? stackalloc byte[length]
-                    : new SpanOwner<byte>(length);
-
-                source.CopyTo(destination.Span, out length);
-                return parser(destination.Span.Slice(0, length), arg);
+                    length = source.CopyTo(destination.Span, out consumed);
+                    return parser(destination.Span.Slice(0, length), arg);
+                }
             }
             finally
             {
-                reader.AdvanceTo(source.GetPosition(length));
+                reader.AdvanceTo(consumed);
             }
         }
     }
@@ -422,15 +428,16 @@ public static async ValueTask<int> ReadAtLeastAsync(this PipeReader reader, Memo
         static int Read(PipeReader reader, ReadOnlySequence<byte> source, in Memory<byte> destination, int minimumSize)
         {
             var readCount = 0;
+            var consumed = source.Start;
             try
             {
-                source.CopyTo(destination.Span, out readCount);
+                readCount = source.CopyTo(destination.Span, out consumed);
                 if (minimumSize > readCount)
                     throw new EndOfStreamException();
             }
             finally
             {
-                reader.AdvanceTo(source.GetPosition(readCount));
+                reader.AdvanceTo(consumed);
             }
 
             return readCount;

src/DotNext/IO/ReadOnlyMemoryStream.cs

@@ -53,11 +53,7 @@ public override void SetLength(long value)
     }
 
     public override int Read(Span<byte> buffer)
-    {
-        RemainingSequence.CopyTo(buffer, out int writtenCount);
-        position = sequence.GetPosition(writtenCount, position);
-        return writtenCount;
-    }
+        => RemainingSequence.CopyTo(buffer, out position);
 
     public override long Seek(long offset, SeekOrigin origin)
     {