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310 lines
7.4 KiB
310 lines
7.4 KiB
// Copyright 2014-2021 Ulrich Kunitz. 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 lzma
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import (
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"errors"
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"fmt"
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"github.com/ulikunitz/xz/internal/hash"
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)
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/* For compression we need to find byte sequences that match the byte
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* sequence at the dictionary head. A hash table is a simple method to
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* provide this capability.
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*/
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// maxMatches limits the number of matches requested from the Matches
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// function. This controls the speed of the overall encoding.
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const maxMatches = 16
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// shortDists defines the number of short distances supported by the
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// implementation.
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const shortDists = 8
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// The minimum is somehow arbitrary but the maximum is limited by the
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// memory requirements of the hash table.
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const (
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minTableExponent = 9
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maxTableExponent = 20
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)
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// newRoller contains the function used to create an instance of the
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// hash.Roller.
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var newRoller = func(n int) hash.Roller { return hash.NewCyclicPoly(n) }
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// hashTable stores the hash table including the rolling hash method.
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//
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// We implement chained hashing into a circular buffer. Each entry in
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// the circular buffer stores the delta distance to the next position with a
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// word that has the same hash value.
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type hashTable struct {
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dict *encoderDict
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// actual hash table
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t []int64
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// circular list data with the offset to the next word
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data []uint32
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front int
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// mask for computing the index for the hash table
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mask uint64
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// hash offset; initial value is -int64(wordLen)
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hoff int64
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// length of the hashed word
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wordLen int
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// hash roller for computing the hash values for the Write
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// method
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wr hash.Roller
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// hash roller for computing arbitrary hashes
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hr hash.Roller
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// preallocated slices
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p [maxMatches]int64
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distances [maxMatches + shortDists]int
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}
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// hashTableExponent derives the hash table exponent from the dictionary
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// capacity.
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func hashTableExponent(n uint32) int {
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e := 30 - nlz32(n)
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switch {
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case e < minTableExponent:
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e = minTableExponent
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case e > maxTableExponent:
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e = maxTableExponent
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}
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return e
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}
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// newHashTable creates a new hash table for words of length wordLen
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func newHashTable(capacity int, wordLen int) (t *hashTable, err error) {
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if !(0 < capacity) {
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return nil, errors.New(
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"newHashTable: capacity must not be negative")
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}
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exp := hashTableExponent(uint32(capacity))
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if !(1 <= wordLen && wordLen <= 4) {
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return nil, errors.New("newHashTable: " +
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"argument wordLen out of range")
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}
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n := 1 << uint(exp)
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if n <= 0 {
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panic("newHashTable: exponent is too large")
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}
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t = &hashTable{
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t: make([]int64, n),
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data: make([]uint32, capacity),
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mask: (uint64(1) << uint(exp)) - 1,
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hoff: -int64(wordLen),
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wordLen: wordLen,
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wr: newRoller(wordLen),
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hr: newRoller(wordLen),
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}
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return t, nil
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}
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func (t *hashTable) SetDict(d *encoderDict) { t.dict = d }
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// buffered returns the number of bytes that are currently hashed.
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func (t *hashTable) buffered() int {
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n := t.hoff + 1
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switch {
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case n <= 0:
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return 0
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case n >= int64(len(t.data)):
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return len(t.data)
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}
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return int(n)
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}
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// addIndex adds n to an index ensuring that is stays inside the
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// circular buffer for the hash chain.
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func (t *hashTable) addIndex(i, n int) int {
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i += n - len(t.data)
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if i < 0 {
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i += len(t.data)
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}
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return i
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}
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// putDelta puts the delta instance at the current front of the circular
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// chain buffer.
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func (t *hashTable) putDelta(delta uint32) {
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t.data[t.front] = delta
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t.front = t.addIndex(t.front, 1)
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}
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// putEntry puts a new entry into the hash table. If there is already a
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// value stored it is moved into the circular chain buffer.
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func (t *hashTable) putEntry(h uint64, pos int64) {
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if pos < 0 {
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return
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}
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i := h & t.mask
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old := t.t[i] - 1
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t.t[i] = pos + 1
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var delta int64
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if old >= 0 {
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delta = pos - old
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if delta > 1<<32-1 || delta > int64(t.buffered()) {
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delta = 0
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}
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}
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t.putDelta(uint32(delta))
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}
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// WriteByte converts a single byte into a hash and puts them into the hash
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// table.
