432 lines
11 KiB
Go
432 lines
11 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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"math/bits"
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)
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// hash4 returns the hash of the lowest 4 bytes of u to fit in a hash table with h bits.
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// Preferably h should be a constant and should always be <32.
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func hash4(u uint64, h uint8) uint32 {
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const prime4bytes = 2654435761
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return (uint32(u) * prime4bytes) >> ((32 - h) & 31)
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}
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// hash5 returns the hash of the lowest 5 bytes of u to fit in a hash table with h bits.
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// Preferably h should be a constant and should always be <64.
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func hash5(u uint64, h uint8) uint32 {
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const prime5bytes = 889523592379
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return uint32(((u << (64 - 40)) * prime5bytes) >> ((64 - h) & 63))
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}
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// hash7 returns the hash of the lowest 7 bytes of u to fit in a hash table with h bits.
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// Preferably h should be a constant and should always be <64.
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func hash7(u uint64, h uint8) uint32 {
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const prime7bytes = 58295818150454627
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return uint32(((u << (64 - 56)) * prime7bytes) >> ((64 - h) & 63))
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}
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// hash8 returns the hash of u to fit in a hash table with h bits.
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// Preferably h should be a constant and should always be <64.
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func hash8(u uint64, h uint8) uint32 {
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const prime8bytes = 0xcf1bbcdcb7a56463
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return uint32((u * prime8bytes) >> ((64 - h) & 63))
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}
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// encodeBlockBetter 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 encodeBlockBetterGo(dst, src []byte) (d int) {
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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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// Initialize the hash tables.
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const (
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// Long hash matches.
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lTableBits = 16
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maxLTableSize = 1 << lTableBits
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// Short hash matches.
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sTableBits = 14
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maxSTableSize = 1 << sTableBits
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)
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var lTable [maxLTableSize]uint32
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var sTable [maxSTableSize]uint32
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// Bail if we can't compress to at least this.
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dstLimit := len(src) - len(src)>>5 - 6
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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 initialize repeat to 0, so we never match on first attempt
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repeat := 0
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for {
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candidateL := 0
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nextS := 0
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for {
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// Next src position to check
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nextS = s + (s-nextEmit)>>7 + 1
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if nextS > sLimit {
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goto emitRemainder
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}
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hashL := hash7(cv, lTableBits)
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hashS := hash4(cv, sTableBits)
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candidateL = int(lTable[hashL])
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candidateS := int(sTable[hashS])
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lTable[hashL] = uint32(s)
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sTable[hashS] = uint32(s)
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// Check repeat at offset checkRep.
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const checkRep = 1
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if false && uint32(cv>>(checkRep*8)) == load32(src, s-repeat+checkRep) {
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base := s + checkRep
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// Extend back
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for i := base - repeat; base > nextEmit && i > 0 && src[i-1] == src[base-1]; {
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i--
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base--
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}
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d += emitLiteral(dst[d:], src[nextEmit:base])
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// Extend forward
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candidate := s - repeat + 4 + checkRep
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s += 4 + checkRep
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for s < len(src) {
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if len(src)-s < 8 {
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if src[s] == src[candidate] {
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s++
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candidate++
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continue
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}
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break
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}
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if diff := load64(src, s) ^ load64(src, candidate); diff != 0 {
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s += bits.TrailingZeros64(diff) >> 3
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break
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}
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s += 8
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candidate += 8
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}
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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:], repeat, s-base)
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} else {
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// First match, cannot be repeat.
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d += emitCopy(dst[d:], repeat, s-base)
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}
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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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cv = load64(src, s)
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continue
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}
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if uint32(cv) == load32(src, candidateL) {
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break
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}
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// Check our short candidate
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if uint32(cv) == load32(src, candidateS) {
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// Try a long candidate at s+1
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hashL = hash7(cv>>8, lTableBits)
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candidateL = int(lTable[hashL])
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lTable[hashL] = uint32(s + 1)
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if uint32(cv>>8) == load32(src, candidateL) {
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s++
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break
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}
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// Use our short candidate.
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candidateL = candidateS
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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
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for candidateL > 0 && s > nextEmit && src[candidateL-1] == src[s-1] {
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candidateL--
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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 := base - candidateL
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// Extend the 4-byte match as long as possible.
