golang deflatefast 代码
文件路径：/src/compress/flate/deflatefast.go
/ Copyright 2016 The 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 flate
import "math"
/ This encoding algorithm, which prioritizes speed over output size, is
/ based on Snappy's LZ77-style encoder: github.com/golang/snappy
const (
tableBits = 14 / Bits used in the table.
tableSize = 1 << tableBits / Size of the table.
tableMask = tableSize - 1 / Mask for table indices. Redundant, but can eliminate bounds checks.
tableShift = 32 - tableBits / Right-shift to get the tableBits most significant bits of a uint32.
/ Reset the buffer offset when reaching this.
/ Offsets are stored between blocks as int32 values.
/ Since the offset we are checking against is at the beginning
/ of the buffer, we need to subtract the current and input
/ buffer to not risk overflowing the int32.
bufferReset = math.MaxInt32 - maxStoreBlockSize*2
)
func load32(b []byte, i int32) uint32 {
b = b[i : i+4 : len(b)] / Help the compiler eliminate bounds checks on the next line.
return uint32(b[0]) | uint32(b[1])<<8 | uint32(b[2])<<16 | uint32(b[3])<<24
}
func load64(b []byte, i int32) uint64 {
b = b[i : i+8 : len(b)] / Help the compiler eliminate bounds checks on the next line.
return uint64(b[0]) | uint64(b[1])<<8 | uint64(b[2])<<16 | uint64(b[3])<<24 |
uint64(b[4])<<32 | uint64(b[5])<<40 | uint64(b[6])<<48 | uint64(b[7])<<56
}
func hash(u uint32) uint32 {
return (u * 0x1e35a7bd) >> tableShift
}
/ These constants are defined by the Snappy implementation so that its
/ assembly implementation can fast-path some 16-bytes-at-a-time copies. They
/ aren't necessary in the pure Go implementation, as we don't use those same
/ optimizations, but using the same thresholds doesn't really hurt.
const (
inputMargin = 16 - 1
minNonLiteralBlockSize = 1 + 1 + inputMargin
)
type tableEntry struct {
val uint32 / Value at destination
offset int32
}
/ deflateFast maintains the table for matches,
/ and the previous byte block for cross block matching.
type deflateFast struct {
table [tableSize]tableEntry
prev []byte / Previous block, zero length if unknown.
cur int32 / Current match offset.
}
func newDeflateFast() *deflateFast {
return &deflateFast{cur: maxStoreBlockSize, prev: make([]byte, 0, maxStoreBlockSize)}
}
/ encode encodes a block given in src and appends tokens
/ to dst and returns the result.
func (e *deflateFast) encode(dst []token, src []byte) []token {
/ Ensure that e.cur doesn't wrap.
if e.cur >= bufferReset {
e.shiftOffsets()
}
/ This check isn't in the Snappy implementation, but there, the caller
/ instead of the callee handles this case.
if len(src) < minNonLiteralBlockSize {
e.cur += maxStoreBlockSize
e.prev = e.prev[:0]
return emitLiteral(dst, src)
}
/ sLimit is when to stop looking for offset/length copies. The inputMargin
/ lets us use a fast path for emitLiteral in the main loop, while we are
/ looking for copies.
sLimit := int32(len(src) - inputMargin)
/ nextEmit is where in src the next emitLiteral should start from.
nextEmit := int32(0)
s := int32(0)
cv := load32(src, s)
nextHash := hash(cv)
for {
/ Copied from the C++ snappy implementation:
/
/ Heuristic match skipping: If 32 bytes are scanned with no matches
/ found, start looking only at every other byte. If 32 more bytes are
/ scanned (or skipped), look at every third byte, etc.. When a match
/ is found, immediately go back to looking at every byte. This is a
/ small loss (~5% performance, ~0.1% density) for compressible data
/ due to more bookkeeping, but for non-compressible data (such as
/ JPEG) it's a huge win since the compressor quickly "realizes" the
/ data is incompressible and doesn't bother looking for matches
/ everywhere.
/
/ The "skip" variable keeps track of how many bytes there are since
/ the last match; dividing it by 32 (ie. right-shifting by five) gives
/ the number of bytes to move ahead for each iteration.
skip := int32(32)
nextS := s
var candidate tableEntry
for {
s = nextS
bytesBetweenHashLookups := skip >> 5
nextS = s + bytesBetweenHashLookups
skip += bytesBetweenHashLookups
if nextS > sLimit {
goto emitRemainder
}
candidate = e.table[nextHash&tableMask]
now := load32(src, nextS)
e.table[nextHash&tableMask] = tableEntry{offset: s + e.cur, val: cv}
nextHash = hash(now)
offset := s - (candidate.offset - e.cur)
if offset > maxMatchOffset || cv != candidate.val {
/ Out of range or not matched.
cv = now
continue
}
break
}
/ A 4-byte match has been found. We'll later see if more than 4 bytes
/ match. But, prior to the match, src[nextEmit:s] are unmatched. Emit
/ them as literal bytes.
dst = emitLiteral(dst, src[nextEmit:s])
/ Call emitCopy, and then see if another emitCopy could be our next
/ move. Repeat until we find no match for the input immediately after
/ what was consumed by the last emitCopy call.
