well-goknown/vendor/github.com/klauspost/compress/zstd/enc_best.go

561 lines
16 KiB
Go
Raw Permalink Normal View History

2024-09-26 01:59:44 +00:00
// Copyright 2019+ Klaus Post. All rights reserved.
// License information can be found in the LICENSE file.
// Based on work by Yann Collet, released under BSD License.
package zstd
import (
"bytes"
"fmt"
"github.com/klauspost/compress"
)
const (
bestLongTableBits = 22 // Bits used in the long match table
bestLongTableSize = 1 << bestLongTableBits // Size of the table
bestLongLen = 8 // Bytes used for table hash
// Note: Increasing the short table bits or making the hash shorter
// can actually lead to compression degradation since it will 'steal' more from the
// long match table and match offsets are quite big.
// This greatly depends on the type of input.
bestShortTableBits = 18 // Bits used in the short match table
bestShortTableSize = 1 << bestShortTableBits // Size of the table
bestShortLen = 4 // Bytes used for table hash
)
type match struct {
offset int32
s int32
length int32
rep int32
est int32
}
const highScore = maxMatchLen * 8
// estBits will estimate output bits from predefined tables.
func (m *match) estBits(bitsPerByte int32) {
mlc := mlCode(uint32(m.length - zstdMinMatch))
var ofc uint8
if m.rep < 0 {
ofc = ofCode(uint32(m.s-m.offset) + 3)
} else {
ofc = ofCode(uint32(m.rep) & 3)
}
// Cost, excluding
ofTT, mlTT := fsePredefEnc[tableOffsets].ct.symbolTT[ofc], fsePredefEnc[tableMatchLengths].ct.symbolTT[mlc]
// Add cost of match encoding...
m.est = int32(ofTT.outBits + mlTT.outBits)
m.est += int32(ofTT.deltaNbBits>>16 + mlTT.deltaNbBits>>16)
// Subtract savings compared to literal encoding...
m.est -= (m.length * bitsPerByte) >> 10
if m.est > 0 {
// Unlikely gain..
m.length = 0
m.est = highScore
}
}
// bestFastEncoder uses 2 tables, one for short matches (5 bytes) and one for long matches.
// The long match table contains the previous entry with the same hash,
// effectively making it a "chain" of length 2.
// When we find a long match we choose between the two values and select the longest.
// When we find a short match, after checking the long, we check if we can find a long at n+1
// and that it is longer (lazy matching).
type bestFastEncoder struct {
fastBase
table [bestShortTableSize]prevEntry
longTable [bestLongTableSize]prevEntry
dictTable []prevEntry
dictLongTable []prevEntry
}
// Encode improves compression...
func (e *bestFastEncoder) Encode(blk *blockEnc, src []byte) {
const (
// Input margin is the number of bytes we read (8)
// and the maximum we will read ahead (2)
inputMargin = 8 + 4
minNonLiteralBlockSize = 16
)
// Protect against e.cur wraparound.
for e.cur >= e.bufferReset-int32(len(e.hist)) {
if len(e.hist) == 0 {
e.table = [bestShortTableSize]prevEntry{}
e.longTable = [bestLongTableSize]prevEntry{}
e.cur = e.maxMatchOff
break
}
// Shift down everything in the table that isn't already too far away.
minOff := e.cur + int32(len(e.hist)) - e.maxMatchOff
for i := range e.table[:] {
v := e.table[i].offset
v2 := e.table[i].prev
if v < minOff {
v = 0
v2 = 0
} else {
v = v - e.cur + e.maxMatchOff
if v2 < minOff {
v2 = 0
} else {
v2 = v2 - e.cur + e.maxMatchOff
}
}
e.table[i] = prevEntry{
offset: v,
prev: v2,
}
}
for i := range e.longTable[:] {
v := e.longTable[i].offset
v2 := e.longTable[i].prev
if v < minOff {
v = 0
v2 = 0
} else {
v = v - e.cur + e.maxMatchOff
if v2 < minOff {
v2 = 0
} else {
v2 = v2 - e.cur + e.maxMatchOff
}
}
e.longTable[i] = prevEntry{
offset: v,
prev: v2,
}
}
e.cur = e.maxMatchOff
break
}
// Add block to history
s := e.addBlock(src)
blk.size = len(src)
// Check RLE first
if len(src) > zstdMinMatch {
ml := matchLen(src[1:], src)
if ml == len(src)-1 {
blk.literals = append(blk.literals, src[0])
blk.sequences = append(blk.sequences, seq{litLen: 1, matchLen: uint32(len(src)-1) - zstdMinMatch, offset: 1 + 3})
return
}
}
if len(src) < minNonLiteralBlockSize {
blk.extraLits = len(src)
blk.literals = blk.literals[:len(src)]
copy(blk.literals, src)
return
}
// Use this to estimate literal cost.
