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 snappy 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.
20package snappy // import "github.com/golang/snappy"
21
22import (
23	"hash/crc32"
24)
25
26/*
27Each encoded block begins with the varint-encoded length of the decoded data,
28followed by a sequence of chunks. Chunks begin and end on byte boundaries. The
29first byte of each chunk is broken into its 2 least and 6 most significant bits
30called l and m: l ranges in [0, 4) and m ranges in [0, 64). l is the chunk tag.
31Zero means a literal tag. All other values mean a copy tag.
32
33For 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
38For copy tags, length bytes are copied from offset bytes ago, in the style of
39Lempel-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*/
51const (
52	tagLiteral = 0x00
53	tagCopy1   = 0x01
54	tagCopy2   = 0x02
55	tagCopy4   = 0x03
56)
57
58const (
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
84const (
85	chunkTypeCompressedData   = 0x00
86	chunkTypeUncompressedData = 0x01
87	chunkTypePadding          = 0xfe
88	chunkTypeStreamIdentifier = 0xff
89)
90
91var 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
95func crc(b []byte) uint32 {
96	c := crc32.Update(0, crcTable, b)
97	return uint32(c>>15|c<<17) + 0xa282ead8
98}
99