/usr/share/gocode/src/github.com/golang/snappy/encode.go is in golang-github-golang-snappy-dev 0.0+git20150730.723cc1e-3.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 | // 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
import (
"encoding/binary"
"io"
)
// We limit how far copy back-references can go, the same as the C++ code.
const maxOffset = 1 << 15
// emitLiteral writes a literal chunk and returns the number of bytes written.
func emitLiteral(dst, lit []byte) int {
i, n := 0, uint(len(lit)-1)
switch {
case n < 60:
dst[0] = uint8(n)<<2 | tagLiteral
i = 1
case n < 1<<8:
dst[0] = 60<<2 | tagLiteral
dst[1] = uint8(n)
i = 2
case n < 1<<16:
dst[0] = 61<<2 | tagLiteral
dst[1] = uint8(n)
dst[2] = uint8(n >> 8)
i = 3
case n < 1<<24:
dst[0] = 62<<2 | tagLiteral
dst[1] = uint8(n)
dst[2] = uint8(n >> 8)
dst[3] = uint8(n >> 16)
i = 4
case int64(n) < 1<<32:
dst[0] = 63<<2 | tagLiteral
dst[1] = uint8(n)
dst[2] = uint8(n >> 8)
dst[3] = uint8(n >> 16)
dst[4] = uint8(n >> 24)
i = 5
default:
panic("snappy: source buffer is too long")
}
if copy(dst[i:], lit) != len(lit) {
panic("snappy: destination buffer is too short")
}
return i + len(lit)
}
// emitCopy writes a copy chunk and returns the number of bytes written.
func emitCopy(dst []byte, offset, length int) int {
i := 0
for length > 0 {
x := length - 4
if 0 <= x && x < 1<<3 && offset < 1<<11 {
dst[i+0] = uint8(offset>>8)&0x07<<5 | uint8(x)<<2 | tagCopy1
dst[i+1] = uint8(offset)
i += 2
break
}
x = length
if x > 1<<6 {
x = 1 << 6
}
dst[i+0] = uint8(x-1)<<2 | tagCopy2
dst[i+1] = uint8(offset)
dst[i+2] = uint8(offset >> 8)
i += 3
length -= x
}
return i
}
// Encode returns the encoded form of src. The returned slice may be a sub-
// slice of dst if dst was large enough to hold the entire encoded block.
// Otherwise, a newly allocated slice will be returned.
// It is valid to pass a nil dst.
func Encode(dst, src []byte) []byte {
if n := MaxEncodedLen(len(src)); len(dst) < n {
dst = make([]byte, n)
}
// The block starts with the varint-encoded length of the decompressed bytes.
d := binary.PutUvarint(dst, uint64(len(src)))
// Return early if src is short.
if len(src) <= 4 {
if len(src) != 0 {
d += emitLiteral(dst[d:], src)
}
return dst[:d]
}
// Initialize the hash table. Its size ranges from 1<<8 to 1<<14 inclusive.
const maxTableSize = 1 << 14
shift, tableSize := uint(32-8), 1<<8
for tableSize < maxTableSize && tableSize < len(src) {
shift--
tableSize *= 2
}
var table [maxTableSize]int
// Iterate over the source bytes.
var (
s int // The iterator position.
t int // The last position with the same hash as s.
lit int // The start position of any pending literal bytes.
)
for s+3 < len(src) {
// Update the hash table.
b0, b1, b2, b3 := src[s], src[s+1], src[s+2], src[s+3]
h := uint32(b0) | uint32(b1)<<8 | uint32(b2)<<16 | uint32(b3)<<24
p := &table[(h*0x1e35a7bd)>>shift]
// We need to to store values in [-1, inf) in table. To save
// some initialization time, (re)use the table's zero value
// and shift the values against this zero: add 1 on writes,
// subtract 1 on reads.
t, *p = *p-1, s+1
// If t is invalid or src[s:s+4] differs from src[t:t+4], accumulate a literal byte.
if t < 0 || s-t >= maxOffset || b0 != src[t] || b1 != src[t+1] || b2 != src[t+2] || b3 != src[t+3] {
s++
continue
}
// Otherwise, we have a match. First, emit any pending literal bytes.
