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flate: Improve level 7-9 (again) #471

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merged 2 commits into from Jan 18, 2022
Merged

flate: Improve level 7-9 (again) #471

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b20a20b
flate: Improve level 7-9 (again)
klauspost Jan 17, 2022
d5d9778
Remove "extended window"
klauspost Jan 18, 2022
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32 changes: 21 additions & 11 deletions flate/deflate.go
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Expand Up @@ -10,9 +10,6 @@ import (
"fmt"
"io"
"math"
"math/bits"

comp "github.com/klauspost/compress"
)

const (
Expand Down Expand Up @@ -110,6 +107,7 @@ type advancedState struct {
type compressor struct {
compressionLevel

h *huffmanEncoder
w *huffmanBitWriter

// compression algorithm
Expand Down Expand Up @@ -271,7 +269,7 @@ func (d *compressor) fillWindow(b []byte) {
// Try to find a match starting at index whose length is greater than prevSize.
// We only look at chainCount possibilities before giving up.
// pos = s.index, prevHead = s.chainHead-s.hashOffset, prevLength=minMatchLength-1, lookahead
func (d *compressor) findMatch(pos int, prevHead int, lookahead, bpb int) (length, offset int, ok bool) {
func (d *compressor) findMatch(pos int, prevHead int, lookahead int) (length, offset int, ok bool) {
minMatchLook := maxMatchLength
if lookahead < minMatchLook {
minMatchLook = lookahead
Expand All @@ -297,14 +295,19 @@ func (d *compressor) findMatch(pos int, prevHead int, lookahead, bpb int) (lengt
}
offset = 0

// Some like it higher (CSV), some like it lower (JSON)
const baseCost = 6
// Base is 4 bytes at with an additional cost.
// Matches must be better than this.
cGain := minMatchLength*bpb - 12
cGain := 0
for i := prevHead; tries > 0; tries-- {
if wEnd == win[i+length] {
n := matchLen(win[i:i+minMatchLook], wPos)
if n > length {
newGain := n*bpb - bits.Len32(uint32(pos-i))
// Calculate gain. Estimate
newGain := d.h.bitLengthRaw(wPos[:n]) - int(offsetExtraBits[offsetCode(uint32(pos-i))]) - baseCost - int(lengthExtraBits[lengthCodes[(n-3)&255]])

//fmt.Println(n, "gain:", newGain, "prev:", cGain, "raw:", d.h.bitLengthRaw(wPos[:n]))
if newGain > cGain {
length = n
offset = pos - i
Expand Down Expand Up @@ -389,10 +392,17 @@ func (d *compressor) deflateLazy() {
if d.windowEnd-s.index < minMatchLength+maxMatchLength && !d.sync {
return
}
s.estBitsPerByte = 8
if !d.sync {
s.estBitsPerByte = comp.ShannonEntropyBits(d.window[s.index:d.windowEnd])
s.estBitsPerByte = int(1 + float64(s.estBitsPerByte)/float64(d.windowEnd-s.index))
if d.windowEnd != s.index {
// Get literal huffman coder.
if d.h == nil {
d.h = newHuffmanEncoder(maxFlateBlockTokens)
}
var tmp [256]uint16
for _, v := range d.window[s.index:d.windowEnd] {
tmp[v]++
}
d.h.generate(tmp[:], 15)
//s.estBitsPerByte = d.h.bitLength(tmp[:])/(d.windowEnd-s.index) + 1
}

s.maxInsertIndex = d.windowEnd - (minMatchLength - 1)
Expand Down Expand Up @@ -446,7 +456,7 @@ func (d *compressor) deflateLazy() {
}

if s.chainHead-s.hashOffset >= minIndex && lookahead > prevLength && prevLength < d.lazy {
if newLength, newOffset, ok := d.findMatch(s.index, s.chainHead-s.hashOffset, lookahead, s.estBitsPerByte); ok {
if newLength, newOffset, ok := d.findMatch(s.index, s.chainHead-s.hashOffset, lookahead); ok {
s.length = newLength
s.offset = newOffset
}
Expand Down
10 changes: 4 additions & 6 deletions flate/huffman_bit_writer.go
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Expand Up @@ -52,18 +52,18 @@ var lengthBase = [32]uint8{
}

