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// Copyright (C) 2022-2023 Luke Shumaker <lukeshu@lukeshu.com>
//
// SPDX-License-Identifier: GPL-2.0-or-later
package rebuildmappings
import (
"errors"
"git.lukeshu.com/btrfs-progs-ng/lib/diskio"
)
type kmpPattern[K ~int64 | ~int, V comparable] interface {
PatLen() K
// Get the value at 'pos' in the sequence. Positions start at
// 0 and increment naturally. It is invalid to call Get(pos)
// with a pos that is >= Len(). If there is a gap/wildcard at
// pos, ok is false.
PatGet(pos K) (v V, ok bool)
}
func kmpSelfEq[K ~int64 | ~int, V comparable](s kmpPattern[K, V], aI K, bI K) bool {
aV, aOK := s.PatGet(aI)
bV, bOK := s.PatGet(bI)
if !aOK || !bOK {
return true
}
return aV == bV
}
// buildKMPTable takes the string 'substr', and returns a table such
// that 'table[matchLen-1]' is the largest value 'val' for which 'val < matchLen' and
// 'substr[:val] == substr[matchLen-val:matchLen]'.
func buildKMPTable[K ~int64 | ~int, V comparable](substr kmpPattern[K, V]) []K {
substrLen := substr.PatLen()
table := make([]K, substrLen)
for j := K(0); j < substrLen; j++ {
if j == 0 {
// First entry must always be 0 (in order to
// satisfy 'val < matchLen').
continue
}
val := table[j-1]
// not a match; go back
for val > 0 && !kmpSelfEq(substr, j, val) {
val = table[val-1]
}
// is a match; go forward
if kmpSelfEq(substr, val, j) {
val++
}
table[j] = val
}
return table
}
func kmpEq[K ~int64 | ~int, V comparable](aV V, bS kmpPattern[K, V], bI K) bool {
bV, ok := bS.PatGet(bI)
if !ok {
return true
}
return aV == bV
}
// indexAll returns the starting-position of all possibly-overlapping
// occurrences of 'substr' in the 'str' sequence.
//
// Will hop around in 'substr', but will only get the natural sequence
// [0...) in order from 'str'.
//
// Will panic if the length of 'substr' is 0.
//
// The 'substr' may include wildcard characters by returning
// ErrWildcard for a position.
//
// Uses the Knuth-Morris-Pratt algorithm.
func indexAll[K ~int64 | ~int, V comparable](str diskio.Sequence[K, V], substr kmpPattern[K, V]) []K {
table := buildKMPTable(substr)
substrLen := K(len(table))
if substrLen == 0 {
panic(errors.New("rebuildmappings.IndexAll: empty substring"))
}
var matches []K
var curMatchBeg K
var curMatchLen K
strLen := str.SeqLen()
for pos := K(0); pos < strLen; pos++ {
chr := str.SeqGet(pos)
// Consider 'chr'
for curMatchLen > 0 && !kmpEq(chr, substr, curMatchLen) { // shorten the match
overlap := table[curMatchLen-1]
curMatchBeg += curMatchLen - overlap
curMatchLen = overlap
}
if kmpEq(chr, substr, curMatchLen) { // lengthen the match
if curMatchLen == 0 {
curMatchBeg = pos
}
curMatchLen++
if curMatchLen == substrLen {
matches = append(matches, curMatchBeg)
overlap := table[curMatchLen-1]
curMatchBeg += curMatchLen - overlap
curMatchLen = overlap
}
}
}
return matches
}
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