if lit != s {
d += emitLiteral(dst[d:], src[lit:s])
}
// Extend the match to be as long as possible.
s0 := s
s, t = s+4, t+4
for s < len(src) && src[s] == src[t] {
s++
t++
}
// Emit the copied bytes.
d += emitCopy(dst[d:], s-t, s-s0)
lit = s
}
// Emit any final pending literal bytes and return.
if lit != len(src) {
d += emitLiteral(dst[d:], src[lit:])
}
return dst[:d]
}
// MaxEncodedLen returns the maximum length of a snappy block, given its
// uncompressed length.
func MaxEncodedLen(srcLen int) int {
// Compressed data can be defined as:
// compressed := item* literal*
// item := literal* copy
//
// The trailing literal sequence has a space blowup of at most 62/60
// since a literal of length 60 needs one tag byte + one extra byte
// for length information.
//
// Item blowup is trickier to measure. Suppose the "copy" op copies
// 4 bytes of data. Because of a special check in the encoding code,
// we produce a 4-byte copy only if the offset is < 65536. Therefore
// the copy op takes 3 bytes to encode, and this type of item leads
// to at most the 62/60 blowup for representing literals.
//
// Suppose the "copy" op copies 5 bytes of data. If the offset is big
// enough, it will take 5 bytes to encode the copy op. Therefore the
// worst case here is a one-byte literal followed by a five-byte copy.
// That is, 6 bytes of input turn into 7 bytes of "compressed" data.
//
// This last factor dominates the blowup, so the final estimate is:
return 32 + srcLen + srcLen/6
}
// NewWriter returns a new Writer that compresses to w, using the framing
// format described at
// https://github.com/google/snappy/blob/master/framing_format.txt
func NewWriter(w io.Writer) *Writer {
return &Writer{
w: w,
enc: make([]byte, MaxEncodedLen(maxUncompressedChunkLen)),
}
}
// Writer is an io.Writer than can write Snappy-compressed bytes.
type Writer struct {
w io.Writer
err error
enc []byte
buf [checksumSize + chunkHeaderSize]byte
wroteHeader bool
}
// Reset discards the writer's state and switches the Snappy writer to write to
// w. This permits reusing a Writer rather than allocating a new one.
func (w *Writer) Reset(writer io.Writer) {
w.w = writer
w.err = nil
w.wroteHeader = false
}
// Write satisfies the io.Writer interface.
func (w *Writer) Write(p []byte) (n int, errRet error) {
if w.err != nil {
return 0, w.err
}
if !w.wroteHeader {
copy(w.enc, magicChunk)
if _, err := w.w.Write(w.enc[:len(magicChunk)]); err != nil {
w.err = err
return n, err
}
w.wroteHeader = true
}
for len(p) > 0 {
var uncompressed []byte
if len(p) > maxUncompressedChunkLen {
uncompressed, p = p[:maxUncompressedChunkLen], p[maxUncompressedChunkLen:]
} else {
uncompressed, p = p, nil
}
checksum := crc(uncompressed)
// Compress the buffer, discarding the result if the improvement
// isn't at least 12.5%.
chunkType := uint8(chunkTypeCompressedData)
chunkBody := Encode(w.enc, uncompressed)
if len(chunkBody) >= len(uncompressed)-len(uncompressed)/8 {
chunkType, chunkBody = chunkTypeUncompressedData, uncompressed
}
chunkLen := 4 + len(chunkBody)
w.buf[0] = chunkType
w.buf[1] = uint8(chunkLen >> 0)
w.buf[2] = uint8(chunkLen >> 8)
w.buf[3] = uint8(chunkLen >> 16)
w.buf[4] = uint8(checksum >> 0)
w.buf[5] = uint8(checksum >> 8)
w.buf[6] = uint8(checksum >> 16)
w.buf[7] = uint8(checksum >> 24)
if _, err := w.w.Write(w.buf[:]); err != nil {
w.err = err
return n, err
}
if _, err := w.w.Write(chunkBody); err != nil {
w.err = err
return n, err
}
n += len(uncompressed)
}
return n, nil
}
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