cubefs

1
// Copyright 2011 The Snappy-Go Authors. All rights reserved.
2
// Use of this source code is governed by a BSD-style
3
// license that can be found in the LICENSE file.
4

5
// Package snapref implements the Snappy compression format. It aims for very
6
// high speeds and reasonable compression.
7
//
8
// There are actually two Snappy formats: block and stream. They are related,
9
// but different: trying to decompress block-compressed data as a Snappy stream
10
// will fail, and vice versa. The block format is the Decode and Encode
11
// functions and the stream format is the Reader and Writer types.
12
//
13
// The block format, the more common case, is used when the complete size (the
14
// number of bytes) of the original data is known upfront, at the time
15
// compression starts. The stream format, also known as the framing format, is
16
// for when that isn't always true.
17
//
18
// The canonical, C++ implementation is at https://github.com/google/snappy and
19
// it only implements the block format.
20
package snapref
21

22
import (
23
	"hash/crc32"
24
)
25

26
/*
27
Each encoded block begins with the varint-encoded length of the decoded data,
28
followed by a sequence of chunks. Chunks begin and end on byte boundaries. The
29
first byte of each chunk is broken into its 2 least and 6 most significant bits
30
called l and m: l ranges in [0, 4) and m ranges in [0, 64). l is the chunk tag.
31
Zero means a literal tag. All other values mean a copy tag.
32

33
For literal tags:
34
  - If m < 60, the next 1 + m bytes are literal bytes.
35
  - Otherwise, let n be the little-endian unsigned integer denoted by the next
36
    m - 59 bytes. The next 1 + n bytes after that are literal bytes.
37

38
For copy tags, length bytes are copied from offset bytes ago, in the style of
39
Lempel-Ziv compression algorithms. In particular:
40
  - For l == 1, the offset ranges in [0, 1<<11) and the length in [4, 12).
41
    The length is 4 + the low 3 bits of m. The high 3 bits of m form bits 8-10
42
    of the offset. The next byte is bits 0-7 of the offset.
43
  - For l == 2, the offset ranges in [0, 1<<16) and the length in [1, 65).
44
    The length is 1 + m. The offset is the little-endian unsigned integer
45
    denoted by the next 2 bytes.
46
  - For l == 3, this tag is a legacy format that is no longer issued by most
47
    encoders. Nonetheless, the offset ranges in [0, 1<<32) and the length in
48
    [1, 65). The length is 1 + m. The offset is the little-endian unsigned
49
    integer denoted by the next 4 bytes.
50
*/
51
const (
52
	tagLiteral = 0x00
53
	tagCopy1   = 0x01
54
	tagCopy2   = 0x02
55
	tagCopy4   = 0x03
56
)
57

58
const (
59
	checksumSize    = 4
60
	chunkHeaderSize = 4
61
	magicChunk      = "\xff\x06\x00\x00" + magicBody
62
	magicBody       = "sNaPpY"
63

64
	// maxBlockSize is the maximum size of the input to encodeBlock. It is not
65
	// part of the wire format per se, but some parts of the encoder assume
66
	// that an offset fits into a uint16.
67
	//
68
	// Also, for the framing format (Writer type instead of Encode function),
69
	// https://github.com/google/snappy/blob/master/framing_format.txt says
70
	// that "the uncompressed data in a chunk must be no longer than 65536
71
	// bytes".
72
	maxBlockSize = 65536
73

74
	// maxEncodedLenOfMaxBlockSize equals MaxEncodedLen(maxBlockSize), but is
75
	// hard coded to be a const instead of a variable, so that obufLen can also
76
	// be a const. Their equivalence is confirmed by
77
	// TestMaxEncodedLenOfMaxBlockSize.
78
	maxEncodedLenOfMaxBlockSize = 76490
79

80
	obufHeaderLen = len(magicChunk) + checksumSize + chunkHeaderSize
81
	obufLen       = obufHeaderLen + maxEncodedLenOfMaxBlockSize
82
)
83

84
const (
85
	chunkTypeCompressedData   = 0x00
86
	chunkTypeUncompressedData = 0x01
87
	chunkTypePadding          = 0xfe
88
	chunkTypeStreamIdentifier = 0xff
89
)
90

91
var crcTable = crc32.MakeTable(crc32.Castagnoli)
92

93
// crc implements the checksum specified in section 3 of
94
// https://github.com/google/snappy/blob/master/framing_format.txt
95
func crc(b []byte) uint32 {
96
	c := crc32.Update(0, crcTable, b)
97
	return uint32(c>>15|c<<17) + 0xa282ead8
98
}
99