605 lines
15 KiB
Go
605 lines
15 KiB
Go
// Copyright 2016 The Snappy-Go Authors. All rights reserved.
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// Copyright (c) 2019 Klaus Post. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package s2
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import (
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"fmt"
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"math/bits"
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)
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// encodeBlockBest encodes a non-empty src to a guaranteed-large-enough dst. It
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// assumes that the varint-encoded length of the decompressed bytes has already
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// been written.
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//
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// It also assumes that:
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// len(dst) >= MaxEncodedLen(len(src)) &&
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// minNonLiteralBlockSize <= len(src) && len(src) <= maxBlockSize
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func encodeBlockBest(dst, src []byte) (d int) {
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// Initialize the hash tables.
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const (
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// Long hash matches.
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lTableBits = 19
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maxLTableSize = 1 << lTableBits
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// Short hash matches.
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sTableBits = 16
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maxSTableSize = 1 << sTableBits
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inputMargin = 8 + 2
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)
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// sLimit is when to stop looking for offset/length copies. The inputMargin
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// lets us use a fast path for emitLiteral in the main loop, while we are
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// looking for copies.
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sLimit := len(src) - inputMargin
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if len(src) < minNonLiteralBlockSize {
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return 0
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}
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var lTable [maxLTableSize]uint64
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var sTable [maxSTableSize]uint64
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// Bail if we can't compress to at least this.
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dstLimit := len(src) - 5
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// nextEmit is where in src the next emitLiteral should start from.
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nextEmit := 0
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// The encoded form must start with a literal, as there are no previous
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// bytes to copy, so we start looking for hash matches at s == 1.
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s := 1
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cv := load64(src, s)
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// We search for a repeat at -1, but don't output repeats when nextEmit == 0
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repeat := 1
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const lowbitMask = 0xffffffff
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getCur := func(x uint64) int {
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return int(x & lowbitMask)
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}
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getPrev := func(x uint64) int {
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return int(x >> 32)
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}
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const maxSkip = 64
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for {
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type match struct {
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offset int
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s int
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length int
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score int
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rep bool
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}
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var best match
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for {
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// Next src position to check
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nextS := (s-nextEmit)>>8 + 1
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if nextS > maxSkip {
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nextS = s + maxSkip
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} else {
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nextS += s
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}
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if nextS > sLimit {
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goto emitRemainder
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}
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hashL := hash8(cv, lTableBits)
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hashS := hash4(cv, sTableBits)
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candidateL := lTable[hashL]
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candidateS := sTable[hashS]
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score := func(m match) int {
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// Matches that are longer forward are penalized since we must emit it as a literal.
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score := m.length - m.s
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if nextEmit == m.s {
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// If we do not have to emit literals, we save 1 byte
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score++
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}
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offset := m.s - m.offset
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if m.rep {
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return score - emitRepeatSize(offset, m.length)
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}
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return score - emitCopySize(offset, m.length)
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}
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matchAt := func(offset, s int, first uint32, rep bool) match {
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if best.length != 0 && best.s-best.offset == s-offset {
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// Don't retest if we have the same offset.
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return match{offset: offset, s: s}
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}
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if load32(src, offset) != first {
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return match{offset: offset, s: s}
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}
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m := match{offset: offset, s: s, length: 4 + offset, rep: rep}
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s += 4
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for s <= sLimit {
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if diff := load64(src, s) ^ load64(src, m.length); diff != 0 {
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m.length += bits.TrailingZeros64(diff) >> 3
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break
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}
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s += 8
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m.length += 8
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}
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m.length -= offset
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m.score = score(m)
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if m.score <= -m.s {
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// Eliminate if no savings, we might find a better one.
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m.length = 0
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}
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return m
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}
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bestOf := func(a, b match) match {
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if b.length == 0 {
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return a
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}
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if a.length == 0 {
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return b
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}
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as := a.score + b.s
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bs := b.score + a.s
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if as >= bs {
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return a
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}
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return b
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}
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best = bestOf(matchAt(getCur(candidateL), s, uint32(cv), false), matchAt(getPrev(candidateL), s, uint32(cv), false))
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best = bestOf(best, matchAt(getCur(candidateS), s, uint32(cv), false))
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best = bestOf(best, matchAt(getPrev(candidateS), s, uint32(cv), false))
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{
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best = bestOf(best, matchAt(s-repeat+1, s+1, uint32(cv>>8), true))
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if best.length > 0 {
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// s+1
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nextShort := sTable[hash4(cv>>8, sTableBits)]
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s := s + 1
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cv := load64(src, s)
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nextLong := lTable[hash8(cv, lTableBits)]
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best = bestOf(best, matchAt(getCur(nextShort), s, uint32(cv), false))
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best = bestOf(best, matchAt(getPrev(nextShort), s, uint32(cv), false))
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best = bestOf(best, matchAt(getCur(nextLong), s, uint32(cv), false))
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best = bestOf(best, matchAt(getPrev(nextLong), s, uint32(cv), false))
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// Repeat at + 2
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best = bestOf(best, matchAt(s-repeat+1, s+1, uint32(cv>>8), true))
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// s+2
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if true {
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nextShort = sTable[hash4(cv>>8, sTableBits)]
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s++
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cv = load64(src, s)
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nextLong = lTable[hash8(cv, lTableBits)]
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best = bestOf(best, matchAt(getCur(nextShort), s, uint32(cv), false))
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best = bestOf(best, matchAt(getPrev(nextShort), s, uint32(cv), false))
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best = bestOf(best, matchAt(getCur(nextLong), s, uint32(cv), false))
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best = bestOf(best, matchAt(getPrev(nextLong), s, uint32(cv), false))
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}
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// Search for a match at best match end, see if that is better.
