mirror of
https://codeberg.org/forgejo/forgejo.git
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486 lines
13 KiB
Go
Vendored
486 lines
13 KiB
Go
Vendored
// Copyright 2019+ Klaus Post. All rights reserved.
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// License information can be found in the LICENSE file.
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// Based on work by Yann Collet, released under BSD License.
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package zstd
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import (
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"fmt"
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"math/bits"
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)
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const (
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bestLongTableBits = 20 // Bits used in the long match table
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bestLongTableSize = 1 << bestLongTableBits // Size of the table
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// Note: Increasing the short table bits or making the hash shorter
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// can actually lead to compression degradation since it will 'steal' more from the
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// long match table and match offsets are quite big.
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// This greatly depends on the type of input.
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bestShortTableBits = 16 // Bits used in the short match table
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bestShortTableSize = 1 << bestShortTableBits // Size of the table
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)
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// bestFastEncoder uses 2 tables, one for short matches (5 bytes) and one for long matches.
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// The long match table contains the previous entry with the same hash,
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// effectively making it a "chain" of length 2.
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// When we find a long match we choose between the two values and select the longest.
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// When we find a short match, after checking the long, we check if we can find a long at n+1
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// and that it is longer (lazy matching).
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type bestFastEncoder struct {
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fastBase
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table [bestShortTableSize]prevEntry
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longTable [bestLongTableSize]prevEntry
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dictTable []prevEntry
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dictLongTable []prevEntry
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}
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// Encode improves compression...
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func (e *bestFastEncoder) Encode(blk *blockEnc, src []byte) {
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const (
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// Input margin is the number of bytes we read (8)
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// and the maximum we will read ahead (2)
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inputMargin = 8 + 4
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minNonLiteralBlockSize = 16
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)
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// Protect against e.cur wraparound.
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for e.cur >= bufferReset {
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if len(e.hist) == 0 {
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for i := range e.table[:] {
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e.table[i] = prevEntry{}
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}
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for i := range e.longTable[:] {
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e.longTable[i] = prevEntry{}
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}
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e.cur = e.maxMatchOff
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break
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}
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// Shift down everything in the table that isn't already too far away.
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minOff := e.cur + int32(len(e.hist)) - e.maxMatchOff
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for i := range e.table[:] {
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v := e.table[i].offset
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v2 := e.table[i].prev
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if v < minOff {
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v = 0
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v2 = 0
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} else {
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v = v - e.cur + e.maxMatchOff
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if v2 < minOff {
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v2 = 0
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} else {
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v2 = v2 - e.cur + e.maxMatchOff
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}
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}
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e.table[i] = prevEntry{
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offset: v,
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prev: v2,
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}
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}
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for i := range e.longTable[:] {
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v := e.longTable[i].offset
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v2 := e.longTable[i].prev
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if v < minOff {
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v = 0
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v2 = 0
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} else {
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v = v - e.cur + e.maxMatchOff
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if v2 < minOff {
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v2 = 0
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} else {
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v2 = v2 - e.cur + e.maxMatchOff
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}
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}
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e.longTable[i] = prevEntry{
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offset: v,
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prev: v2,
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}
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}
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e.cur = e.maxMatchOff
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break
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}
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s := e.addBlock(src)
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blk.size = len(src)
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if len(src) < minNonLiteralBlockSize {
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blk.extraLits = len(src)
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blk.literals = blk.literals[:len(src)]
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copy(blk.literals, src)
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return
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}
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// Override src
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src = e.hist
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sLimit := int32(len(src)) - inputMargin
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const kSearchStrength = 10
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// nextEmit is where in src the next emitLiteral should start from.
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nextEmit := s
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cv := load6432(src, s)
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// Relative offsets
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offset1 := int32(blk.recentOffsets[0])
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offset2 := int32(blk.recentOffsets[1])
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offset3 := int32(blk.recentOffsets[2])
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addLiterals := func(s *seq, until int32) {
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if until == nextEmit {
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return
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}
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blk.literals = append(blk.literals, src[nextEmit:until]...)
