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authorLibravatar Rutger Broekhoff2024-01-02 18:56:31 +0100
committerLibravatar Rutger Broekhoff2024-01-02 18:56:31 +0100
commit8db41da676ac8368ef7c2549d56239a5ff5eedde (patch)
tree09c427fd66de2ec1ebffc8342f5fdbb84b0701b5 /vendor/github.com/klauspost/compress/s2/s2.go
parentd4f75fb6db22e57577867445a022227e70958931 (diff)
downloadgitolfs3-8db41da676ac8368ef7c2549d56239a5ff5eedde.tar.gz
gitolfs3-8db41da676ac8368ef7c2549d56239a5ff5eedde.zip
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1// Copyright 2011 The Snappy-Go Authors. All rights reserved.
2// Copyright (c) 2019 Klaus Post. All rights reserved.
3// Use of this source code is governed by a BSD-style
4// license that can be found in the LICENSE file.
5
6// Package s2 implements the S2 compression format.
7//
8// S2 is an extension of Snappy. Similar to Snappy S2 is aimed for high throughput,
9// which is why it features concurrent compression for bigger payloads.
10//
11// Decoding is compatible with Snappy compressed content,
12// but content compressed with S2 cannot be decompressed by Snappy.
13//
14// For more information on Snappy/S2 differences see README in: https://github.com/klauspost/compress/tree/master/s2
15//
16// There are actually two S2 formats: block and stream. They are related,
17// but different: trying to decompress block-compressed data as a S2 stream
18// will fail, and vice versa. The block format is the Decode and Encode
19// functions and the stream format is the Reader and Writer types.
20//
21// A "better" compression option is available. This will trade some compression
22// speed
23//
24// The block format, the more common case, is used when the complete size (the
25// number of bytes) of the original data is known upfront, at the time
26// compression starts. The stream format, also known as the framing format, is
27// for when that isn't always true.
28//
29// Blocks to not offer much data protection, so it is up to you to
30// add data validation of decompressed blocks.
31//
32// Streams perform CRC validation of the decompressed data.
33// Stream compression will also be performed on multiple CPU cores concurrently
34// significantly improving throughput.
35package s2
36
37import (
38 "bytes"
39 "hash/crc32"
40)
41
42/*
43Each encoded block begins with the varint-encoded length of the decoded data,
44followed by a sequence of chunks. Chunks begin and end on byte boundaries. The
45first byte of each chunk is broken into its 2 least and 6 most significant bits
46called l and m: l ranges in [0, 4) and m ranges in [0, 64). l is the chunk tag.
47Zero means a literal tag. All other values mean a copy tag.
48
49For literal tags:
50 - If m < 60, the next 1 + m bytes are literal bytes.
51 - Otherwise, let n be the little-endian unsigned integer denoted by the next
52 m - 59 bytes. The next 1 + n bytes after that are literal bytes.
53
54For copy tags, length bytes are copied from offset bytes ago, in the style of
55Lempel-Ziv compression algorithms. In particular:
56 - For l == 1, the offset ranges in [0, 1<<11) and the length in [4, 12).
57 The length is 4 + the low 3 bits of m. The high 3 bits of m form bits 8-10
58 of the offset. The next byte is bits 0-7 of the offset.
59 - For l == 2, the offset ranges in [0, 1<<16) and the length in [1, 65).
60 The length is 1 + m. The offset is the little-endian unsigned integer
61 denoted by the next 2 bytes.
62 - For l == 3, the offset ranges in [0, 1<<32) and the length in
63 [1, 65). The length is 1 + m. The offset is the little-endian unsigned
64 integer denoted by the next 4 bytes.
65*/
66const (
67 tagLiteral = 0x00
68 tagCopy1 = 0x01
69 tagCopy2 = 0x02
70 tagCopy4 = 0x03
71)
72
73const (
74 checksumSize = 4
75 chunkHeaderSize = 4
76 magicChunk = "\xff\x06\x00\x00" + magicBody
77 magicChunkSnappy = "\xff\x06\x00\x00" + magicBodySnappy
78 magicBodySnappy = "sNaPpY"
79 magicBody = "S2sTwO"
80
81 // maxBlockSize is the maximum size of the input to encodeBlock.
82 //
83 // For the framing format (Writer type instead of Encode function),
84 // this is the maximum uncompressed size of a block.
85 maxBlockSize = 4 << 20
86
87 // minBlockSize is the minimum size of block setting when creating a writer.
88 minBlockSize = 4 << 10
89
90 skippableFrameHeader = 4
91 maxChunkSize = 1<<24 - 1 // 16777215
92
93 // Default block size
94 defaultBlockSize = 1 << 20
95
96 // maxSnappyBlockSize is the maximum snappy block size.
97 maxSnappyBlockSize = 1 << 16
98
99 obufHeaderLen = checksumSize + chunkHeaderSize
100)
101
102const (
103 chunkTypeCompressedData = 0x00
104 chunkTypeUncompressedData = 0x01
105 ChunkTypeIndex = 0x99
106 chunkTypePadding = 0xfe
107 chunkTypeStreamIdentifier = 0xff
108)
109
110var crcTable = crc32.MakeTable(crc32.Castagnoli)
111
112// crc implements the checksum specified in section 3 of
113// https://github.com/google/snappy/blob/master/framing_format.txt
114func crc(b []byte) uint32 {
115 c := crc32.Update(0, crcTable, b)
116 return c>>15 | c<<17 + 0xa282ead8
117}
118
119// literalExtraSize returns the extra size of encoding n literals.
120// n should be >= 0 and <= math.MaxUint32.
121func literalExtraSize(n int64) int64 {
122 if n == 0 {
123 return 0
124 }
125 switch {
126 case n < 60:
127 return 1
128 case n < 1<<8:
129 return 2
130 case n < 1<<16:
131 return 3
132 case n < 1<<24:
133 return 4
134 default:
135 return 5
136 }
137}
138
139type byter interface {
140 Bytes() []byte
141}
142
143var _ byter = &bytes.Buffer{}