sbox-public/engine/Sandbox.Engine/Systems/Networking/System/Channel/Connection.Wire.cs

using Sandbox.Compression;
using Sandbox.Network;
using System.Buffers.Binary;
using System.IO;

namespace Sandbox;

public abstract partial class Connection
{
	// Transport-level wire flags — the first byte of every packet sent or received.
	internal const byte FlagRaw = 0; // [FlagRaw:1][data:N]               — no origLen needed
	internal const byte FlagCompressed = 1; // [FlagCompressed:1][origLen:4][lz4:N]
	internal const byte FlagChunk = 2; // [FlagChunk:1][index:4][total:4][data:N]

	private const int MinimumCompressionByteCount = 128;

	/// <summary>
	/// Upper bound on decompressed payload size. Any <see cref="FlagCompressed"/> packet claiming a
	/// larger original length is treated as corrupt/malicious and rejected.
	/// </summary>
	private const int MaxDecompressedByteSize = 256 * 1024 * 1024;

	// Static decode buffer — all decoding is synchronous on the network tick thread,
	// so a single shared buffer with grow-to-high-water-mark is safe and zero-GC after warmup.
	static byte[] _decodeBuffer;

	/// <summary>
	/// Encode a <see cref="ByteStream"/> into a wire-ready packet, applying LZ4 when beneficial.
	/// Wire format: <c>[FlagRaw][data]</c> or <c>[FlagCompressed][origLen:4][lz4data]</c>.
	/// Returns a heap-allocated <see cref="byte"/>[] that transports may store beyond the call.
	/// </summary>
	internal static byte[] Encode( ByteStream stream )
	{
		var src = stream.ToSpan();

		if ( src.Length > MinimumCompressionByteCount )
		{
			var compressed = LZ4.CompressBlock( src );

			if ( compressed.Length < src.Length )
			{
				var outputSize = 1 + sizeof( int ) + compressed.Length;
				var output = GC.AllocateUninitializedArray<byte>( outputSize );
				output[0] = FlagCompressed;
				BinaryPrimitives.WriteInt32LittleEndian( output.AsSpan( 1 ), src.Length );
				compressed.CopyTo( output.AsSpan( 1 + sizeof( int ) ) );
				return output;
			}
		}

		var result = GC.AllocateUninitializedArray<byte>( 1 + src.Length );
		result[0] = FlagRaw;
		src.CopyTo( result.AsSpan( 1 ) );
		return result;
	}

	/// <summary>
	/// Decode a wire payload (<see cref="FlagRaw"/> or <see cref="FlagCompressed"/>).
	/// <see cref="FlagChunk"/> packets must be fully reassembled by <see cref="OnRawPacketReceived"/>
	/// before reaching this method.
	/// Returns a span into either the source data (raw) or the static <see cref="_decodeBuffer"/> (compressed).
	/// The span is only valid until the next call to <see cref="Decode"/>.
	/// </summary>
	internal static ReadOnlySpan<byte> Decode( ReadOnlySpan<byte> data )
	{
		if ( data.Length < 1 )
			return ReadOnlySpan<byte>.Empty;

		switch ( data[0] )
		{
			case FlagRaw:
				return data.Slice( 1 );
			case FlagCompressed:
				{
					const int headerSize = 1 + sizeof( int ); // flag + origLen
					if ( data.Length < headerSize )
						throw new InvalidDataException( $"Compressed packet too short ({data.Length}b, need {headerSize}b)" );

					var origLen = BinaryPrimitives.ReadInt32LittleEndian( data.Slice( 1, sizeof( int ) ) );
					if ( origLen <= 0 || origLen > MaxDecompressedByteSize )
						throw new InvalidDataException( $"Compressed origLen {origLen} out of range (1..{MaxDecompressedByteSize})" );

					// Grow to high-water mark; never returned — zero GC after warmup.
					if ( _decodeBuffer == null || _decodeBuffer.Length < origLen )
						_decodeBuffer = GC.AllocateUninitializedArray<byte>( origLen );

					var lz4Data = data.Slice( headerSize );
					int written = LZ4.DecompressBlock( lz4Data, _decodeBuffer );

					if ( written != origLen )
						throw new InvalidDataException( $"LZ4 decompressed {written}b but header claimed {origLen}b" );

					Networking.TryRecordMessage( _decodeBuffer.AsSpan( 0, origLen ) );
					return _decodeBuffer.AsSpan( 0, origLen );
				}
			default:
				throw new InvalidOperationException( $"Unknown wire flag {data[0]}" );
		}
	}

	/// <summary>
	/// Build a transport-level chunk packet: <c>[FlagChunk:1][index:4][total:4][data:N]</c>.
	/// Chunks carry raw slices of an already-encoded (possibly compressed) payload.
	/// Chunk header is 9 bytes vs the old 14-byte envelope.
	/// </summary>
	internal static byte[] BuildChunkPacket( byte[] encoded, int offset, int length, int chunkIndex, int totalChunks )
	{
		var result = GC.AllocateUninitializedArray<byte>( 1 + sizeof( uint ) + sizeof( uint ) + length ); // 9-byte header
		result[0] = FlagChunk;
		BinaryPrimitives.WriteUInt32LittleEndian( result.AsSpan( 1 ), (uint)chunkIndex );
		BinaryPrimitives.WriteUInt32LittleEndian( result.AsSpan( 5 ), (uint)totalChunks );
		encoded.AsSpan( offset, length ).CopyTo( result.AsSpan( 9 ) );
		return result;
	}
}