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.
20 package snappy
21 22 /*
23 Each encoded block begins with the varint-encoded length of the decoded data,
24 followed by a sequence of chunks. Chunks begin and end on byte boundaries. The
25 first byte of each chunk is broken into its 2 least and 6 most significant bits
26 called l and m: l ranges in [0, 4) and m ranges in [0, 64). l is the chunk tag.
27 Zero means a literal tag. All other values mean a copy tag.
28 29 For literal tags:
30 - If m < 60, the next 1 + m bytes are literal bytes.
31 - Otherwise, let n be the little-endian unsigned integer denoted by the next
32 m - 59 bytes. The next 1 + n bytes after that are literal bytes.
33 34 For copy tags, length bytes are copied from offset bytes ago, in the style of
35 Lempel-Ziv compression algorithms. In particular:
36 - For l == 1, the offset ranges in [0, 1<<11) and the length in [4, 12).
37 The length is 4 + the low 3 bits of m. The high 3 bits of m form bits 8-10
38 of the offset. The next byte is bits 0-7 of the offset.
39 - For l == 2, the offset ranges in [0, 1<<16) and the length in [1, 65).
40 The length is 1 + m. The offset is the little-endian unsigned integer
41 denoted by the next 2 bytes.
42 - For l == 3, this tag is a legacy format that is no longer issued by most
43 encoders. Nonetheless, the offset ranges in [0, 1<<32) and the length in
44 [1, 65). The length is 1 + m. The offset is the little-endian unsigned
45 integer denoted by the next 4 bytes.
46 */
47