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s.litLen = uint32(until - nextEmit)
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}
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_ = addLiterals
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if debug {
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println("recent offsets:", blk.recentOffsets)
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}
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encodeLoop:
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for {
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// We allow the encoder to optionally turn off repeat offsets across blocks
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canRepeat := len(blk.sequences) > 2
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if debugAsserts && canRepeat && offset1 == 0 {
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panic("offset0 was 0")
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}
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type match struct {
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offset int32
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s int32
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length int32
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rep int32
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}
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matchAt := func(offset int32, s int32, first uint32, rep int32) match {
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if s-offset >= e.maxMatchOff || load3232(src, offset) != first {
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return match{offset: offset, s: s}
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}
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return match{offset: offset, s: s, length: 4 + e.matchlen(s+4, offset+4, src), rep: rep}
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}
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bestOf := func(a, b match) match {
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aScore := b.s - a.s + a.length
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bScore := a.s - b.s + b.length
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if a.rep < 0 {
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aScore = aScore - int32(bits.Len32(uint32(a.offset)))/8
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}
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if b.rep < 0 {
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bScore = bScore - int32(bits.Len32(uint32(b.offset)))/8
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}
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if aScore >= bScore {
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return a
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}
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return b
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}
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const goodEnough = 100
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nextHashL := hash8(cv, bestLongTableBits)
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nextHashS := hash4x64(cv, bestShortTableBits)
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candidateL := e.longTable[nextHashL]
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candidateS := e.table[nextHashS]
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best := bestOf(matchAt(candidateL.offset-e.cur, s, uint32(cv), -1), matchAt(candidateL.prev-e.cur, s, uint32(cv), -1))
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best = bestOf(best, matchAt(candidateS.offset-e.cur, s, uint32(cv), -1))
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best = bestOf(best, matchAt(candidateS.prev-e.cur, s, uint32(cv), -1))
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if canRepeat && best.length < goodEnough {
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best = bestOf(best, matchAt(s-offset1+1, s+1, uint32(cv>>8), 1))
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best = bestOf(best, matchAt(s-offset2+1, s+1, uint32(cv>>8), 2))
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best = bestOf(best, matchAt(s-offset3+1, s+1, uint32(cv>>8), 3))
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if best.length > 0 {
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best = bestOf(best, matchAt(s-offset1+3, s+3, uint32(cv>>24), 1))
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best = bestOf(best, matchAt(s-offset2+3, s+3, uint32(cv>>24), 2))
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best = bestOf(best, matchAt(s-offset3+3, s+3, uint32(cv>>24), 3))
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}
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}
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// Load next and check...
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e.longTable[nextHashL] = prevEntry{offset: s + e.cur, prev: candidateL.offset}
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e.table[nextHashS] = prevEntry{offset: s + e.cur, prev: candidateS.offset}
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// Look far ahead, unless we have a really long match already...
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if best.length < goodEnough {
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// No match found, move forward on input, no need to check forward...
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if best.length < 4 {
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s += 1 + (s-nextEmit)>>(kSearchStrength-1)
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if s >= sLimit {
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break encodeLoop
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}
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cv = load6432(src, s)
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continue
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}
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s++
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candidateS = e.table[hash4x64(cv>>8, bestShortTableBits)]
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cv = load6432(src, s)
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cv2 := load6432(src, s+1)
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candidateL = e.longTable[hash8(cv, bestLongTableBits)]
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candidateL2 := e.longTable[hash8(cv2, bestLongTableBits)]
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best = bestOf(best, matchAt(candidateS.offset-e.cur, s, uint32(cv), -1))
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best = bestOf(best, matchAt(candidateL.offset-e.cur, s, uint32(cv), -1))
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best = bestOf(best, matchAt(candidateL.prev-e.cur, s, uint32(cv), -1))
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best = bestOf(best, matchAt(candidateL2.offset-e.cur, s+1, uint32(cv2), -1))
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best = bestOf(best, matchAt(candidateL2.prev-e.cur, s+1, uint32(cv2), -1))
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}
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// We have a match, we can store the forward value
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if best.rep > 0 {
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s = best.s
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var seq seq
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seq.matchLen = uint32(best.length - zstdMinMatch)
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// We might be able to match backwards.
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// Extend as long as we can.
