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Scott Bakere7144bc2019-10-01 14:16:47 -07001// Package lz4 implements reading and writing lz4 compressed data (a frame),
2// as specified in http://fastcompression.blogspot.fr/2013/04/lz4-streaming-format-final.html.
3//
4// Although the block level compression and decompression functions are exposed and are fully compatible
5// with the lz4 block format definition, they are low level and should not be used directly.
6// For a complete description of an lz4 compressed block, see:
7// http://fastcompression.blogspot.fr/2011/05/lz4-explained.html
8//
9// See https://github.com/Cyan4973/lz4 for the reference C implementation.
10//
11package lz4
12
13const (
14 // Extension is the LZ4 frame file name extension
15 Extension = ".lz4"
16 // Version is the LZ4 frame format version
17 Version = 1
18
19 frameMagic uint32 = 0x184D2204
20 frameSkipMagic uint32 = 0x184D2A50
21
22 // The following constants are used to setup the compression algorithm.
23 minMatch = 4 // the minimum size of the match sequence size (4 bytes)
24 winSizeLog = 16 // LZ4 64Kb window size limit
25 winSize = 1 << winSizeLog
26 winMask = winSize - 1 // 64Kb window of previous data for dependent blocks
27 compressedBlockFlag = 1 << 31
28 compressedBlockMask = compressedBlockFlag - 1
29
30 // hashLog determines the size of the hash table used to quickly find a previous match position.
31 // Its value influences the compression speed and memory usage, the lower the faster,
32 // but at the expense of the compression ratio.
33 // 16 seems to be the best compromise.
34 hashLog = 16
35 hashTableSize = 1 << hashLog
36 hashShift = uint((minMatch * 8) - hashLog)
37
38 mfLimit = 8 + minMatch // The last match cannot start within the last 12 bytes.
39 skipStrength = 6 // variable step for fast scan
40)
41
42// map the block max size id with its value in bytes: 64Kb, 256Kb, 1Mb and 4Mb.
43var (
44 bsMapID = map[byte]int{4: 64 << 10, 5: 256 << 10, 6: 1 << 20, 7: 4 << 20}
45 bsMapValue = make(map[int]byte, len(bsMapID))
46)
47
48// Reversed.
49func init() {
50 for i, v := range bsMapID {
51 bsMapValue[v] = i
52 }
53}
54
55// Header describes the various flags that can be set on a Writer or obtained from a Reader.
56// The default values match those of the LZ4 frame format definition
57// (http://fastcompression.blogspot.com/2013/04/lz4-streaming-format-final.html).
58//
59// NB. in a Reader, in case of concatenated frames, the Header values may change between Read() calls.
60// It is the caller responsibility to check them if necessary.
61type Header struct {
62 BlockChecksum bool // Compressed blocks checksum flag.
63 NoChecksum bool // Frame checksum flag.
64 BlockMaxSize int // Size of the uncompressed data block (one of [64KB, 256KB, 1MB, 4MB]). Default=4MB.
65 Size uint64 // Frame total size. It is _not_ computed by the Writer.
66 CompressionLevel int // Compression level (higher is better, use 0 for fastest compression).
67 done bool // Header processed flag (Read or Write and checked).
68}