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.
5 // Package snappy implements the snappy block-based compression format.
6 // It aims for very high speeds and reasonable compression.
8 // The C++ snappy implementation is at https://github.com/google/snappy
9 package snappy // import "github.com/golang/snappy"
16 Each encoded block begins with the varint-encoded length of the decoded data,
17 followed by a sequence of chunks. Chunks begin and end on byte boundaries. The
18 first byte of each chunk is broken into its 2 least and 6 most significant bits
19 called l and m: l ranges in [0, 4) and m ranges in [0, 64). l is the chunk tag.
20 Zero means a literal tag. All other values mean a copy tag.
23 - If m < 60, the next 1 + m bytes are literal bytes.
24 - Otherwise, let n be the little-endian unsigned integer denoted by the next
25 m - 59 bytes. The next 1 + n bytes after that are literal bytes.
27 For copy tags, length bytes are copied from offset bytes ago, in the style of
28 Lempel-Ziv compression algorithms. In particular:
29 - For l == 1, the offset ranges in [0, 1<<11) and the length in [4, 12).
30 The length is 4 + the low 3 bits of m. The high 3 bits of m form bits 8-10
31 of the offset. The next byte is bits 0-7 of the offset.
32 - For l == 2, the offset ranges in [0, 1<<16) and the length in [1, 65).
33 The length is 1 + m. The offset is the little-endian unsigned integer
34 denoted by the next 2 bytes.
35 - For l == 3, this tag is a legacy format that is no longer issued by most
36 encoders. Nonetheless, the offset ranges in [0, 1<<32) and the length in
37 [1, 65). The length is 1 + m. The offset is the little-endian unsigned
38 integer denoted by the next 4 bytes.
50 magicChunk = "\xff\x06\x00\x00" + magicBody
53 // maxBlockSize is the maximum size of the input to encodeBlock. It is not
54 // part of the wire format per se, but some parts of the encoder assume
55 // that an offset fits into a uint16.
57 // Also, for the framing format (Writer type instead of Encode function),
58 // https://github.com/google/snappy/blob/master/framing_format.txt says
59 // that "the uncompressed data in a chunk must be no longer than 65536
63 // maxEncodedLenOfMaxBlockSize equals MaxEncodedLen(maxBlockSize), but is
64 // hard coded to be a const instead of a variable, so that obufLen can also
65 // be a const. Their equivalence is confirmed by
66 // TestMaxEncodedLenOfMaxBlockSize.
67 maxEncodedLenOfMaxBlockSize = 76490
69 obufHeaderLen = len(magicChunk) + checksumSize + chunkHeaderSize
70 obufLen = obufHeaderLen + maxEncodedLenOfMaxBlockSize
74 chunkTypeCompressedData = 0x00
75 chunkTypeUncompressedData = 0x01
76 chunkTypePadding = 0xfe
77 chunkTypeStreamIdentifier = 0xff
80 var crcTable = crc32.MakeTable(crc32.Castagnoli)
82 // crc implements the checksum specified in section 3 of
83 // https://github.com/google/snappy/blob/master/framing_format.txt
84 func crc(b []byte) uint32 {
85 c := crc32.Update(0, crcTable, b)
86 return uint32(c>>15|c<<17) + 0xa282ead8