1 files changed, 113 insertions, 0 deletions
diff --git a/cmd/btrfs-rec/inspect/rebuildmappings/kmp.go b/cmd/btrfs-rec/inspect/rebuildmappings/kmp.go
new file mode 100644
index 0000000..20772ba
--- /dev/null
+++ b/cmd/btrfs-rec/inspect/rebuildmappings/kmp.go
@@ -0,0 +1,113 @@
+// 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
+}