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if sAt := best.s + best.length; sAt < sLimit {
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sBack := best.s
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backL := best.length
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// Load initial values
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cv = load64(src, sBack)
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// Search for mismatch
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next := lTable[hash8(load64(src, sAt), lTableBits)]
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//next := sTable[hash4(load64(src, sAt), sTableBits)]
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if checkAt := getCur(next) - backL; checkAt > 0 {
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best = bestOf(best, matchAt(checkAt, sBack, uint32(cv), false))
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}
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if checkAt := getPrev(next) - backL; checkAt > 0 {
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best = bestOf(best, matchAt(checkAt, sBack, uint32(cv), false))
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}
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}
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}
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}
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// Update table
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lTable[hashL] = uint64(s) | candidateL<<32
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sTable[hashS] = uint64(s) | candidateS<<32
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if best.length > 0 {
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break
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}
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cv = load64(src, nextS)
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s = nextS
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}
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// Extend backwards, not needed for repeats...
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s = best.s
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if !best.rep {
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for best.offset > 0 && s > nextEmit && src[best.offset-1] == src[s-1] {
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best.offset--
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best.length++
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s--
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}
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}
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if false && best.offset >= s {
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panic(fmt.Errorf("t %d >= s %d", best.offset, s))
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}
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// Bail if we exceed the maximum size.
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if d+(s-nextEmit) > dstLimit {
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return 0
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}
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base := s
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offset := s - best.offset
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s += best.length
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if offset > 65535 && s-base <= 5 && !best.rep {
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// Bail if the match is equal or worse to the encoding.
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s = best.s + 1
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if s >= sLimit {
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goto emitRemainder
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}
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cv = load64(src, s)
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continue
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}
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d += emitLiteral(dst[d:], src[nextEmit:base])
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if best.rep {
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if nextEmit > 0 {
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// same as `add := emitCopy(dst[d:], repeat, s-base)` but skips storing offset.
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d += emitRepeat(dst[d:], offset, best.length)
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} else {
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// First match, cannot be repeat.
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d += emitCopy(dst[d:], offset, best.length)
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}
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} else {
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d += emitCopy(dst[d:], offset, best.length)
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}
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repeat = offset
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nextEmit = s
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if s >= sLimit {
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goto emitRemainder
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}
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if d > dstLimit {
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// Do we have space for more, if not bail.
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return 0
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}
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// Fill tables...
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for i := best.s + 1; i < s; i++ {
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cv0 := load64(src, i)
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long0 := hash8(cv0, lTableBits)
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short0 := hash4(cv0, sTableBits)
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lTable[long0] = uint64(i) | lTable[long0]<<32
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sTable[short0] = uint64(i) | sTable[short0]<<32
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}
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cv = load64(src, s)
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}
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emitRemainder:
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if nextEmit < len(src) {
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// Bail if we exceed the maximum size.
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if d+len(src)-nextEmit > dstLimit {
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return 0
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}
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d += emitLiteral(dst[d:], src[nextEmit:])
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}
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return d
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}
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// encodeBlockBestSnappy encodes a non-empty src to a guaranteed-large-enough dst. It
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// assumes that the varint-encoded length of the decompressed bytes has already
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// been written.
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//
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// It also assumes that:
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// len(dst) >= MaxEncodedLen(len(src)) &&
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// minNonLiteralBlockSize <= len(src) && len(src) <= maxBlockSize
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func encodeBlockBestSnappy(dst, src []byte) (d int) {
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// Initialize the hash tables.
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const (
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// Long hash matches.
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lTableBits = 19
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maxLTableSize = 1 << lTableBits
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// Short hash matches.
