Span-based (non-stream) compression APIs

[timmydo]

Background and Motivation

Currently to compress/decompress in .NET, you typically would use GZipStream or DeflateStream. However, there are a lot of times where you have an array of bytes and you want to get your output in an array of bytes. It would be simpler/faster to not have to have a bunch of stream redirection to the underlying byte array.

The request is that we add some methods that operate on bytes.

Proposed API

Roughly something like this

namespace System.IO.Compression
{
public class GZipCompressor
public class DeflateCompressor
{
 private object state;
 public ctor(CompressionLevel)
 public void Compress(ReadOnlySpan<byte> in, Span<byte> out, enum flushType, out uint outputBytesWritten);
}
}

Usage Examples

I'm just sort of pseudo-coding this out:

var input = new List<byte[]> {...};
var output = ArrayPool.Shared.Rent(GetMaxCompressedSize(input.Sum(x => x.Length)));
var compressor = new GZipCompressor(CompressionLevel.Optimal);
var outputIndex = 0;
foreach (var array in input)
{
compressor.Compress(new ReadOnlySpan<byte>(array), new Span<byte>(output, outputIndex, output.Length - outputIndex), Z.NoFlush, out var written)
outputIndex += written;
}

compressor.Compress(default(), new Span<byte>(output, outputIndex, output.Length - outputIndex, Z.FinalFlush, out var finalWrite);

 var finalOutput = new Span<byte>(output, 0, outputIndex + finalWrite);
// do stuff with finalOutput

}

[FiniteReality]

These should probably use the OperationStatus enum to communicate its result, like the BrotliEncoder/Decoder. The names should also follow suit. So, something like this would make sense for DEFLATE, I believe:

class DeflateEncoder : IDisposable
{
    public DeflateEncoder(CompressionLevel level);
    public OperationStatus Compress(ReadOnlySpan<byte> source, ReadOnlySpan<byte> destination, out int bytesConsumed, out int bytesWritten, bool isFinalBlock);
    public static int GetMaxCompressedLength(int inputSize);
    // IDisposable implementation
}
class DeflateDecoder : IDisposable
{
    public DeflateDecoder(CompressionLevel level);
    public OperationStatus Decompress(ReadOnlySpan<byte> source, ReadOnlySpan<byte> destination, out int bytesConsumed, out int bytesWritten, bool isFinalBlock);
    // IDisposable implementation
}

// Likewise for GZip

Since we're here, could we potentially also add one for ZLib? It would have the same API as above, but instead of producing raw DEFLATE blocks or GZip blocks, it would produce ZLib blocks instead.

[timmydo]

Related #16923

[wegylexy]

I read about ZLib compression level was between 0 and 9, but now may be 19 or even higher. Need a way to specify that too.