99 lines
3.9 KiB
Go
99 lines
3.9 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 compression format. It aims for very
|
|
// high speeds and reasonable compression.
|
|
//
|
|
// There are actually two Snappy formats: block and stream. They are related,
|
|
// but different: trying to decompress block-compressed data as a Snappy stream
|
|
// will fail, and vice versa. The block format is the Decode and Encode
|
|
// functions and the stream format is the Reader and Writer types.
|
|
//
|
|
// The block format, the more common case, is used when the complete size (the
|
|
// number of bytes) of the original data is known upfront, at the time
|
|
// compression starts. The stream format, also known as the framing format, is
|
|
// for when that isn't always true.
|
|
//
|
|
// The canonical, C++ implementation is at https://github.com/google/snappy and
|
|
// it only implements the block format.
|
|
package snappy // import "github.com/golang/snappy"
|
|
|
|
import (
|
|
"hash/crc32"
|
|
)
|
|
|
|
/*
|
|
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 issued by most
|
|
encoders. Nonetheless, the offset ranges in [0, 1<<32) and the length in
|
|
[1, 65). The length is 1 + m. The offset is the little-endian unsigned
|
|
integer denoted by the next 4 bytes.
|
|
*/
|
|
const (
|
|
tagLiteral = 0x00
|
|
tagCopy1 = 0x01
|
|
tagCopy2 = 0x02
|
|
tagCopy4 = 0x03
|
|
)
|
|
|
|
const (
|
|
checksumSize = 4
|
|
chunkHeaderSize = 4
|
|
magicChunk = "\xff\x06\x00\x00" + magicBody
|
|
magicBody = "sNaPpY"
|
|
|
|
// maxBlockSize is the maximum size of the input to encodeBlock. It is not
|
|
// part of the wire format per se, but some parts of the encoder assume
|
|
// that an offset fits into a uint16.
|
|
//
|
|
// Also, for the framing format (Writer type instead of Encode function),
|
|
// https://github.com/google/snappy/blob/master/framing_format.txt says
|
|
// that "the uncompressed data in a chunk must be no longer than 65536
|
|
// bytes".
|
|
maxBlockSize = 65536
|
|
|
|
// maxEncodedLenOfMaxBlockSize equals MaxEncodedLen(maxBlockSize), but is
|
|
// hard coded to be a const instead of a variable, so that obufLen can also
|
|
// be a const. Their equivalence is confirmed by
|
|
// TestMaxEncodedLenOfMaxBlockSize.
|
|
maxEncodedLenOfMaxBlockSize = 76490
|
|
|
|
obufHeaderLen = len(magicChunk) + checksumSize + chunkHeaderSize
|
|
obufLen = obufHeaderLen + maxEncodedLenOfMaxBlockSize
|
|
)
|
|
|
|
const (
|
|
chunkTypeCompressedData = 0x00
|
|
chunkTypeUncompressedData = 0x01
|
|
chunkTypePadding = 0xfe
|
|
chunkTypeStreamIdentifier = 0xff
|
|
)
|
|
|
|
var crcTable = crc32.MakeTable(crc32.Castagnoli)
|
|
|
|
// crc implements the checksum specified in section 3 of
|
|
// https://github.com/google/snappy/blob/master/framing_format.txt
|
|
func crc(b []byte) uint32 {
|
|
c := crc32.Update(0, crcTable, b)
|
|
return uint32(c>>15|c<<17) + 0xa282ead8
|
|
}
|