217 lines
6.0 KiB
Go
217 lines
6.0 KiB
Go
// Copyright 2011 The Snappy-Go Authors. All rights reserved.
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// Modified for deflate by Klaus Post (c) 2015.
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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 flate
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import (
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"encoding/binary"
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"fmt"
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"math/bits"
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)
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type fastEnc interface {
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Encode(dst *tokens, src []byte)
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Reset()
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}
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func newFastEnc(level int) fastEnc {
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switch level {
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case 1:
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return &fastEncL1{fastGen: fastGen{cur: maxStoreBlockSize}}
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case 2:
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return &fastEncL2{fastGen: fastGen{cur: maxStoreBlockSize}}
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case 3:
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return &fastEncL3{fastGen: fastGen{cur: maxStoreBlockSize}}
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case 4:
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return &fastEncL4{fastGen: fastGen{cur: maxStoreBlockSize}}
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case 5:
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return &fastEncL5{fastGen: fastGen{cur: maxStoreBlockSize}}
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case 6:
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return &fastEncL6{fastGen: fastGen{cur: maxStoreBlockSize}}
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default:
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panic("invalid level specified")
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}
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}
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const (
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tableBits = 15 // Bits used in the table
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tableSize = 1 << tableBits // Size of the table
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tableShift = 32 - tableBits // Right-shift to get the tableBits most significant bits of a uint32.
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baseMatchOffset = 1 // The smallest match offset
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baseMatchLength = 3 // The smallest match length per the RFC section 3.2.5
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maxMatchOffset = 1 << 15 // The largest match offset
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bTableBits = 17 // Bits used in the big tables
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bTableSize = 1 << bTableBits // Size of the table
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allocHistory = maxStoreBlockSize * 5 // Size to preallocate for history.
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bufferReset = (1 << 31) - allocHistory - maxStoreBlockSize - 1 // Reset the buffer offset when reaching this.
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)
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const (
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prime3bytes = 506832829
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prime4bytes = 2654435761
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prime5bytes = 889523592379
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prime6bytes = 227718039650203
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prime7bytes = 58295818150454627
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prime8bytes = 0xcf1bbcdcb7a56463
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)
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func load3232(b []byte, i int32) uint32 {
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return binary.LittleEndian.Uint32(b[i:])
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}
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func load6432(b []byte, i int32) uint64 {
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return binary.LittleEndian.Uint64(b[i:])
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}
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type tableEntry struct {
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offset int32
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}
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// fastGen maintains the table for matches,
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// and the previous byte block for level 2.
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// This is the generic implementation.
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type fastGen struct {
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hist []byte
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cur int32
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}
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func (e *fastGen) addBlock(src []byte) int32 {
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// check if we have space already
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if len(e.hist)+len(src) > cap(e.hist) {
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if cap(e.hist) == 0 {
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e.hist = make([]byte, 0, allocHistory)
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} else {
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if cap(e.hist) < maxMatchOffset*2 {
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panic("unexpected buffer size")
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}
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// Move down
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offset := int32(len(e.hist)) - maxMatchOffset
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// copy(e.hist[0:maxMatchOffset], e.hist[offset:])
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*(*[maxMatchOffset]byte)(e.hist) = *(*[maxMatchOffset]byte)(e.hist[offset:])
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e.cur += offset
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e.hist = e.hist[:maxMatchOffset]
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}
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}
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s := int32(len(e.hist))
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e.hist = append(e.hist, src...)
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return s
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}
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type tableEntryPrev struct {
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Cur tableEntry
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Prev tableEntry
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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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return uint32(((u << (64 - 56)) * prime7bytes) >> ((64 - h) & reg8SizeMask64))
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}
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// hashLen returns a hash of the lowest mls bytes of with length output bits.
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// mls must be >=3 and <=8. Any other value will return hash for 4 bytes.
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// length should always be < 32.
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// Preferably length and mls should be a constant for inlining.
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func hashLen(u uint64, length, mls uint8) uint32 {
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switch mls {
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case 3:
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return (uint32(u<<8) * prime3bytes) >> (32 - length)
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case 5:
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return uint32(((u << (64 - 40)) * prime5bytes) >> (64 - length))
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case 6:
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return uint32(((u << (64 - 48)) * prime6bytes) >> (64 - length))
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case 7:
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return uint32(((u << (64 - 56)) * prime7bytes) >> (64 - length))
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case 8:
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return uint32((u * prime8bytes) >> (64 - length))
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default:
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return (uint32(u) * prime4bytes) >> (32 - length)
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}
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}
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// matchlen will return the match length between offsets and t in src.
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// The maximum length returned is maxMatchLength - 4.
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// It is assumed that s > t, that t >=0 and s < len(src).
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func (e *fastGen) matchlen(s, t int32, src []byte) int32 {
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if debugDecode {
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if t >= s {
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panic(fmt.Sprint("t >=s:", t, s))
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}
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if int(s) >= len(src) {
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panic(fmt.Sprint("s >= len(src):", s, len(src)))
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}
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if t < 0 {
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panic(fmt.Sprint("t < 0:", t))
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}
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if s-t > maxMatchOffset {
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panic(fmt.Sprint(s, "-", t, "(", s-t, ") > maxMatchLength (", maxMatchOffset, ")"))
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}
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}
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s1 := int(s) + maxMatchLength - 4
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if s1 > len(src) {
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s1 = len(src)
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}
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// Extend the match to be as long as possible.
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return int32(matchLen(src[s:s1], src[t:]))
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}
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// matchlenLong will return the match length between offsets and t in src.
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// It is assumed that s > t, that t >=0 and s < len(src).
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func (e *fastGen) matchlenLong(s, t int32, src []byte) int32 {
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if debugDeflate {
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if t >= s {
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panic(fmt.Sprint("t >=s:", t, s))
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}
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if int(s) >= len(src) {
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panic(fmt.Sprint("s >= len(src):", s, len(src)))
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}
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if t < 0 {
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panic(fmt.Sprint("t < 0:", t))
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}
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if s-t > maxMatchOffset {
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panic(fmt.Sprint(s, "-", t, "(", s-t, ") > maxMatchLength (", maxMatchOffset, ")"))
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}
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}
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// Extend the match to be as long as possible.
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return int32(matchLen(src[s:], src[t:]))
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}
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// Reset the encoding table.
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func (e *fastGen) Reset() {
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if cap(e.hist) < allocHistory {
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e.hist = make([]byte, 0, allocHistory)
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}
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// We offset current position so everything will be out of reach.
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// If we are above the buffer reset it will be cleared anyway since len(hist) == 0.
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if e.cur <= bufferReset {
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e.cur += maxMatchOffset + int32(len(e.hist))
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}
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e.hist = e.hist[:0]
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}
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// matchLen returns the maximum length.
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// 'a' must be the shortest of the two.
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func matchLen(a, b []byte) int {
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var checked int
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for len(a) >= 8 {
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if diff := binary.LittleEndian.Uint64(a) ^ binary.LittleEndian.Uint64(b); diff != 0 {
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return checked + (bits.TrailingZeros64(diff) >> 3)
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}
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checked += 8
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a = a[8:]
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b = b[8:]
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}
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b = b[:len(a)]
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for i := range a {
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if a[i] != b[i] {
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return i + checked
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}
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}
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return len(a) + checked
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}
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