mirror of
https://github.com/superseriousbusiness/gotosocial.git
synced 2024-12-15 03:36:37 +00:00
151 lines
4.9 KiB
Go
151 lines
4.9 KiB
Go
// Copyright 2011 The Snappy-Go Authors. All rights reserved.
|
|
// Copyright (c) 2019 Klaus Post. All rights reserved.
|
|
// Use of this source code is governed by a BSD-style
|
|
// license that can be found in the LICENSE file.
|
|
|
|
// Package s2 implements the S2 compression format.
|
|
//
|
|
// S2 is an extension of Snappy. Similar to Snappy S2 is aimed for high throughput,
|
|
// which is why it features concurrent compression for bigger payloads.
|
|
//
|
|
// Decoding is compatible with Snappy compressed content,
|
|
// but content compressed with S2 cannot be decompressed by Snappy.
|
|
//
|
|
// For more information on Snappy/S2 differences see README in: https://github.com/klauspost/compress/tree/master/s2
|
|
//
|
|
// There are actually two S2 formats: block and stream. They are related,
|
|
// but different: trying to decompress block-compressed data as a S2 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.
|
|
//
|
|
// A "better" compression option is available. This will trade some compression
|
|
// speed
|
|
//
|
|
// 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.
|
|
//
|
|
// Blocks to not offer much data protection, so it is up to you to
|
|
// add data validation of decompressed blocks.
|
|
//
|
|
// Streams perform CRC validation of the decompressed data.
|
|
// Stream compression will also be performed on multiple CPU cores concurrently
|
|
// significantly improving throughput.
|
|
package s2
|
|
|
|
import (
|
|
"bytes"
|
|
"hash/crc32"
|
|
|
|
"github.com/klauspost/compress/internal/race"
|
|
)
|
|
|
|
/*
|
|
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, 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
|
|
magicChunkSnappy = "\xff\x06\x00\x00" + magicBodySnappy
|
|
magicBodySnappy = "sNaPpY"
|
|
magicBody = "S2sTwO"
|
|
|
|
// maxBlockSize is the maximum size of the input to encodeBlock.
|
|
//
|
|
// For the framing format (Writer type instead of Encode function),
|
|
// this is the maximum uncompressed size of a block.
|
|
maxBlockSize = 4 << 20
|
|
|
|
// minBlockSize is the minimum size of block setting when creating a writer.
|
|
minBlockSize = 4 << 10
|
|
|
|
skippableFrameHeader = 4
|
|
maxChunkSize = 1<<24 - 1 // 16777215
|
|
|
|
// Default block size
|
|
defaultBlockSize = 1 << 20
|
|
|
|
// maxSnappyBlockSize is the maximum snappy block size.
|
|
maxSnappyBlockSize = 1 << 16
|
|
|
|
obufHeaderLen = checksumSize + chunkHeaderSize
|
|
)
|
|
|
|
const (
|
|
chunkTypeCompressedData = 0x00
|
|
chunkTypeUncompressedData = 0x01
|
|
ChunkTypeIndex = 0x99
|
|
chunkTypePadding = 0xfe
|
|
chunkTypeStreamIdentifier = 0xff
|
|
)
|
|
|
|
var (
|
|
crcTable = crc32.MakeTable(crc32.Castagnoli)
|
|
magicChunkSnappyBytes = []byte(magicChunkSnappy) // Can be passed to functions where it escapes.
|
|
magicChunkBytes = []byte(magicChunk) // Can be passed to functions where it escapes.
|
|
)
|
|
|
|
// crc implements the checksum specified in section 3 of
|
|
// https://github.com/google/snappy/blob/master/framing_format.txt
|
|
func crc(b []byte) uint32 {
|
|
race.ReadSlice(b)
|
|
|
|
c := crc32.Update(0, crcTable, b)
|
|
return c>>15 | c<<17 + 0xa282ead8
|
|
}
|
|
|
|
// literalExtraSize returns the extra size of encoding n literals.
|
|
// n should be >= 0 and <= math.MaxUint32.
|
|
func literalExtraSize(n int64) int64 {
|
|
if n == 0 {
|
|
return 0
|
|
}
|
|
switch {
|
|
case n < 60:
|
|
return 1
|
|
case n < 1<<8:
|
|
return 2
|
|
case n < 1<<16:
|
|
return 3
|
|
case n < 1<<24:
|
|
return 4
|
|
default:
|
|
return 5
|
|
}
|
|
}
|
|
|
|
type byter interface {
|
|
Bytes() []byte
|
|
}
|
|
|
|
var _ byter = &bytes.Buffer{}
|