309 lines
8.2 KiB
Go
309 lines
8.2 KiB
Go
package matchfinder
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import (
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"encoding/binary"
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"math/bits"
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"runtime"
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)
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// M4 is an implementation of the MatchFinder
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// interface that uses a hash table to find matches,
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// optional match chains,
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// and the advanced parsing technique from
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// https://fastcompression.blogspot.com/2011/12/advanced-parsing-strategies.html.
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type M4 struct {
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// MaxDistance is the maximum distance (in bytes) to look back for
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// a match. The default is 65535.
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MaxDistance int
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// MinLength is the length of the shortest match to return.
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// The default is 4.
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MinLength int
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// HashLen is the number of bytes to use to calculate the hashes.
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// The maximum is 8 and the default is 6.
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HashLen int
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// TableBits is the number of bits in the hash table indexes.
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// The default is 17 (128K entries).
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TableBits int
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// ChainLength is how many entries to search on the "match chain" of older
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// locations with the same hash as the current location.
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ChainLength int
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// DistanceBitCost is used when comparing two matches to see
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// which is better. The comparison is primarily based on the length
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// of the matches, but it can also take the distance into account,
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// in terms of the number of bits needed to represent the distance.
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// One byte of length is given a score of 256, so 32 (256/8) would
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// be a reasonable first guess for the value of one bit.
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// (The default is 0, which bases the comparison solely on length.)
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DistanceBitCost int
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table []uint32
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chain []uint16
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history []byte
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}
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func (q *M4) Reset() {
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for i := range q.table {
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q.table[i] = 0
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}
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q.history = q.history[:0]
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q.chain = q.chain[:0]
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}
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func (q *M4) score(m absoluteMatch) int {
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return (m.End-m.Start)*256 + (bits.LeadingZeros32(uint32(m.Start-m.Match))-32)*q.DistanceBitCost
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}
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func (q *M4) FindMatches(dst []Match, src []byte) []Match {
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if q.MaxDistance == 0 {
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q.MaxDistance = 65535
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}
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if q.MinLength == 0 {
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q.MinLength = 4
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}
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if q.HashLen == 0 {
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q.HashLen = 6
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}
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if q.TableBits == 0 {
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q.TableBits = 17
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}
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if len(q.table) < 1<<q.TableBits {
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q.table = make([]uint32, 1<<q.TableBits)
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}
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e := matchEmitter{Dst: dst}
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if len(q.history) > q.MaxDistance*2 {
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// Trim down the history buffer.
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delta := len(q.history) - q.MaxDistance
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copy(q.history, q.history[delta:])
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q.history = q.history[:q.MaxDistance]
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if q.ChainLength > 0 {
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q.chain = q.chain[:q.MaxDistance]
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}
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for i, v := range q.table {
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newV := int(v) - delta
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if newV < 0 {
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newV = 0
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}
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q.table[i] = uint32(newV)
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}
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}
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// Append src to the history buffer.
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e.NextEmit = len(q.history)
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q.history = append(q.history, src...)
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if q.ChainLength > 0 {
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q.chain = append(q.chain, make([]uint16, len(src))...)
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}
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src = q.history
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// matches stores the matches that have been found but not emitted,
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// in reverse order. (matches[0] is the most recent one.)
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var matches [3]absoluteMatch
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for i := e.NextEmit; i < len(src)-7; i++ {
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if matches[0] != (absoluteMatch{}) && i >= matches[0].End {
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// We have found some matches, and we're far enough along that we probably
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// won't find overlapping matches, so we might as well emit them.
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if matches[1] != (absoluteMatch{}) {
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if matches[1].End > matches[0].Start {
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matches[1].End = matches[0].Start
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}
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if matches[1].End-matches[1].Start >= q.MinLength && q.score(matches[1]) > 0 {
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e.emit(matches[1])
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}
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}
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e.emit(matches[0])
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matches = [3]absoluteMatch{}
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}
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// Calculate and store the hash.
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h := ((binary.LittleEndian.Uint64(src[i:]) & (1<<(8*q.HashLen) - 1)) * hashMul64) >> (64 - q.TableBits)
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candidate := int(q.table[h])
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q.table[h] = uint32(i)
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if q.ChainLength > 0 && candidate != 0 {
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delta := i - candidate
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if delta < 1<<16 {
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q.chain[i] = uint16(delta)
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}
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}
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if i < matches[0].End && i != matches[0].End+2-q.HashLen {
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continue
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}
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if candidate == 0 || i-candidate > q.MaxDistance {
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continue
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}
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// Look for a match.
