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|
package flate
import (
"encoding/binary"
"fmt"
"math/bits"
)
// fastGen maintains the table for matches,
// and the previous byte block for level 2.
// This is the generic implementation.
type fastEncL1 struct {
fastGen
table [tableSize]tableEntry
}
// EncodeL1 uses a similar algorithm to level 1
func (e *fastEncL1) Encode(dst *tokens, src []byte) {
const (
inputMargin = 12 - 1
minNonLiteralBlockSize = 1 + 1 + inputMargin
)
if debugDeflate && e.cur < 0 {
panic(fmt.Sprint("e.cur < 0: ", e.cur))
}
// Protect against e.cur wraparound.
for e.cur >= bufferReset {
if len(e.hist) == 0 {
for i := range e.table[:] {
e.table[i] = tableEntry{}
}
e.cur = maxMatchOffset
break
}
// Shift down everything in the table that isn't already too far away.
minOff := e.cur + int32(len(e.hist)) - maxMatchOffset
for i := range e.table[:] {
v := e.table[i].offset
if v <= minOff {
v = 0
} else {
v = v - e.cur + maxMatchOffset
}
e.table[i].offset = v
}
e.cur = maxMatchOffset
}
s := e.addBlock(src)
// This check isn't in the Snappy implementation, but there, the caller
// instead of the callee handles this case.
if len(src) < minNonLiteralBlockSize {
// We do not fill the token table.
// This will be picked up by caller.
dst.n = uint16(len(src))
return
}
// Override src
src = e.hist
nextEmit := s
// 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.
cv := load3232(src, s)
for {
const skipLog = 5
const doEvery = 2
nextS := s
var candidate tableEntry
for {
nextHash := hash(cv)
candidate = e.table[nextHash]
nextS = s + doEvery + (s-nextEmit)>>skipLog
if nextS > sLimit {
goto emitRemainder
}
now := load6432(src, nextS)
e.table[nextHash] = tableEntry{offset: s + e.cur}
nextHash = hash(uint32(now))
offset := s - (candidate.offset - e.cur)
if offset < maxMatchOffset && cv == load3232(src, candidate.offset-e.cur) {
e.table[nextHash] = tableEntry{offset: nextS + e.cur}
break
}
// Do one right away...
cv = uint32(now)
s = nextS
nextS++
candidate = e.table[nextHash]
now >>= 8
e.table[nextHash] = tableEntry{offset: s + e.cur}
offset = s - (candidate.offset - e.cur)
if offset < maxMatchOffset && cv == load3232(src, candidate.offset-e.cur) {
e.table[nextHash] = tableEntry{offset: nextS + e.cur}
break
}
cv = uint32(now)
s = nextS
}
// 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.
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.
t := candidate.offset - e.cur
var l = int32(4)
if false {
l = e.matchlenLong(s+4, t+4, src) + 4
} else {
// inlined:
a := src[s+4:]
b := src[t+4:]
for len(a) >= 8 {
if diff := binary.LittleEndian.Uint64(a) ^ binary.LittleEndian.Uint64(b); diff != 0 {
l += int32(bits.TrailingZeros64(diff) >> 3)
break
}
l += 8
a = a[8:]
b = b[8:]
}
if len(a) < 8 {
b = b[:len(a)]
for i := range a {
if a[i] != b[i] {
break
}
l++
}
}
}
// Extend backwards
for t > 0 && s > nextEmit && src[t-1] == src[s-1] {
s--
t--
l++
}
if nextEmit < s {
emitLiteral(dst, src[nextEmit:s])
}
// Save the match found
if false {
dst.AddMatchLong(l, uint32(s-t-baseMatchOffset))
} else {
// Inlined...
xoffset := uint32(s - t - baseMatchOffset)
xlength := l
oc := offsetCode(xoffset)
xoffset |= oc << 16
for xlength > 0 {
xl := xlength
if xl > 258 {
// We need to have at least baseMatchLength left over for next loop.
xl = 258 - baseMatchLength
}
xlength -= xl
xl -= baseMatchLength
dst.extraHist[lengthCodes1[uint8(xl)]]++
dst.offHist[oc]++
dst.tokens[dst.n] = token(matchType | uint32(xl)<<lengthShift | xoffset)
dst.n++
}
}
s += l
nextEmit = s
if nextS >= s {
s = nextS + 1
}
if s >= sLimit {
// Index first pair after match end.
if int(s+l+4) < len(src) {
cv := load3232(src, s)
e.table[hash(cv)] = tableEntry{offset: s + e.cur}
}
goto emitRemainder
}
// We could immediately start working at s now, but to improve
// compression we first update the hash table at s-2 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 := load6432(src, s-2)
o := e.cur + s - 2
prevHash := hash(uint32(x))
e.table[prevHash] = tableEntry{offset: o}
x >>= 16
currHash := hash(uint32(x))
candidate = e.table[currHash]
e.table[currHash] = tableEntry{offset: o + 2}
offset := s - (candidate.offset - e.cur)
if offset > maxMatchOffset || uint32(x) != load3232(src, candidate.offset-e.cur) {
cv = uint32(x >> 8)
s++
break
}
}
}
emitRemainder:
if int(nextEmit) < len(src) {
// If nothing was added, don't encode literals.
if dst.n == 0 {
return
}
emitLiteral(dst, src[nextEmit:])
}
}
|
