247 lines
6.4 KiB
Go
247 lines
6.4 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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"sort"
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)
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// MultiHash is an implementation of the MatchFinder
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// interface that uses multiple hashes of different lengths.
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type MultiHash 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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// HashLengths is a list of the hashes to use, with the number of
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// bytes to use for each. For example, to to use 4-byte, 7-byte, and
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// 10-byte hashes, set HashLengths to []int{4, 7, 10}.
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// The minimum length is 4.
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HashLengths []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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// 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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tables [][]uint32
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history []byte
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}
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func (q *MultiHash) Reset() {
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for _, t := range q.tables {
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for i := range t {
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t[i] = 0
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}
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}
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q.history = q.history[:0]
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}
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func (q *MultiHash) score(m absoluteMatch) int {
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return (m.End-m.Start)*256 + bits.LeadingZeros32(uint32(m.Start-m.Match))*q.DistanceBitCost
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}
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func (q *MultiHash) 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.TableBits == 0 {
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q.TableBits = 17
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}
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if len(q.tables) < len(q.HashLengths) {
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q.tables = make([][]uint32, len(q.HashLengths))
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for i := range q.tables {
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q.tables[i] = make([]uint32, 1<<q.TableBits)
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}
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}
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sort.Ints(q.HashLengths)
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maxHashLen := q.HashLengths[len(q.HashLengths)-1]
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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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for _, t := range q.tables {
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for i, v := range t {
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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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t[i] = uint32(newV)
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}
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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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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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candidates := make([]int, len(q.HashLengths))
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for i := e.NextEmit; i < len(src)-maxHashLen; 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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e.trim(matches[1], matches[0].Start, q.MinLength)
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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 hashes.
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h := uint32(0x811c9dc5) // FNV-32 offset basis
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nb := 0
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for j, hashLen := range q.HashLengths {
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for nb < hashLen {
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h ^= uint32(src[i+nb])
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h *= 0x01000193 // FNV-32 prime
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nb++
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}
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index := h >> (32 - q.TableBits)
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candidates[j] = int(q.tables[j][index])
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q.tables[j][index] = uint32(i)
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}
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// Look for a match.
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var currentMatch absoluteMatch
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if i < matches[0].End {
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// If we're looking for an overlapping match, we only need to check the
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// hash that ends 2 bytes after the end of the previous match.
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for j, candidate := range candidates {
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if i+q.HashLengths[j] != matches[0].End+2 {
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continue
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}
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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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break
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}
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m := extendMatch2(src, i, candidate, e.NextEmit)
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if m.End-m.Start >= q.HashLengths[j] {
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currentMatch = m
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}
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}
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} else {
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for j, candidate := range candidates {
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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 i-candidate == matches[0].Start-matches[0].Match {
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// Don't bother to check for the same match we already have.
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continue
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}
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if currentMatch.End-currentMatch.Start > q.HashLengths[j] {
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// Don't bother with hashes that are shorter than the current match.
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continue
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}
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if binary.LittleEndian.Uint32(src[candidate:]) != binary.LittleEndian.Uint32(src[i:]) {
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break
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}
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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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e.trim(matches[2], matches[1].Start, q.MinLength)
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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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e.trim(matches[1], matches[0].Start, q.MinLength)
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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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