Add support for zstd compressed depot chunks

Fixes #1503.

Author: xPaw

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, verifyChecksum: false );
                 }
                 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="3.2.52" />
     <PackageReference Include="System.IO.Hashing" Version="9.0.4" />
+    <PackageReference Include="ZstdSharp.Port" Version="0.8.5" />
   </ItemGroup>
 
 </Project>

SteamKit2/SteamKit2/Util/VZstdUtil.cs

@@ -0,0 +1,53 @@
+using System;
+using System.Diagnostics;
+using System.IO;
+using System.IO.Hashing;
+using System.Runtime.InteropServices;
+
+namespace SteamKit2
+{
+    static class VZstdUtil
+    {
+        private const uint VZstdHeader = 0x615A5356;
+
+        public static int Decompress( ReadOnlySpan<byte> buffer, byte[] destination, bool verifyChecksum = true )
+        {
+            if ( MemoryMarshal.Read<uint>( buffer ) != VZstdHeader )
+            {
+                throw new InvalidDataException( "Expecting VZstdHeader at start of stream" );
+            }
+
+            var crc32 = MemoryMarshal.Read<int>( buffer[ 4.. ] );
+            var crc32_footer = MemoryMarshal.Read<int>( buffer[ ^15.. ] );
+            var sizeDecompressed = MemoryMarshal.Read<int>( buffer[ ^11.. ] );
+
+            Debug.Assert( crc32 == crc32_footer ); // They write CRC32 twice?
+
+            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[ 8..^15 ];
+
+            if ( !zstdDecompressor.TryUnwrap( input, destination, out var sizeWritten ) || sizeDecompressed != sizeWritten )
+            {
+                throw new InvalidDataException( $"Failed to decompress Zstd (expected {sizeDecompressed} bytes, got {sizeWritten})." );
+            }
+
+            if ( verifyChecksum && Crc32.HashToUInt32( destination.AsSpan()[ ..sizeDecompressed ] ) != crc32_footer )
+            {
+                throw new InvalidDataException( "CRC does not match decompressed data. VZstd data may be corrupted." );
+            }
+
+            return sizeDecompressed;
+        }
+    }
+}

SteamKit2/Tests/DepotChunkFacts.cs

@@ -74,5 +74,37 @@ public void DecryptsAndDecompressesDepotChunkVZip()
             var hash = Convert.ToHexString( SHA1.HashData( destination ) );
             Assert.Equal( "7B8567D9B3C09295CDBF4978C32B348D8E76C750", hash );
         }
+
+        [Fact]
+        public void DecryptsAndDecompressesDepotChunkZStd()
+        {
+            var assembly = Assembly.GetExecutingAssembly();
+            using var stream = assembly.GetManifestResourceStream( "Tests.Files.depot_3441461_chunk_9e72678e305540630a665b93e1463bc3983eb55a.bin" );
+            using var ms = new MemoryStream();
+            stream.CopyTo( ms );
+            var chunkData = ms.ToArray();
+
+            var chunk = new DepotManifest.ChunkData(
+                id: [], // id is not needed herein 
+                checksum: 3753325726,
+                offset: 0,
+                comp_length: 176,
+                uncomp_length: 156
+            );
+
+            var destination = new byte[ chunk.UncompressedLength ];
+            var writtenLength = DepotChunk.Process( chunk, chunkData, destination, [
+                0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08,
+                0x09, 0x0A, 0x0B, 0x0C, 0x0D, 0x0E, 0x0F, 0x10,
+                0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17, 0x18,
+                0x19, 0x1A, 0x1B, 0x1C, 0x1D, 0x1E, 0x1F, 0x20
+            ] );
+
+            Assert.Equal( chunk.CompressedLength, ( uint )chunkData.Length );
+            Assert.Equal( chunk.UncompressedLength, ( uint )writtenLength );
+
+            var hash = Convert.ToHexString( SHA1.HashData( destination ) );
+            Assert.Equal( "9E72678E305540630A665B93E1463BC3983EB55A", hash );
+        }
     }
 }

SteamKit2/Tests/Files/depot_3441461_chunk_9e72678e305540630a665b93e1463bc3983eb55a.bin

@@ -0,0 +1 @@
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