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start := best.s
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// We end the search early, so we don't risk 0 literals
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// and have to do special offset treatment.
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startLimit := nextEmit + 1
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tMin := s - e.maxMatchOff
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if tMin < 0 {
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tMin = 0
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}
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repIndex := best.offset
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for repIndex > tMin && start > startLimit && src[repIndex-1] == src[start-1] && seq.matchLen < maxMatchLength-zstdMinMatch-1 {
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repIndex--
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start--
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seq.matchLen++
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}
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addLiterals(&seq, start)
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// rep 0
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seq.offset = uint32(best.rep)
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if debugSequences {
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println("repeat sequence", seq, "next s:", s)
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}
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blk.sequences = append(blk.sequences, seq)
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// Index match start+1 (long) -> s - 1
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index0 := s
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s = best.s + best.length
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nextEmit = s
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if s >= sLimit {
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if debug {
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println("repeat ended", s, best.length)
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}
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break encodeLoop
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}
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// Index skipped...
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off := index0 + e.cur
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for index0 < s-1 {
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cv0 := load6432(src, index0)
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h0 := hash8(cv0, bestLongTableBits)
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h1 := hash4x64(cv0, bestShortTableBits)
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e.longTable[h0] = prevEntry{offset: off, prev: e.longTable[h0].offset}
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e.table[h1] = prevEntry{offset: off, prev: e.table[h1].offset}
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off++
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index0++
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}
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switch best.rep {
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case 2:
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offset1, offset2 = offset2, offset1
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case 3:
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offset1, offset2, offset3 = offset3, offset1, offset2
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}
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cv = load6432(src, s)
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continue
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}
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// A 4-byte match has been found. Update recent offsets.
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// We'll later see if more than 4 bytes.
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s = best.s
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t := best.offset
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offset1, offset2, offset3 = s-t, offset1, offset2
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if debugAsserts && s <= t {
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panic(fmt.Sprintf("s (%d) <= t (%d)", s, t))
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}
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if debugAsserts && canRepeat && int(offset1) > len(src) {
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panic("invalid offset")
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}
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// Extend the n-byte match as long as possible.
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l := best.length
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// Extend backwards
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tMin := s - e.maxMatchOff
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if tMin < 0 {
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tMin = 0
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}
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for t > tMin && s > nextEmit && src[t-1] == src[s-1] && l < maxMatchLength {
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s--
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t--
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l++
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}
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// Write our sequence
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var seq seq
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seq.litLen = uint32(s - nextEmit)
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seq.matchLen = uint32(l - zstdMinMatch)
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if seq.litLen > 0 {
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blk.literals = append(blk.literals, src[nextEmit:s]...)
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}
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seq.offset = uint32(s-t) + 3
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s += l
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if debugSequences {
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println("sequence", seq, "next s:", s)
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}
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blk.sequences = append(blk.sequences, seq)
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nextEmit = s
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if s >= sLimit {
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break encodeLoop
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}
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// Index match start+1 (long) -> s - 1
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index0 := s - l + 1
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// every entry
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for index0 < s-1 {
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cv0 := load6432(src, index0)
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h0 := hash8(cv0, bestLongTableBits)
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h1 := hash4x64(cv0, bestShortTableBits)
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off := index0 + e.cur
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e.longTable[h0] = prevEntry{offset: off, prev: e.longTable[h0].offset}
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e.table[h1] = prevEntry{offset: off, prev: e.table[h1].offset}
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index0++
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}
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cv = load6432(src, s)
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if !canRepeat {
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continue
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}
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// Check offset 2
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for {
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o2 := s - offset2
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if load3232(src, o2) != uint32(cv) {
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// Do regular search
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break
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}
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// Store this, since we have it.
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nextHashS := hash4x64(cv, bestShortTableBits)
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nextHashL := hash8(cv, bestLongTableBits)
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// We have at least 4 byte match.
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// No need to check backwards. We come straight from a match
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l := 4 + e.matchlen(s+4, o2+4, src)
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e.longTable[nextHashL] = prevEntry{offset: s + e.cur, prev: e.longTable[nextHashL].offset}
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e.table[nextHashS] = prevEntry{offset: s + e.cur, prev: e.table[nextHashS].offset}
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seq.matchLen = uint32(l) - zstdMinMatch
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seq.litLen = 0
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// Since litlen is always 0, this is offset 1.