/
/ If we exit this loop normally then we need to call emitLiteral next,
/ though we don't yet know how big the literal will be. We handle that
/ by proceeding to the next iteration of the main loop. We also can
/ exit this loop via goto if we get close to exhausting the input.
for {
/ Invariant: we have a 4-byte match at s, and no need to emit any
/ literal bytes prior to s.
/ Extend the 4-byte match as long as possible.
/
s += 4
t := candidate.offset - e.cur + 4
l := e.matchLen(s, t, src)
/ matchToken is flate's equivalent of Snappy's emitCopy. (length,offset)
dst = append(dst, matchToken(uint32(l+4-baseMatchLength), uint32(s-t-baseMatchOffset)))
s += l
nextEmit = s
if s >= sLimit {
goto emitRemainder
}
/ We could immediately start working at s now, but to improve
/ compression we first update the hash table at s-1 and at s. If
/ another emitCopy is not our next move, also calculate nextHash
/ at s+1. At least on GOARCH=amd64, these three hash calculations
/ are faster as one load64 call (with some shifts) instead of
/ three load32 calls.
x := load64(src, s-1)
prevHash := hash(uint32(x))
e.table[prevHash&tableMask] = tableEntry{offset: e.cur + s - 1, val: uint32(x)}
x >>= 8
currHash := hash(uint32(x))
candidate = e.table[currHash&tableMask]
e.table[currHash&tableMask] = tableEntry{offset: e.cur + s, val: uint32(x)}
offset := s - (candidate.offset - e.cur)
if offset > maxMatchOffset || uint32(x) != candidate.val {
cv = uint32(x >> 8)
nextHash = hash(cv)
s++
break
}
}
}
emitRemainder:
if int(nextEmit) < len(src) {
dst = emitLiteral(dst, src[nextEmit:])
}
e.cur += int32(len(src))
e.prev = e.prev[:len(src)]
copy(e.prev, src)
return dst
}
func emitLiteral(dst []token, lit []byte) []token {
for _, v := range lit {
dst = append(dst, literalToken(uint32(v)))
}
return dst
}
/ matchLen returns the match length between src[s:] and src[t:].
/ t can be negative to indicate the match is starting in e.prev.
/ We assume that src[s-4:s] and src[t-4:t] already match.
func (e *deflateFast) matchLen(s, t int32, src []byte) int32 {
s1 := int(s) + maxMatchLength - 4
if s1 > len(src) {
s1 = len(src)
}
/ If we are inside the current block
if t >= 0 {
b := src[t:]
a := src[s:s1]
b = b[:len(a)]
/ Extend the match to be as long as possible.
for i := range a {
if a[i] != b[i] {
return int32(i)
}
}
return int32(len(a))
}
/ We found a match in the previous block.
tp := int32(len(e.prev)) + t
if tp < 0 {
return 0
}
/ Extend the match to be as long as possible.
a := src[s:s1]
b := e.prev[tp:]
if len(b) > len(a) {
b = b[:len(a)]
}
a = a[:len(b)]
for i := range b {
if a[i] != b[i] {
return int32(i)
}
}
/ If we reached our limit, we matched everything we are
/ allowed to in the previous block and we return.
n := int32(len(b))
if int(s+n) == s1 {
return n
}
/ Continue looking for more matches in the current block.
a = src[s+n : s1]
b = src[:len(a)]
for i := range a {
if a[i] != b[i] {
return int32(i) + n
}
}
return int32(len(a)) + n
}
/ Reset resets the encoding history.
/ This ensures that no matches are made to the previous block.
func (e *deflateFast) reset() {
e.prev = e.prev[:0]
/ Bump the offset, so all matches will fail distance check.
/ Nothing should be >= e.cur in the table.
e.cur += maxMatchOffset
/ Protect against e.cur wraparound.
if e.cur >= bufferReset {
e.shiftOffsets()
}
}
/ shiftOffsets will shift down all match offset.
/ This is only called in rare situations to prevent integer overflow.
/
/ See https://golang.org/issue/18636 and https://github.com/golang/go/issues/34121.
func (e *deflateFast) shiftOffsets() {
if len(e.prev) == 0 {
/ We have no history; just clear the table.
for i := range e.table[:] {
e.table[i] = tableEntry{}
}
e.cur = maxMatchOffset + 1
return
}
/ Shift down everything in the table that isn't already too far away.
for i := range e.table[:] {
v := e.table[i].offset - e.cur + maxMatchOffset + 1
if v < 0 {
/ We want to reset e.cur to maxMatchOffset + 1, so we need to shift
/ all table entries down by (e.cur - (maxMatchOffset + 1)).
/ Because we ignore matches > maxMatchOffset, we can cap
/ any negative offsets at 0.
v = 0
}
e.table[i].offset = v
}
e.cur = maxMatchOffset + 1
}