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s += 4
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candidateL += 4
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for s < len(src) {
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if len(src)-s < 8 {
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if src[s] == src[candidateL] {
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s++
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candidateL++
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continue
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}
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break
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}
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if diff := load64(src, s) ^ load64(src, candidateL); diff != 0 {
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s += bits.TrailingZeros64(diff) >> 3
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break
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}
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s += 8
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candidateL += 8
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}
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if offset > 65535 && s-base <= 5 && repeat != offset {
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// Bail if the match is equal or worse to the encoding.
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s = nextS + 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 repeat == offset {
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d += emitRepeat(dst[d:], offset, s-base)
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} else {
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d += emitCopy(dst[d:], offset, s-base)
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repeat = offset
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}
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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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// Index match start+1 (long) and start+2 (short)
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index0 := base + 1
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// Index match end-2 (long) and end-1 (short)
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index1 := s - 2
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cv0 := load64(src, index0)
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cv1 := load64(src, index1)
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cv = load64(src, s)
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lTable[hash7(cv0, lTableBits)] = uint32(index0)
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lTable[hash7(cv0>>8, lTableBits)] = uint32(index0 + 1)
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lTable[hash7(cv1, lTableBits)] = uint32(index1)
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lTable[hash7(cv1>>8, lTableBits)] = uint32(index1 + 1)
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sTable[hash4(cv0>>8, sTableBits)] = uint32(index0 + 1)
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sTable[hash4(cv0>>16, sTableBits)] = uint32(index0 + 2)
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sTable[hash4(cv1>>8, sTableBits)] = uint32(index1 + 1)
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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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// encodeBlockBetterSnappyGo 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 encodeBlockBetterSnappyGo(dst, src []byte) (d int) {
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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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// Initialize the hash tables.
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const (
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// Long hash matches.
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lTableBits = 16
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maxLTableSize = 1 << lTableBits
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// Short hash matches.
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sTableBits = 14
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maxSTableSize = 1 << sTableBits
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)
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var lTable [maxLTableSize]uint32
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var sTable [maxSTableSize]uint32
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// Bail if we can't compress to at least this.
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dstLimit := len(src) - len(src)>>5 - 6
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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 initialize repeat to 0, so we never match on first attempt
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repeat := 0
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const maxSkip = 100
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for {
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candidateL := 0
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nextS := 0
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for {
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// Next src position to check
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nextS = (s-nextEmit)>>7 + 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 := hash7(cv, lTableBits)
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hashS := hash4(cv, sTableBits)
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candidateL = int(lTable[hashL])
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candidateS := int(sTable[hashS])
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lTable[hashL] = uint32(s)
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sTable[hashS] = uint32(s)
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if uint32(cv) == load32(src, candidateL) {
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break
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}
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// Check our short candidate
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if uint32(cv) == load32(src, candidateS) {
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// Try a long candidate at s+1
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hashL = hash7(cv>>8, lTableBits)
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candidateL = int(lTable[hashL])
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lTable[hashL] = uint32(s + 1)
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if uint32(cv>>8) == load32(src, candidateL) {
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s++
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break
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}
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// Use our short candidate.
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candidateL = candidateS
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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
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for candidateL > 0 && s > nextEmit && src[candidateL-1] == src[s-1] {
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candidateL--
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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 := base - candidateL
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// Extend the 4-byte match as long as possible.
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s += 4
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candidateL += 4
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for s < len(src) {
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if len(src)-s < 8 {
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if src[s] == src[candidateL] {
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s++
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candidateL++
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continue
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}
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break
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}
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if diff := load64(src, s) ^ load64(src, candidateL); diff != 0 {
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s += bits.TrailingZeros64(diff) >> 3
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break
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}
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s += 8
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candidateL += 8
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}
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if offset > 65535 && s-base <= 5 && repeat != offset {
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// Bail if the match is equal or worse to the encoding.
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s = nextS + 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, s-base)
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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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// Index match start+1 (long) and start+2 (short)
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index0 := base + 1
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// Index match end-2 (long) and end-1 (short)
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index1 := s - 2
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cv0 := load64(src, index0)
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cv1 := load64(src, index1)
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cv = load64(src, s)
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lTable[hash7(cv0, lTableBits)] = uint32(index0)
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lTable[hash7(cv0>>8, lTableBits)] = uint32(index0 + 1)
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lTable[hash7(cv1, lTableBits)] = uint32(index1)
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lTable[hash7(cv1>>8, lTableBits)] = uint32(index1 + 1)
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sTable[hash4(cv0>>8, sTableBits)] = uint32(index0 + 1)
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sTable[hash4(cv0>>16, sTableBits)] = uint32(index0 + 2)
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sTable[hash4(cv1>>8, sTableBits)] = uint32(index1 + 1)
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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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