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sTableBits = 16
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maxSTableSize = 1 << sTableBits
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inputMargin = 8 + 2
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)
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// sLimit is when to stop looking for offset/length copies. The inputMargin
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// lets us use a fast path for emitLiteral in the main loop, while we are
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// looking for copies.
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sLimit := len(src) - inputMargin
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if len(src) < minNonLiteralBlockSize {
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return 0
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}
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var lTable [maxLTableSize]uint64
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var sTable [maxSTableSize]uint64
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// Bail if we can't compress to at least this.
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dstLimit := len(src) - 5
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// nextEmit is where in src the next emitLiteral should start from.
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nextEmit := 0
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// The encoded form must start with a literal, as there are no previous
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// bytes to copy, so we start looking for hash matches at s == 1.
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s := 1
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cv := load64(src, s)
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// We search for a repeat at -1, but don't output repeats when nextEmit == 0
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repeat := 1
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const lowbitMask = 0xffffffff
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getCur := func(x uint64) int {
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return int(x & lowbitMask)
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}
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getPrev := func(x uint64) int {
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return int(x >> 32)
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}
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const maxSkip = 64
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for {
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type match struct {
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offset int
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s int
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length int
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score int
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}
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var best match
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for {
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// Next src position to check
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nextS := (s-nextEmit)>>8 + 1
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if nextS > maxSkip {
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nextS = s + maxSkip
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} else {
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nextS += s
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}
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if nextS > sLimit {
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goto emitRemainder
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}
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hashL := hash8(cv, lTableBits)
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hashS := hash4(cv, sTableBits)
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candidateL := lTable[hashL]
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candidateS := sTable[hashS]
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score := func(m match) int {
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// Matches that are longer forward are penalized since we must emit it as a literal.
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score := m.length - m.s
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if nextEmit == m.s {
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// If we do not have to emit literals, we save 1 byte
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score++
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}
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offset := m.s - m.offset
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return score - emitCopySize(offset, m.length)
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}
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matchAt := func(offset, s int, first uint32) match {
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if best.length != 0 && best.s-best.offset == s-offset {
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// Don't retest if we have the same offset.
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return match{offset: offset, s: s}
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}
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if load32(src, offset) != first {
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return match{offset: offset, s: s}
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}
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m := match{offset: offset, s: s, length: 4 + offset}
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s += 4
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for s <= sLimit {
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if diff := load64(src, s) ^ load64(src, m.length); diff != 0 {
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m.length += bits.TrailingZeros64(diff) >> 3
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break
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}
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s += 8
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m.length += 8
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}
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m.length -= offset
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m.score = score(m)
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if m.score <= -m.s {
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// Eliminate if no savings, we might find a better one.
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m.length = 0
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}
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return m
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}
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bestOf := func(a, b match) match {
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if b.length == 0 {
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return a
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}
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if a.length == 0 {
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return b
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}
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as := a.score + b.s
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bs := b.score + a.s
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if as >= bs {
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return a
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}
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return b
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}
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best = bestOf(matchAt(getCur(candidateL), s, uint32(cv)), matchAt(getPrev(candidateL), s, uint32(cv)))
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best = bestOf(best, matchAt(getCur(candidateS), s, uint32(cv)))
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best = bestOf(best, matchAt(getPrev(candidateS), s, uint32(cv)))
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{
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best = bestOf(best, matchAt(s-repeat+1, s+1, uint32(cv>>8)))
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if best.length > 0 {
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// s+1
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nextShort := sTable[hash4(cv>>8, sTableBits)]
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s := s + 1
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cv := load64(src, s)
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nextLong := lTable[hash8(cv, lTableBits)]
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best = bestOf(best, matchAt(getCur(nextShort), s, uint32(cv)))
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best = bestOf(best, matchAt(getPrev(nextShort), s, uint32(cv)))
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best = bestOf(best, matchAt(getCur(nextLong), s, uint32(cv)))
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best = bestOf(best, matchAt(getPrev(nextLong), s, uint32(cv)))
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// Repeat at + 2
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best = bestOf(best, matchAt(s-repeat+1, s+1, uint32(cv>>8)))
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// s+2
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if true {
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nextShort = sTable[hash4(cv>>8, sTableBits)]
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s++
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cv = load64(src, s)
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nextLong = lTable[hash8(cv, lTableBits)]
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best = bestOf(best, matchAt(getCur(nextShort), s, uint32(cv)))
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best = bestOf(best, matchAt(getPrev(nextShort), s, uint32(cv)))
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best = bestOf(best, matchAt(getCur(nextLong), s, uint32(cv)))
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best = bestOf(best, matchAt(getPrev(nextLong), s, uint32(cv)))
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}
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// Search for a match at best match end, see if that is better.