// Scaled by 10 bits.
bitsPerByte := int32((compress.ShannonEntropyBits(src) * 1024) / len(src))
// Huffman can never go < 1 bit/byte
if bitsPerByte < 1024 {
bitsPerByte = 1024
}
// Override src
src = e.hist
sLimit := int32(len(src)) - inputMargin
const kSearchStrength = 10
// nextEmit is where in src the next emitLiteral should start from.
nextEmit := s
// Relative offsets
offset1 := int32(blk.recentOffsets[0])
offset2 := int32(blk.recentOffsets[1])
offset3 := int32(blk.recentOffsets[2])
addLiterals := func(s *seq, until int32) {
if until == nextEmit {
return
}
blk.literals = append(blk.literals, src[nextEmit:until]...)
s.litLen = uint32(until - nextEmit)
}
if debugEncoder {
println("recent offsets:", blk.recentOffsets)
}
encodeLoop:
for {
// We allow the encoder to optionally turn off repeat offsets across blocks
canRepeat := len(blk.sequences) > 2
if debugAsserts && canRepeat && offset1 == 0 {
panic("offset0 was 0")
}
const goodEnough = 250
cv := load6432(src, s)
nextHashL := hashLen(cv, bestLongTableBits, bestLongLen)
nextHashS := hashLen(cv, bestShortTableBits, bestShortLen)
candidateL := e.longTable[nextHashL]
candidateS := e.table[nextHashS]
// Set m to a match at offset if it looks like that will improve compression.
improve := func(m *match, offset int32, s int32, first uint32, rep int32) {
delta := s - offset
if delta >= e.maxMatchOff || delta <= 0 || load3232(src, offset) != first {
return
}
// Try to quick reject if we already have a long match.
if m.length > 16 {
left := len(src) - int(m.s+m.length)
// If we are too close to the end, keep as is.
if left <= 0 {
return
}
checkLen := m.length - (s - m.s) - 8
if left > 2 && checkLen > 4 {
// Check 4 bytes, 4 bytes from the end of the current match.
a := load3232(src, offset+checkLen)
b := load3232(src, s+checkLen)
if a != b {
return
}
}
}
l := 4 + e.matchlen(s+4, offset+4, src)
if m.rep <= 0 {
// Extend candidate match backwards as far as possible.
// Do not extend repeats as we can assume they are optimal
// and offsets change if s == nextEmit.
tMin := s - e.maxMatchOff
if tMin < 0 {
tMin = 0
}
for offset > tMin && s > nextEmit && src[offset-1] == src[s-1] && l < maxMatchLength {
s--
offset--
l++
}
}
if debugAsserts {
if offset >= s {
panic(fmt.Sprintf("offset: %d - s:%d - rep: %d - cur :%d - max: %d", offset, s, rep, e.cur, e.maxMatchOff))
}
if !bytes.Equal(src[s:s+l], src[offset:offset+l]) {
panic(fmt.Sprintf("second match mismatch: %v != %v, first: %08x", src[s:s+4], src[offset:offset+4], first))
}
}
cand := match{offset: offset, s: s, length: l, rep: rep}
cand.estBits(bitsPerByte)
if m.est >= highScore || cand.est-m.est+(cand.s-m.s)*bitsPerByte>>10 < 0 {
*m = cand
}
}
best := match{s: s, est: highScore}
improve(&best, candidateL.offset-e.cur, s, uint32(cv), -1)
improve(&best, candidateL.prev-e.cur, s, uint32(cv), -1)
improve(&best, candidateS.offset-e.cur, s, uint32(cv), -1)
improve(&best, candidateS.prev-e.cur, s, uint32(cv), -1)
if canRepeat && best.length < goodEnough {
if s == nextEmit {
// Check repeats straight after a match.
improve(&best, s-offset2, s, uint32(cv), 1|4)
improve(&best, s-offset3, s, uint32(cv), 2|4)
if offset1 > 1 {
improve(&best, s-(offset1-1), s, uint32(cv), 3|4)
}
}
// If either no match or a non-repeat match, check at + 1
if best.rep <= 0 {
cv32 := uint32(cv >> 8)
spp := s + 1
improve(&best, spp-offset1, spp, cv32, 1)
improve(&best, spp-offset2, spp, cv32, 2)
improve(&best, spp-offset3, spp, cv32, 3)
if best.rep < 0 {
cv32 = uint32(cv >> 24)
spp += 2
improve(&best, spp-offset1, spp, cv32, 1)
improve(&best, spp-offset2, spp, cv32, 2)
improve(&best, spp-offset3, spp, cv32, 3)
}
}
}
// Load next and check...
