Add speculative support for zstd compressed chunks

SteamKit2/SteamKit2/Steam/CDN/DepotChunk.cs

@@ -61,7 +61,7 @@ public static int Process( DepotManifest.ChunkData info, ReadOnlySpan<byte> data
 
                 if ( buffer[ 0 ] == 'V' && buffer[ 1 ] == 'S' && buffer[ 2 ] == 'Z' && buffer[ 3 ] == 'a' ) // Zstd
                 {
-                    throw new NotImplementedException( "Zstd compressed chunks are not yet implemented in SteamKit." );
+                    writtenDecompressed = VZstdUtil.Decompress( buffer.AsSpan( 0, written ), destination );
                 }
                 else if ( buffer[ 0 ] == 'V' && buffer[ 1 ] == 'Z' && buffer[ 2 ] == 'a' ) // LZMA
                 {

SteamKit2/SteamKit2/SteamKit2.csproj

@@ -55,6 +55,7 @@
     </PackageReference>
     <PackageReference Include="protobuf-net" Version="3.2.46" />
     <PackageReference Include="System.IO.Hashing" Version="9.0.2" />
+    <PackageReference Include="ZstdSharp.Port" Version="0.8.4" />
   </ItemGroup>
 
 </Project>

SteamKit2/SteamKit2/Util/VZstdUtil.cs

@@ -0,0 +1,44 @@
+using System;
+using System.Buffers;
+using System.IO;
+using System.Runtime.InteropServices;
+
+namespace SteamKit2
+{
+    static class VZstdUtil
+    {
+        private const uint VZstdHeader = 0x56535A61;
+
+        public static int Decompress( ReadOnlySpan<byte> buffer, byte[] destination )
+        {
+            if ( MemoryMarshal.Read<uint>( buffer ) != VZstdHeader )
+            {
+                throw new InvalidDataException( "Expecting VZstdHeader at start of stream" );
+            }
+
+            var sizeCompressed = MemoryMarshal.Read<int>( buffer[ ^15.. ] ); // TODO: I am not convinced this is correct -- maybe its the frame size
+            var sizeDecompressed = MemoryMarshal.Read<int>( buffer[ ^11.. ] );
+
+            if ( buffer[ ^3 ] != 'z' || buffer[ ^2 ] != 's' || buffer[ ^1 ] != 'v' )
+            {
+                throw new InvalidDataException( "Expecting VZstdFooter at end of stream" );
+            }
+
+            if ( destination.Length < sizeDecompressed )
+            {
+                throw new ArgumentException( "The destination buffer is smaller than the decompressed data size.", nameof( destination ) );
+            }
+
+            using var zstdDecompressor = new ZstdSharp.Decompressor();
+
+            var input = buffer[ 4..^12 ];
+
+            if ( !zstdDecompressor.TryUnwrap( input, destination, out var sizeWritten ) || sizeDecompressed != sizeWritten )
+            {
+                throw new InvalidDataException( $"Failed to decompress Zstd (expected {sizeDecompressed} bytes, got {sizeWritten})." );
+            }
+
+            return sizeDecompressed;
+        }
+    }
+}