mirror of
https://github.com/astaxie/beego.git
synced 2024-12-25 16:20:48 +00:00
39 lines
1.6 KiB
Go
39 lines
1.6 KiB
Go
|
// Copyright 2011 The Snappy-Go Authors. All rights reserved.
|
||
|
|
// Use of this source code is governed by a BSD-style
|
||
|
|
// license that can be found in the LICENSE file.
|
||
|
|
|
||
|
|
// Package snappy implements the snappy block-based compression format.
|
||
|
|
// It aims for very high speeds and reasonable compression.
|
||
|
|
//
|
||
|
|
// The C++ snappy implementation is at http://code.google.com/p/snappy/
|
||
|
|
package snappy
|
||
|
|
|
||
|
|
/*
|
||
|
|
Each encoded block begins with the varint-encoded length of the decoded data,
|
||
|
|
followed by a sequence of chunks. Chunks begin and end on byte boundaries. The
|
||
|
|
first byte of each chunk is broken into its 2 least and 6 most significant bits
|
||
|
|
called l and m: l ranges in [0, 4) and m ranges in [0, 64). l is the chunk tag.
|
||
|
|
Zero means a literal tag. All other values mean a copy tag.
|
||
|
|
|
||
|
|
For literal tags:
|
||
|
|
- If m < 60, the next 1 + m bytes are literal bytes.
|
||
|
|
- Otherwise, let n be the little-endian unsigned integer denoted by the next
|
||
|
|
m - 59 bytes. The next 1 + n bytes after that are literal bytes.
|
||
|
|
|
||
|
|
For copy tags, length bytes are copied from offset bytes ago, in the style of
|
||
|
|
Lempel-Ziv compression algorithms. In particular:
|
||
|
|
- For l == 1, the offset ranges in [0, 1<<11) and the length in [4, 12).
|
||
|
|
The length is 4 + the low 3 bits of m. The high 3 bits of m form bits 8-10
|
||
|
|
of the offset. The next byte is bits 0-7 of the offset.
|
||
|
|
- For l == 2, the offset ranges in [0, 1<<16) and the length in [1, 65).
|
||
|
|
The length is 1 + m. The offset is the little-endian unsigned integer
|
||
|
|
denoted by the next 2 bytes.
|
||
|
|
- For l == 3, this tag is a legacy format that is no longer supported.
|
||
|
|
*/
|
||
|
|
const (
|
||
|
|
tagLiteral = 0x00
|
||
|
|
tagCopy1 = 0x01
|
||
|
|
tagCopy2 = 0x02
|
||
|
|
tagCopy4 = 0x03
|
||
|
|
)
|