e.longTable[nextHashL] = prevEntry{offset: s + e.cur, prev: candidateL.offset}
e.table[nextHashS] = prevEntry{offset: s + e.cur, prev: candidateS.offset}
index0 := s + 1
// Look far ahead, unless we have a really long match already...
if best.length < goodEnough {
// No match found, move forward on input, no need to check forward...
if best.length < 4 {
s += 1 + (s-nextEmit)>>(kSearchStrength-1)
if s >= sLimit {
break encodeLoop
}
continue
}
candidateS = e.table[hashLen(cv>>8, bestShortTableBits, bestShortLen)]
cv = load6432(src, s+1)
cv2 := load6432(src, s+2)
candidateL = e.longTable[hashLen(cv, bestLongTableBits, bestLongLen)]
candidateL2 := e.longTable[hashLen(cv2, bestLongTableBits, bestLongLen)]
// Short at s+1
improve(&best, candidateS.offset-e.cur, s+1, uint32(cv), -1)
// Long at s+1, s+2
improve(&best, candidateL.offset-e.cur, s+1, uint32(cv), -1)
improve(&best, candidateL.prev-e.cur, s+1, uint32(cv), -1)
improve(&best, candidateL2.offset-e.cur, s+2, uint32(cv2), -1)
improve(&best, candidateL2.prev-e.cur, s+2, uint32(cv2), -1)
if false {
// Short at s+3.
// Too often worse...
improve(&best, e.table[hashLen(cv2>>8, bestShortTableBits, bestShortLen)].offset-e.cur, s+3, uint32(cv2>>8), -1)
}
// Start check at a fixed offset to allow for a few mismatches.
// For this compression level 2 yields the best results.
// We cannot do this if we have already indexed this position.
const skipBeginning = 2
if best.s > s-skipBeginning {
// See if we can find a better match by checking where the current best ends.
// Use that offset to see if we can find a better full match.
if sAt := best.s + best.length; sAt < sLimit {
nextHashL := hashLen(load6432(src, sAt), bestLongTableBits, bestLongLen)
candidateEnd := e.longTable[nextHashL]
if off := candidateEnd.offset - e.cur - best.length + skipBeginning; off >= 0 {
improve(&best, off, best.s+skipBeginning, load3232(src, best.s+skipBeginning), -1)
if off := candidateEnd.prev - e.cur - best.length + skipBeginning; off >= 0 {
improve(&best, off, best.s+skipBeginning, load3232(src, best.s+skipBeginning), -1)
}
}
}
}
}
if debugAsserts {
if best.offset >= best.s {
panic(fmt.Sprintf("best.offset > s: %d >= %d", best.offset, best.s))
}
if best.s < nextEmit {
panic(fmt.Sprintf("s %d < nextEmit %d", best.s, nextEmit))
}
if best.offset < s-e.maxMatchOff {
panic(fmt.Sprintf("best.offset < s-e.maxMatchOff: %d < %d", best.offset, s-e.maxMatchOff))
}
if !bytes.Equal(src[best.s:best.s+best.length], src[best.offset:best.offset+best.length]) {
panic(fmt.Sprintf("match mismatch: %v != %v", src[best.s:best.s+best.length], src[best.offset:best.offset+best.length]))
}
}
// We have a match, we can store the forward value
s = best.s
if best.rep > 0 {
var seq seq
seq.matchLen = uint32(best.length - zstdMinMatch)
addLiterals(&seq, best.s)
// Repeat. If bit 4 is set, this is a non-lit repeat.
seq.offset = uint32(best.rep & 3)
if debugSequences {
println("repeat sequence", seq, "next s:", best.s, "off:", best.s-best.offset)
}
blk.sequences = append(blk.sequences, seq)
// Index old s + 1 -> s - 1
s = best.s + best.length
nextEmit = s
// Index skipped...
end := s
if s > sLimit+4 {
end = sLimit + 4
}
off := index0 + e.cur
for index0 < end {
cv0 := load6432(src, index0)
h0 := hashLen(cv0, bestLongTableBits, bestLongLen)
h1 := hashLen(cv0, bestShortTableBits, bestShortLen)
e.longTable[h0] = prevEntry{offset: off, prev: e.longTable[h0].offset}
e.table[h1] = prevEntry{offset: off, prev: e.table[h1].offset}
off++
index0++
}
switch best.rep {
case 2, 4 | 1:
offset1, offset2 = offset2, offset1
case 3, 4 | 2:
offset1, offset2, offset3 = offset3, offset1, offset2
case 4 | 3:
offset1, offset2, offset3 = offset1-1, offset1, offset2
}
if s >= sLimit {
if debugEncoder {
println("repeat ended", s, best.length)
}
break encodeLoop
}
continue
}
// A 4-byte match has been found. Update recent offsets.