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func (t *hashTable) WriteByte(b byte) error {
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h := t.wr.RollByte(b)
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t.hoff++
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t.putEntry(h, t.hoff)
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return nil
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}
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// Write converts the bytes provided into hash tables and stores the
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// abbreviated offsets into the hash table. The method will never return an
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// error.
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func (t *hashTable) Write(p []byte) (n int, err error) {
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for _, b := range p {
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// WriteByte doesn't generate an error.
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t.WriteByte(b)
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}
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return len(p), nil
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}
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// getMatches the matches for a specific hash. The functions returns the
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// number of positions found.
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//
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// TODO: Make a getDistances because that we are actually interested in.
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func (t *hashTable) getMatches(h uint64, positions []int64) (n int) {
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if t.hoff < 0 || len(positions) == 0 {
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return 0
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}
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buffered := t.buffered()
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tailPos := t.hoff + 1 - int64(buffered)
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rear := t.front - buffered
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if rear >= 0 {
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rear -= len(t.data)
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}
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// get the slot for the hash
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pos := t.t[h&t.mask] - 1
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delta := pos - tailPos
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for {
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if delta < 0 {
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return n
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}
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positions[n] = tailPos + delta
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n++
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if n >= len(positions) {
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return n
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}
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i := rear + int(delta)
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if i < 0 {
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i += len(t.data)
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}
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u := t.data[i]
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if u == 0 {
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return n
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}
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delta -= int64(u)
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}
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}
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// hash computes the rolling hash for the word stored in p. For correct
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// results its length must be equal to t.wordLen.
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func (t *hashTable) hash(p []byte) uint64 {
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var h uint64
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for _, b := range p {
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h = t.hr.RollByte(b)
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}
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return h
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}
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// Matches fills the positions slice with potential matches. The
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// functions returns the number of positions filled into positions. The
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// byte slice p must have word length of the hash table.
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func (t *hashTable) Matches(p []byte, positions []int64) int {
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if len(p) != t.wordLen {
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panic(fmt.Errorf(
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"byte slice must have length %d", t.wordLen))
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}
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h := t.hash(p)
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return t.getMatches(h, positions)
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}
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// NextOp identifies the next operation using the hash table.
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//
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// TODO: Use all repetitions to find matches.
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func (t *hashTable) NextOp(rep [4]uint32) operation {
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// get positions
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data := t.dict.data[:maxMatchLen]
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n, _ := t.dict.buf.Peek(data)
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data = data[:n]
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var p []int64
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if n < t.wordLen {
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p = t.p[:0]
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} else {
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p = t.p[:maxMatches]
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n = t.Matches(data[:t.wordLen], p)
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p = p[:n]
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}
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// convert positions in potential distances
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head := t.dict.head
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dists := append(t.distances[:0], 1, 2, 3, 4, 5, 6, 7, 8)
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for _, pos := range p {
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dis := int(head - pos)
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if dis > shortDists {
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dists = append(dists, dis)
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}
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}
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// check distances
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var m match
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dictLen := t.dict.DictLen()
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for _, dist := range dists {
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if dist > dictLen {
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continue
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}
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// Here comes a trick. We are only interested in matches
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// that are longer than the matches we have been found
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// before. So before we test the whole byte sequence at
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// the given distance, we test the first byte that would
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// make the match longer. If it doesn't match the byte
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// to match, we don't to care any longer.
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i := t.dict.buf.rear - dist + m.n
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if i < 0 {
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i += len(t.dict.buf.data)
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}
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if t.dict.buf.data[i] != data[m.n] {
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// We can't get a longer match. Jump to the next
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// distance.
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continue
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}
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n := t.dict.buf.matchLen(dist, data)
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switch n {
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case 0:
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continue
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case 1:
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if uint32(dist-minDistance) != rep[0] {
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continue
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}
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}
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if n > m.n {
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m = match{int64(dist), n}
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if n == len(data) {
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// No better match will be found.
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break
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}
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}
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}
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if m.n == 0 {
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return lit{data[0]}
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}
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return m
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}
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