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if sAt := best.s + best.length; sAt < sLimit {
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sBack := best.s
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backL := best.length
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// Load initial values
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cv = load64(src, sBack)
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// Search for mismatch
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next := lTable[hash8(load64(src, sAt), lTableBits)]
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//next := sTable[hash4(load64(src, sAt), sTableBits)]
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if checkAt := getCur(next) - backL; checkAt > 0 {
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best = bestOf(best, matchAt(checkAt, sBack, uint32(cv)))
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}
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if checkAt := getPrev(next) - backL; checkAt > 0 {
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best = bestOf(best, matchAt(checkAt, sBack, uint32(cv)))
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}
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}
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}
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}
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// Update table
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lTable[hashL] = uint64(s) | candidateL<<32
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sTable[hashS] = uint64(s) | candidateS<<32
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if best.length > 0 {
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break
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}
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cv = load64(src, nextS)
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s = nextS
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}
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// Extend backwards, not needed for repeats...
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s = best.s
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if true {
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for best.offset > 0 && s > nextEmit && src[best.offset-1] == src[s-1] {
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best.offset--
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best.length++
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s--
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}
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}
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if false && best.offset >= s {
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panic(fmt.Errorf("t %d >= s %d", best.offset, s))
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}
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// Bail if we exceed the maximum size.
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if d+(s-nextEmit) > dstLimit {
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return 0
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}
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base := s
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offset := s - best.offset
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s += best.length
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if offset > 65535 && s-base <= 5 {
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// Bail if the match is equal or worse to the encoding.
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s = best.s + 1
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if s >= sLimit {
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goto emitRemainder
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}
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cv = load64(src, s)
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continue
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}
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d += emitLiteral(dst[d:], src[nextEmit:base])
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d += emitCopyNoRepeat(dst[d:], offset, best.length)
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repeat = offset
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nextEmit = s
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if s >= sLimit {
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goto emitRemainder
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}
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|
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if d > dstLimit {
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// Do we have space for more, if not bail.
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return 0
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}
|
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// Fill tables...
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|
for i := best.s + 1; i < s; i++ {
|
|
cv0 := load64(src, i)
|
|
long0 := hash8(cv0, lTableBits)
|
|
short0 := hash4(cv0, sTableBits)
|
|
lTable[long0] = uint64(i) | lTable[long0]<<32
|
|
sTable[short0] = uint64(i) | sTable[short0]<<32
|
|
}
|
|
cv = load64(src, s)
|
|
}
|
|
|
|
emitRemainder:
|
|
if nextEmit < len(src) {
|
|
// Bail if we exceed the maximum size.
|
|
if d+len(src)-nextEmit > dstLimit {
|
|
return 0
|
|
}
|
|
d += emitLiteral(dst[d:], src[nextEmit:])
|
|
}
|
|
return d
|
|
}
|
|
|
|
// emitCopySize returns the size to encode the offset+length
|
|
//
|
|
// It assumes that:
|
|
// 1 <= offset && offset <= math.MaxUint32
|
|
// 4 <= length && length <= 1 << 24
|
|
func emitCopySize(offset, length int) int {
|
|
if offset >= 65536 {
|
|
i := 0
|
|
if length > 64 {
|
|
length -= 64
|
|
if length >= 4 {
|
|
// Emit remaining as repeats
|
|
return 5 + emitRepeatSize(offset, length)
|
|
}
|
|
i = 5
|
|
}
|
|
if length == 0 {
|
|
return i
|
|
}
|
|
return i + 5
|
|
}
|
|
|
|
// Offset no more than 2 bytes.
|
|
if length > 64 {
|
|
// Emit remaining as repeats, at least 4 bytes remain.
|
|
return 3 + emitRepeatSize(offset, length-60)
|
|
}
|
|
if length >= 12 || offset >= 2048 {
|
|
return 3
|
|
}
|
|
// Emit the remaining copy, encoded as 2 bytes.
|
|
return 2
|
|
}
|
|
|
|
// emitRepeatSize returns the number of bytes required to encode a repeat.
|
|
// Length must be at least 4 and < 1<<24
|
|
func emitRepeatSize(offset, length int) int {
|
|
// Repeat offset, make length cheaper
|
|
if length <= 4+4 || (length < 8+4 && offset < 2048) {
|
|
return 2
|
|
}
|
|
if length < (1<<8)+4+4 {
|
|
return 3
|
|
}
|
|
if length < (1<<16)+(1<<8)+4 {
|
|
return 4
|
|
}
|
|
const maxRepeat = (1 << 24) - 1
|
|
length -= (1 << 16) - 4
|
|
left := 0
|
|
if length > maxRepeat {
|
|
left = length - maxRepeat + 4
|
|
length = maxRepeat - 4
|
|
}
|
|
if left > 0 {
|
|
return 5 + emitRepeatSize(offset, left)
|
|
}
|
|
return 5
|
|
}
|