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seq.offset = 1
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s += l
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nextEmit = s
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if debugSequences {
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println("sequence", seq, "next s:", s)
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}
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blk.sequences = append(blk.sequences, seq)
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// Swap offset 1 and 2.
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offset1, offset2 = offset2, offset1
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if s >= sLimit {
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// Finished
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break encodeLoop
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}
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cv = load6432(src, s)
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}
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}
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if int(nextEmit) < len(src) {
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blk.literals = append(blk.literals, src[nextEmit:]...)
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blk.extraLits = len(src) - int(nextEmit)
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}
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blk.recentOffsets[0] = uint32(offset1)
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blk.recentOffsets[1] = uint32(offset2)
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blk.recentOffsets[2] = uint32(offset3)
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if debug {
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println("returning, recent offsets:", blk.recentOffsets, "extra literals:", blk.extraLits)
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}
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}
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// EncodeNoHist will encode a block with no history and no following blocks.
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// Most notable difference is that src will not be copied for history and
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// we do not need to check for max match length.
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func (e *bestFastEncoder) EncodeNoHist(blk *blockEnc, src []byte) {
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e.Encode(blk, src)
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}
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// ResetDict will reset and set a dictionary if not nil
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func (e *bestFastEncoder) Reset(d *dict, singleBlock bool) {
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e.resetBase(d, singleBlock)
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if d == nil {
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return
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}
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// Init or copy dict table
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if len(e.dictTable) != len(e.table) || d.id != e.lastDictID {
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if len(e.dictTable) != len(e.table) {
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e.dictTable = make([]prevEntry, len(e.table))
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}
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end := int32(len(d.content)) - 8 + e.maxMatchOff
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for i := e.maxMatchOff; i < end; i += 4 {
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const hashLog = bestShortTableBits
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cv := load6432(d.content, i-e.maxMatchOff)
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nextHash := hash4x64(cv, hashLog) // 0 -> 4
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nextHash1 := hash4x64(cv>>8, hashLog) // 1 -> 5
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nextHash2 := hash4x64(cv>>16, hashLog) // 2 -> 6
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nextHash3 := hash4x64(cv>>24, hashLog) // 3 -> 7
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e.dictTable[nextHash] = prevEntry{
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prev: e.dictTable[nextHash].offset,
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offset: i,
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}
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e.dictTable[nextHash1] = prevEntry{
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prev: e.dictTable[nextHash1].offset,
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offset: i + 1,
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}
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e.dictTable[nextHash2] = prevEntry{
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prev: e.dictTable[nextHash2].offset,
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offset: i + 2,
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}
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e.dictTable[nextHash3] = prevEntry{
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prev: e.dictTable[nextHash3].offset,
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offset: i + 3,
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}
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}
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e.lastDictID = d.id
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}
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// Init or copy dict table
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if len(e.dictLongTable) != len(e.longTable) || d.id != e.lastDictID {
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if len(e.dictLongTable) != len(e.longTable) {
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e.dictLongTable = make([]prevEntry, len(e.longTable))
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}
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if len(d.content) >= 8 {
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cv := load6432(d.content, 0)
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h := hash8(cv, bestLongTableBits)
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e.dictLongTable[h] = prevEntry{
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offset: e.maxMatchOff,
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prev: e.dictLongTable[h].offset,
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}
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end := int32(len(d.content)) - 8 + e.maxMatchOff
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off := 8 // First to read
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for i := e.maxMatchOff + 1; i < end; i++ {
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cv = cv>>8 | (uint64(d.content[off]) << 56)
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h := hash8(cv, bestLongTableBits)
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e.dictLongTable[h] = prevEntry{
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offset: i,
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prev: e.dictLongTable[h].offset,
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}
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off++
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}
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}
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e.lastDictID = d.id
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}
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// Reset table to initial state
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copy(e.longTable[:], e.dictLongTable)
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e.cur = e.maxMatchOff
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// Reset table to initial state
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copy(e.table[:], e.dictTable)
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}
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