sow-tools/internal/nwsync/compressedbuf.go

package nwsync

import (
	"bytes"
	"encoding/binary"
	"fmt"

	"github.com/klauspost/compress/zstd"
)

// NWCompressedBuffer framing, as upstream's neverwinter/compressedbuf.nim
// writes it for NWSync blobs. All fields are little-endian uint32:
//
//	magic "NSYC", version 3, algorithm 2 (zstd), uncompressed size,
//	zstd header version 1, dictionary 0, then the raw zstd frame.
const (
	blobMagic       = 0x4359534E // "NSYC" little-endian
	blobVersion     = 3
	algorithmZstd   = 2
	zstdHeaderVer   = 1
	zstdDictionary  = 0
	blobHeaderBytes = 24
)

var (
	blobEncoder, _ = zstd.NewWriter(nil)
	blobDecoder, _ = zstd.NewReader(nil)
)

// compressBlob wraps data in NWCompressedBuffer framing.
func compressBlob(data []byte) []byte {
	var out bytes.Buffer
	header := []uint32{blobMagic, blobVersion, algorithmZstd, uint32(len(data)), zstdHeaderVer, zstdDictionary}
	for _, field := range header {
		_ = binary.Write(&out, binary.LittleEndian, field)
	}
	out.Write(blobEncoder.EncodeAll(data, nil))
	return out.Bytes()
}

// decompressBlob unwraps NWCompressedBuffer framing. It exists so a blob this
// package wrote — or one upstream wrote — can be compared by its uncompressed
// bytes, which is the only comparison that is meaningful across zstd
// implementations.
func decompressBlob(blob []byte) ([]byte, error) {
	if len(blob) < blobHeaderBytes {
		return nil, fmt.Errorf("blob too small: %d bytes", len(blob))
	}
	header := make([]uint32, 6)
	if err := binary.Read(bytes.NewReader(blob[:blobHeaderBytes]), binary.LittleEndian, header); err != nil {
		return nil, fmt.Errorf("decode blob header: %w", err)
	}
	switch {
	case header[0] != blobMagic:
		return nil, fmt.Errorf("invalid blob magic: %#x", header[0])
	case header[1] != blobVersion:
		return nil, fmt.Errorf("unsupported blob version: %d", header[1])
	case header[2] != algorithmZstd:
		return nil, fmt.Errorf("unsupported compression algorithm: %d", header[2])
	case header[4] != zstdHeaderVer:
		return nil, fmt.Errorf("unsupported zstd header version: %d", header[4])
	case header[5] != zstdDictionary:
		return nil, fmt.Errorf("zstd dictionaries are not supported")
	}
	if header[3] == 0 {
		return nil, nil
	}
	data, err := blobDecoder.DecodeAll(blob[blobHeaderBytes:], nil)
	if err != nil {
		return nil, fmt.Errorf("decompress blob: %w", err)
	}
	if uint32(len(data)) != header[3] {
		return nil, fmt.Errorf("blob size mismatch: header says %d, got %d", header[3], len(data))
	}
	return data, nil
}