mirror of https://github.com/status-im/consul.git
69 lines
2.9 KiB
Go
69 lines
2.9 KiB
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).
|
|
}
|