// We'll later see if more than 4 bytes.
t := best.offset
offset1, offset2, offset3 = s-t, offset1, offset2
if debugAsserts && s <= t {
panic(fmt.Sprintf("s (%d) <= t (%d)", s, t))
}
if debugAsserts && int(offset1) > len(src) {
panic("invalid offset")
}
// Write our sequence
var seq seq
l := best.length
seq.litLen = uint32(s - nextEmit)
seq.matchLen = uint32(l - zstdMinMatch)
if seq.litLen > 0 {
blk.literals = append(blk.literals, src[nextEmit:s]...)
}
seq.offset = uint32(s-t) + 3
s += l
if debugSequences {
println("sequence", seq, "next s:", s)
}
blk.sequences = append(blk.sequences, seq)
nextEmit = s
// Index old s + 1 -> s - 1 or sLimit
end := s
if s > sLimit-4 {
end = sLimit - 4
}
off := index0 + e.cur
for index0 < end {
cv0 := load6432(src, index0)
h0 := hashLen(cv0, bestLongTableBits, bestLongLen)
h1 := hashLen(cv0, bestShortTableBits, bestShortLen)
e.longTable[h0] = prevEntry{offset: off, prev: e.longTable[h0].offset}
e.table[h1] = prevEntry{offset: off, prev: e.table[h1].offset}
index0++
off++
}
if s >= sLimit {
break encodeLoop
}
}
if int(nextEmit) < len(src) {
blk.literals = append(blk.literals, src[nextEmit:]...)
blk.extraLits = len(src) - int(nextEmit)
}
blk.recentOffsets[0] = uint32(offset1)
blk.recentOffsets[1] = uint32(offset2)
blk.recentOffsets[2] = uint32(offset3)
if debugEncoder {
println("returning, recent offsets:", blk.recentOffsets, "extra literals:", blk.extraLits)
}
}
// EncodeNoHist will encode a block with no history and no following blocks.
// Most notable difference is that src will not be copied for history and
// we do not need to check for max match length.
func (e *bestFastEncoder) EncodeNoHist(blk *blockEnc, src []byte) {
e.ensureHist(len(src))
e.Encode(blk, src)
}
// Reset will reset and set a dictionary if not nil
func (e *bestFastEncoder) Reset(d *dict, singleBlock bool) {
e.resetBase(d, singleBlock)
if d == nil {
return
}
// Init or copy dict table
if len(e.dictTable) != len(e.table) || d.id != e.lastDictID {
if len(e.dictTable) != len(e.table) {
e.dictTable = make([]prevEntry, len(e.table))
}
end := int32(len(d.content)) - 8 + e.maxMatchOff
for i := e.maxMatchOff; i < end; i += 4 {
const hashLog = bestShortTableBits
cv := load6432(d.content, i-e.maxMatchOff)
nextHash := hashLen(cv, hashLog, bestShortLen) // 0 -> 4
nextHash1 := hashLen(cv>>8, hashLog, bestShortLen) // 1 -> 5
nextHash2 := hashLen(cv>>16, hashLog, bestShortLen) // 2 -> 6
nextHash3 := hashLen(cv>>24, hashLog, bestShortLen) // 3 -> 7
e.dictTable[nextHash] = prevEntry{
prev: e.dictTable[nextHash].offset,
offset: i,
}
e.dictTable[nextHash1] = prevEntry{
prev: e.dictTable[nextHash1].offset,
offset: i + 1,
}
e.dictTable[nextHash2] = prevEntry{
prev: e.dictTable[nextHash2].offset,
offset: i + 2,
}
e.dictTable[nextHash3] = prevEntry{
prev: e.dictTable[nextHash3].offset,
offset: i + 3,
}
}
e.lastDictID = d.id
}
// Init or copy dict table
if len(e.dictLongTable) != len(e.longTable) || d.id != e.lastDictID {
if len(e.dictLongTable) != len(e.longTable) {
e.dictLongTable = make([]prevEntry, len(e.longTable))
}
if len(d.content) >= 8 {
cv := load6432(d.content, 0)
h := hashLen(cv, bestLongTableBits, bestLongLen)
e.dictLongTable[h] = prevEntry{
offset: e.maxMatchOff,
prev: e.dictLongTable[h].offset,
}
end := int32(len(d.content)) - 8 + e.maxMatchOff
off := 8 // First to read
for i := e.maxMatchOff + 1; i < end; i++ {
cv = cv>>8 | (uint64(d.content[off]) << 56)
h := hashLen(cv, bestLongTableBits, bestLongLen)
e.dictLongTable[h] = prevEntry{
offset: i,
prev: e.dictLongTable[h].offset,
}
off++
}
}
e.lastDictID = d.id
}
// Reset table to initial state
copy(e.longTable[:], e.dictLongTable)
e.cur = e.maxMatchOff
// Reset table to initial state
copy(e.table[:], e.dictTable)
}