Add support for zstd compressed chunks

Author: xPaw

Diff for: SteamKit2/SteamKit2/Steam/CDN/DepotChunk.cs

@@ -59,14 +59,18 @@ public static int Process( DepotManifest.ChunkData info, ReadOnlySpan<byte> data
                     throw new InvalidDataException( $"Not enough data in the decrypted depot chunk (was {buffer.Length} bytes)." );
                 }
 
-                var decryptedStream = new MemoryStream( buffer, 0, written );
-
-                if ( buffer[ 0 ] == 'V' && buffer[ 1 ] == 'Z' && buffer[ 2 ] == 'a' )
+                if ( buffer[ 0 ] == 'V' && buffer[ 1 ] == 'S' && buffer[ 2 ] == 'Z' && buffer[ 3 ] == 'a' ) // Zstd
+                {
+                    writtenDecompressed = VZstdUtil.Decompress( buffer.AsSpan( 0, written ), destination );
+                }
+                else if ( buffer[ 0 ] == 'V' && buffer[ 1 ] == 'Z' && buffer[ 2 ] == 'a' ) // LZMA
                 {
+                    using var decryptedStream = new MemoryStream( buffer, 0, written );
                     writtenDecompressed = VZipUtil.Decompress( decryptedStream, destination, verifyChecksum: false );
                 }
-                else if ( buffer[ 0 ] == 'P' && buffer[ 1 ] == 'K' && buffer[ 2 ] == 0x03 && buffer[ 3 ] == 0x04 )
+                else if ( buffer[ 0 ] == 'P' && buffer[ 1 ] == 'K' && buffer[ 2 ] == 0x03 && buffer[ 3 ] == 0x04 ) // PKzip
                 {
+                    using var decryptedStream = new MemoryStream( buffer, 0, written );
                     writtenDecompressed = ZipUtil.Decompress( decryptedStream, destination, verifyChecksum: false );
                 }
                 else

Diff for: 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;
+        }
+    }
+}