// offset code word extra bits.
var offsetExtraBits = [64]int8{
var offsetExtraBits = [32]int8{
0, 0, 0, 0, 1, 1, 2, 2, 3, 3,
4, 4, 5, 5, 6, 6, 7, 7, 8, 8,
9, 9, 10, 10, 11, 11, 12, 12, 13, 13,
/* extended window */
14, 14, 15, 15, 16, 16, 17, 17, 18, 18, 19, 19, 20, 20,
14, 14,
}

var offsetCombined = [32]uint32{}

func init() {
var offsetBase = [64]uint32{
var offsetBase = [32]uint32{
/* normal deflate */
0x000000, 0x000001, 0x000002, 0x000003, 0x000004,
0x000006, 0x000008, 0x00000c, 0x000010, 0x000018,
Expand All @@ -73,9 +73,7 @@ func init() {
0x001800, 0x002000, 0x003000, 0x004000, 0x006000,

/* extended window */
0x008000, 0x00c000, 0x010000, 0x018000, 0x020000,
0x030000, 0x040000, 0x060000, 0x080000, 0x0c0000,
0x100000, 0x180000, 0x200000, 0x300000,
0x008000, 0x00c000,
}

for i := range offsetCombined[:] {
Expand Down
4 changes: 1 addition & 3 deletions flate/huffman_code.go
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Expand Up @@ -129,9 +129,7 @@ func (h *huffmanEncoder) bitLength(freq []uint16) int {
func (h *huffmanEncoder) bitLengthRaw(b []byte) int {
var total int
for _, f := range b {
if f != 0 {
total += int(h.codes[f].len)
}
total += int(h.codes[f].len)
}
return total
}
Expand Down
19 changes: 8 additions & 11 deletions flate/token.go
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Expand Up @@ -129,11 +129,11 @@ var offsetCodes14 = [256]uint32{
type token uint32

type tokens struct {
nLits int
extraHist [32]uint16 // codes 256->maxnumlit
offHist [32]uint16 // offset codes
litHist [256]uint16 // codes 0->255
n uint16 // Must be able to contain maxStoreBlockSize
nFilled int
n uint16 // Must be able to contain maxStoreBlockSize
tokens [maxStoreBlockSize + 1]token
}

Expand All @@ -142,7 +142,7 @@ func (t *tokens) Reset() {
return
}
t.n = 0
t.nLits = 0
t.nFilled = 0
for i := range t.litHist[:] {
t.litHist[i] = 0
}
Expand All @@ -161,12 +161,12 @@ func (t *tokens) Fill() {
for i, v := range t.litHist[:] {
if v == 0 {
t.litHist[i] = 1
t.nLits++
t.nFilled++
}
}
for i, v := range t.extraHist[:literalCount-256] {
if v == 0 {
t.nLits++
t.nFilled++
t.extraHist[i] = 1
}
}
Expand Down Expand Up @@ -202,14 +202,12 @@ func emitLiteral(dst *tokens, lit []byte) {
dst.litHist[v]++
}
dst.n += uint16(len(lit))
dst.nLits += len(lit)
}

func (t *tokens) AddLiteral(lit byte) {
t.tokens[t.n] = token(lit)
t.litHist[lit]++
t.n++
t.nLits++
}

// from https://stackoverflow.com/a/28730362
Expand All @@ -230,8 +228,9 @@ func (t *tokens) EstimatedBits() int {
shannon := float32(0)
bits := int(0)
nMatches := 0
if t.nLits > 0 {
invTotal := 1.0 / float32(t.nLits)
total := int(t.n) + t.nFilled
if total > 0 {
invTotal := 1.0 / float32(total)
for _, v := range t.litHist[:] {
if v > 0 {
n := float32(v)
Expand Down Expand Up @@ -275,7 +274,6 @@ func (t *tokens) AddMatch(xlength uint32, xoffset uint32) {
}
oCode := offsetCode(xoffset)
xoffset |= oCode << 16
t.nLits++

t.extraHist[lengthCodes1[uint8(xlength)]]++
t.offHist[oCode]++
Expand All @@ -301,7 +299,6 @@ func (t *tokens) AddMatchLong(xlength int32, xoffset uint32) {
}
xlength -= xl
xl -= baseMatchLength
t.nLits++
t.extraHist[lengthCodes1[uint8(xl)]]++
t.offHist[oc]++
t.tokens[t.n] = token(matchType | uint32(xl)<<lengthShift | xoffset)
Expand Down