lz4.go

// Package lz4 implements reading and writing lz4 compressed data (a frame),
// as specified in http://fastcompression.blogspot.fr/2013/04/lz4-streaming-format-final.html.
//
// Although the block level compression and decompression functions are exposed and are fully compatible
// with the lz4 block format definition, they are low level and should not be used directly.
// For a complete description of an lz4 compressed block, see:
// http://fastcompression.blogspot.fr/2011/05/lz4-explained.html
//
// See https://github.com/Cyan4973/lz4 for the reference C implementation.
//
package lz4

const (
	// Extension is the LZ4 frame file name extension
	Extension = ".lz4"
	// Version is the LZ4 frame format version
	Version = 1

	frameMagic     uint32 = 0x184D2204
	frameSkipMagic uint32 = 0x184D2A50

	// The following constants are used to setup the compression algorithm.
	minMatch            = 4  // the minimum size of the match sequence size (4 bytes)
	winSizeLog          = 16 // LZ4 64Kb window size limit
	winSize             = 1 << winSizeLog
	winMask             = winSize - 1 // 64Kb window of previous data for dependent blocks
	compressedBlockFlag = 1 << 31
	compressedBlockMask = compressedBlockFlag - 1

	// hashLog determines the size of the hash table used to quickly find a previous match position.
	// Its value influences the compression speed and memory usage, the lower the faster,
	// but at the expense of the compression ratio.
	// 16 seems to be the best compromise.
	hashLog       = 16
	hashTableSize = 1 << hashLog
	hashShift     = uint((minMatch * 8) - hashLog)

	mfLimit      = 8 + minMatch // The last match cannot start within the last 12 bytes.
	skipStrength = 6            // variable step for fast scan
)

// map the block max size id with its value in bytes: 64Kb, 256Kb, 1Mb and 4Mb.
var (
	bsMapID    = map[byte]int{4: 64 << 10, 5: 256 << 10, 6: 1 << 20, 7: 4 << 20}
	bsMapValue = make(map[int]byte, len(bsMapID))
)

// Reversed.
func init() {
	for i, v := range bsMapID {
		bsMapValue[v] = i
	}
}

// Header describes the various flags that can be set on a Writer or obtained from a Reader.
// The default values match those of the LZ4 frame format definition
// (http://fastcompression.blogspot.com/2013/04/lz4-streaming-format-final.html).
//
// NB. in a Reader, in case of concatenated frames, the Header values may change between Read() calls.
// It is the caller responsibility to check them if necessary.
type Header struct {
	BlockChecksum    bool   // Compressed blocks checksum flag.
	NoChecksum       bool   // Frame checksum flag.
	BlockMaxSize     int    // Size of the uncompressed data block (one of [64KB, 256KB, 1MB, 4MB]). Default=4MB.
	Size             uint64 // Frame total size. It is _not_ computed by the Writer.
	CompressionLevel int    // Compression level (higher is better, use 0 for fastest compression).
	done             bool   // Header processed flag (Read or Write and checked).
}