// Copyright 2014-2017 Ulrich Kunitz. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package lzma
import (
"errors"
"fmt"
"github.com/ulikunitz/xz/internal/hash"
)
/* For compression we need to find byte sequences that match the byte
* sequence at the dictionary head. A hash table is a simple method to
* provide this capability.
*/
// maxMatches limits the number of matches requested from the Matches
// function. This controls the speed of the overall encoding.
const maxMatches = 16
// shortDists defines the number of short distances supported by the
// implementation.
const shortDists = 8
// The minimum is somehow arbitrary but the maximum is limited by the
// memory requirements of the hash table.
const (
minTableExponent = 9
maxTableExponent = 20
)
// newRoller contains the function used to create an instance of the
// hash.Roller.
var newRoller = func(n int) hash.Roller { return hash.NewCyclicPoly(n) }
// hashTable stores the hash table including the rolling hash method.
//
// We implement chained hashing into a circular buffer. Each entry in
// the circular buffer stores the delta distance to the next position with a
// word that has the same hash value.
type hashTable struct {
dict *encoderDict
// actual hash table
t []int64
// circular list data with the offset to the next word
data []uint32
front int
// mask for computing the index for the hash table
mask uint64
// hash offset; initial value is -int64(wordLen)
hoff int64
// length of the hashed word
wordLen int
// hash roller for computing the hash values for the Write
// method
wr hash.Roller
// hash roller for computing arbitrary hashes
hr hash.Roller
// preallocated slices
p [maxMatches]int64
distances [maxMatches + shortDists]int
}
// hashTableExponent derives the hash table exponent from the dictionary
// capacity.
func hashTableExponent(n uint32) int {
e := 30 - nlz32(n)
switch {
case e < minTableExponent:
e = minTableExponent
case e > maxTableExponent:
e = maxTableExponent
}
return e
}
// newHashTable creates a new hash table for words of length wordLen
func newHashTable(capacity int, wordLen int) (t *hashTable, err error) {
if !(0 < capacity) {
return nil, errors.New(
"newHashTable: capacity must not be negative")
}
exp := hashTableExponent(uint32(capacity))
if !(1 <= wordLen && wordLen <= 4) {
return nil, errors.New("newHashTable: " +
"argument wordLen out of range")
}
n := 1 << uint(exp)
if n <= 0 {
panic("newHashTable: exponent is too large")
}
t = &hashTable{
t: make([]int64, n),
data: make([]uint32, capacity),
mask: (uint64(1) << uint(exp)) - 1,
hoff: -int64(wordLen),
wordLen: wordLen,
wr: newRoller(wordLen),
hr: newRoller(wordLen),
}
return t, nil
}
func (t *hashTable) SetDict(d *encoderDict) { t.dict = d }
// buffered returns the number of bytes that are currently hashed.
func (t *hashTable) buffered() int {
n := t.hoff + 1
switch {
case n <= 0:
return 0
case n >= int64(len(t.data)):
return len(t.data)
}
return int(n)
}
// addIndex adds n to an index ensuring that is stays inside the
// circular buffer for the hash chain.
func (t *hashTable) addIndex(i, n int) int {
i += n - len(t.data)
if i < 0 {
i += len(t.data)
}
return i
}
// putDelta puts the delta instance at the current front of the circular
// chain buffer.
func (t *hashTable) putDelta(delta uint32) {
t.data[t.front] = delta
t.front = t.addIndex(t.front, 1)
}
// putEntry puts a new entry into the hash table. If there is already a
// value stored it is moved into the circular chain buffer.
func (t *hashTable) putEntry(h uint64, pos int64) {
if pos < 0 {
return
}
i := h & t.mask
old := t.t[i] - 1
t.t[i] = pos + 1
var delta int64
if old >= 0 {
delta = pos - old
if delta > 1<<32-1 || delta > int64(t.buffered()) {
delta = 0
}
}
t.putDelta(uint32(delta))
}
// WriteByte converts a single byte into a hash and puts them into the hash
// table.
func (t *hashTable) WriteByte(b byte) error {
h := t.wr.RollByte(b)
t.hoff++
t.putEntry(h, t.hoff)
return nil
}
// Write converts the bytes provided into hash tables and stores the
// abbreviated offsets into the hash table. The method will never return an
// error.
func (t *hashTable) Write(p []byte) (n int, err error) {
for _, b := range p {
// WriteByte doesn't generate an error.
t.WriteByte(b)
}
return len(p), nil
}
// getMatches the matches for a specific hash. The functions returns the
// number of positions found.
//
// TODO: Make a getDistances because that we are actually interested in.
func (t *hashTable) getMatches(h uint64, positions []int64) (n int) {
if t.hoff < 0 || len(positions) == 0 {
return 0
}
buffered := t.buffered()
tailPos := t.hoff + 1 - int64(buffered)
rear := t.front - buffered
if rear >= 0 {
rear -= len(t.data)
}
// get the slot for the hash
pos := t.t[h&t.mask] - 1
delta := pos - tailPos
for {
if delta < 0 {
return n
}
positions[n] = tailPos + delta
n++
if n >= len(positions) {
return n
}
i := rear + int(delta)
if i < 0 {
i += len(t.data)
}
u := t.data[i]
if u == 0 {
return n
}
delta -= int64(u)
}
}
// hash computes the rolling hash for the word stored in p. For correct
// results its length must be equal to t.wordLen.
func (t *hashTable) hash(p []byte) uint64 {
var h uint64
for _, b := range p {
h = t.hr.RollByte(b)
}
return h
}
// Matches fills the positions slice with potential matches. The
// functions returns the number of positions filled into positions. The
// byte slice p must have word length of the hash table.
func (t *hashTable) Matches(p []byte, positions []int64) int {
if len(p) != t.wordLen {
panic(fmt.Errorf(
"byte slice must have length %d", t.wordLen))
}
h := t.hash(p)
return t.getMatches(h, positions)
}
// NextOp identifies the next operation using the hash table.
//
// TODO: Use all repetitions to find matches.
func (t *hashTable) NextOp(rep [4]uint32) operation {
// get positions
data := t.dict.data[:maxMatchLen]
n, _ := t.dict.buf.Peek(data)
data = data[:n]
var p []int64
if n < t.wordLen {
p = t.p[:0]
} else {
p = t.p[:maxMatches]
n = t.Matches(data[:t.wordLen], p)
p = p[:n]
}
// convert positions in potential distances
head := t.dict.head
dists := append(t.distances[:0], 1, 2, 3, 4, 5, 6, 7, 8)
for _, pos := range p {
dis := int(head - pos)
if dis > shortDists {
dists = append(dists, dis)
}
}
// check distances
var m match
dictLen := t.dict.DictLen()
for _, dist := range dists {
if dist > dictLen {
continue
}
// Here comes a trick. We are only interested in matches
// that are longer than the matches we have been found
// before. So before we test the whole byte sequence at
// the given distance, we test the first byte that would
// make the match longer. If it doesn't match the byte
// to match, we don't to care any longer.
i := t.dict.buf.rear - dist + m.n
if i < 0 {
i += len(t.dict.buf.data)
}
if t.dict.buf.data[i] != data[m.n] {
// We can't get a longer match. Jump to the next
// distance.
continue
}
n := t.dict.buf.matchLen(dist, data)
switch n {
case 0:
continue
case 1:
if uint32(dist-minDistance) != rep[0] {
continue
}
}
if n > m.n {
m = match{int64(dist), n}
if n == len(data) {
// No better match will be found.
break
}
}
}
if m.n == 0 {
return lit{data[0]}
}
return m
}
