<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net10.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
    <PublishAot>true</PublishAot>
    <InvariantGlobalization>true</InvariantGlobalization>
    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="ZstdSharp.Port" Version="0.8.6" />
  </ItemGroup>

</Project>

using System.Runtime.InteropServices;
using ZstdSharp.Unsafe;

var sourceBytes = new byte[1024*4];
var out1 = new byte[256];
var compressor = new ZStdCompressor(out1.Length);
var random = new Random(0);
random.NextBytes(sourceBytes);
var loopNo = 0;
while (++loopNo <= 1000)
{
    var length1 = random.Next(out1.Length);
    var start1 = random.Next(sourceBytes.Length - length1);
    var toCompress1 = sourceBytes.AsSpan(start1, length1);
    var compressedBytes = compressor.Compress(toCompress1, out1);
    Console.WriteLine($"{loopNo:D4}: {length1}b => {(compressedBytes == 0 ? length1 : compressedBytes)}b");
}

public unsafe class ZStdCompressor
{
    private readonly ZSTD_CCtx_s* _context;
    private readonly nint _scratchPad;
    private const int CCompressionLevel = 19;
    private const int ScratchPadSize = 8 * 1024;
    private int _scratchPadPos = 0;
    private readonly int _scratchPadSafeLimit;
    
    public ZStdCompressor(int maxPacketSize = 256)
    {
        _context = Methods.ZSTD_createCCtx();
        _scratchPad = Marshal.AllocHGlobal(ScratchPadSize);
        Console.WriteLine($"ContextPtr: {(ulong)_context}, ScratchPadPtr: {(ulong)_scratchPad}");
        Window.Clear();
        _scratchPadSafeLimit = ScratchPadSize - maxPacketSize;
        Methods.ZSTD_compressBegin(_context, CCompressionLevel);
    }

    public void Reset()
    {
        Window.Clear();
        _scratchPadPos = 0;
        Methods.ZSTD_CCtx_reset(_context, ZSTD_ResetDirective.ZSTD_reset_session_only);
        Methods.ZSTD_compressBegin(_context, CCompressionLevel);
    }

    public Span<byte> Window => new((void*)_scratchPad, ScratchPadSize);
    public int Compress(ReadOnlySpan<byte> input, Span<byte> output)
    {
        var rbBuffer = new Span<byte>((void*)(_scratchPad + _scratchPadPos), input.Length);
        _scratchPadPos += input.Length;
        if (_scratchPadPos >= _scratchPadSafeLimit) _scratchPadPos = 0;
        input.CopyTo(rbBuffer);
        fixed (void* dstPtr = output)
        fixed (void* srcPtr = rbBuffer)
        {
            var numBytes = (int)Methods.ZSTD_compressBlock(_context, dstPtr, (nuint)output.Length, srcPtr, (nuint)input.Length);
            if (numBytes == 0) 
            {
                rbBuffer.CopyTo(output);
            }

            return numBytes;
        }
    }

    public void Dispose()
    {
        Methods.ZSTD_freeCCtx(_context);
        Marshal.FreeHGlobal(_scratchPad);
    }
}

ContextPtr: 2372961007840, ScratchPadPtr: 2372961013136
0001: 128b => 128b
0002: 3b => 3b
0003: 174b => 174b
0004: 140b => 19b
0005: 181b => 181b
0006: 62b => 62b
----------------------
0994: 158b => 74b
0995: 77b => 9b
0996: 96b => 9b
0997: 35b => 8b
0998: 12b => 8b
0999: 169b => 9b
1000: 179b => 15b

ContextPtr: 1369886658560, ScratchPadPtr: 1369886663856
0001: 128b => 128b
0002: 3b => 3b
0003: 174b => 174b
Process terminated. Access Violation: Attempted to read or write protected memory. This is often an indication that other memory is corrupt. The application will be terminated since this platform does not support throwing an AccessViolationException.
   at System.RuntimeExceptionHelpers.FailFast(String, Exception, String, RhFailFastReason, IntPtr, IntPtr) + 0x137
   at System.RuntimeExceptionHelpers.GetRuntimeException(ExceptionIDs) + 0x165
   at System.Runtime.EH.GetClasslibException(ExceptionIDs, IntPtr) + 0x34
   at ZstdSharp.Unsafe.Methods.ZSTD_litLengthPrice(UInt32, optState_t*, Int32) + 0x35
   at ZstdSharp.Unsafe.Methods.ZSTD_compressBlock_opt_generic(ZSTD_MatchState_t*, SeqStore_t*, UInt32*, Void*, UIntPtr, Int32, ZSTD_dictMode_e) + 0x30f
   at ZstdSharp.Unsafe.Methods.ZSTD_compressBlock_btultra2(ZSTD_MatchState_t*, SeqStore_t*, UInt32*, Void*, UIntPtr) + 0x81
   at ZstdSharp.Unsafe.Methods.ZSTD_buildSeqStore(ZSTD_CCtx_s*, Void*, UIntPtr) + 0x1c5
   at ZstdSharp.Unsafe.Methods.ZSTD_compressBlock_internal(ZSTD_CCtx_s*, Void*, UIntPtr, Void*, UIntPtr, UInt32) + 0x32
   at ZstdSharp.Unsafe.Methods.ZSTD_compressContinue_internal(ZSTD_CCtx_s*, Void*, UIntPtr, Void*, UIntPtr, UInt32, UInt32) + 0x213
   at ZstdSharp.Unsafe.Methods.ZSTD_compressBlock_deprecated(ZSTD_CCtx_s*, Void*, UIntPtr, Void*, UIntPtr) + 0x4b
   at ZStdCompressor.Compress(ReadOnlySpan`1, Span`1) + 0x91
   at Program.<Main>$(String[] args) + 0x2d2