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var currentMatch absoluteMatch
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if binary.LittleEndian.Uint32(src[candidate:]) == binary.LittleEndian.Uint32(src[i:]) {
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m := extendMatch2(src, i, candidate, e.NextEmit)
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if m.End-m.Start > q.MinLength && q.score(m) > 0 {
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currentMatch = m
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}
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}
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for j := 0; j < q.ChainLength; j++ {
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delta := q.chain[candidate]
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if delta == 0 {
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break
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}
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candidate -= int(delta)
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if candidate <= 0 || i-candidate > q.MaxDistance {
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break
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}
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if binary.LittleEndian.Uint32(src[candidate:]) == binary.LittleEndian.Uint32(src[i:]) {
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m := extendMatch2(src, i, candidate, e.NextEmit)
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if m.End-m.Start > q.MinLength && q.score(m) > q.score(currentMatch) {
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currentMatch = m
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}
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}
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}
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if currentMatch.End-currentMatch.Start < q.MinLength {
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continue
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}
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overlapPenalty := 0
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if matches[0] != (absoluteMatch{}) {
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overlapPenalty = 275
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if currentMatch.Start <= matches[1].End {
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// This match would completely replace the previous match,
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// so there is no penalty for overlap.
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overlapPenalty = 0
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}
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}
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if q.score(currentMatch) <= q.score(matches[0])+overlapPenalty {
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continue
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}
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matches = [3]absoluteMatch{
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currentMatch,
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matches[0],
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matches[1],
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}
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if matches[2] == (absoluteMatch{}) {
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continue
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}
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// We have three matches, so it's time to emit one and/or eliminate one.
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switch {
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case matches[0].Start < matches[2].End:
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// The first and third matches overlap; discard the one in between.
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matches = [3]absoluteMatch{
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matches[0],
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matches[2],
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absoluteMatch{},
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}
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case matches[0].Start < matches[2].End+q.MinLength:
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// The first and third matches don't overlap, but there's no room for
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// another match between them. Emit the first match and discard the second.
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e.emit(matches[2])
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matches = [3]absoluteMatch{
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matches[0],
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absoluteMatch{},
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absoluteMatch{},
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}
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default:
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// Emit the first match, shortening it if necessary to avoid overlap with the second.
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if matches[2].End > matches[1].Start {
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matches[2].End = matches[1].Start
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}
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if matches[2].End-matches[2].Start >= q.MinLength && q.score(matches[2]) > 0 {
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e.emit(matches[2])
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}
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matches[2] = absoluteMatch{}
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}
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}
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// We've found all the matches now; emit the remaining ones.
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if matches[1] != (absoluteMatch{}) {
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if matches[1].End > matches[0].Start {
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matches[1].End = matches[0].Start
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}
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if matches[1].End-matches[1].Start >= q.MinLength && q.score(matches[1]) > 0 {
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e.emit(matches[1])
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}
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}
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if matches[0] != (absoluteMatch{}) {
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e.emit(matches[0])
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}
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dst = e.Dst
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if e.NextEmit < len(src) {
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dst = append(dst, Match{
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Unmatched: len(src) - e.NextEmit,
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})
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}
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return dst
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}
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const hashMul64 = 0x1E35A7BD1E35A7BD
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// extendMatch returns the largest k such that k <= len(src) and that
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// src[i:i+k-j] and src[j:k] have the same contents.
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//
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// It assumes that:
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//
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// 0 <= i && i < j && j <= len(src)
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func extendMatch(src []byte, i, j int) int {
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switch runtime.GOARCH {
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case "amd64":
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// As long as we are 8 or more bytes before the end of src, we can load and
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// compare 8 bytes at a time. If those 8 bytes are equal, repeat.
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for j+8 < len(src) {
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iBytes := binary.LittleEndian.Uint64(src[i:])
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jBytes := binary.LittleEndian.Uint64(src[j:])
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if iBytes != jBytes {
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// If those 8 bytes were not equal, XOR the two 8 byte values, and return
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// the index of the first byte that differs. The BSF instruction finds the
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// least significant 1 bit, the amd64 architecture is little-endian, and
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// the shift by 3 converts a bit index to a byte index.
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return j + bits.TrailingZeros64(iBytes^jBytes)>>3
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}
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i, j = i+8, j+8
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}
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case "386":
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// On a 32-bit CPU, we do it 4 bytes at a time.
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for j+4 < len(src) {
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iBytes := binary.LittleEndian.Uint32(src[i:])
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jBytes := binary.LittleEndian.Uint32(src[j:])
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if iBytes != jBytes {
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return j + bits.TrailingZeros32(iBytes^jBytes)>>3
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}
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i, j = i+4, j+4
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}
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}
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for ; j < len(src) && src[i] == src[j]; i, j = i+1, j+1 {
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}
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return j
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}
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// Given a 4-byte match at src[start] and src[candidate], extendMatch2 extends it
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// upward as far as possible, and downward no farther than to min.
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func extendMatch2(src []byte, start, candidate, min int) absoluteMatch {
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end := extendMatch(src, candidate+4, start+4)
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for start > min && candidate > 0 && src[start-1] == src[candidate-1] {
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start--
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candidate--
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}
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return absoluteMatch{
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Start: start,
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End: end,
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Match: candidate,
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}
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}
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