Implement KMP string search

1 files changed, 83 insertions, 0 deletions

diff --git a/lib/kmp/kmp.go b/lib/kmp/kmp.go
new file mode 100644
index 0000000..66b5516
--- /dev/null
+++ b/lib/kmp/kmp.go
@@ -0,0 +1,83 @@
+// Copyright (C) 2022  Luke Shumaker <lukeshu@lukeshu.com>
+//
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+package kmp
+
+import (
+	"errors"
+	"io"
+)
+
+// buildTable 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 buildTable(substr []byte) []int {
+	table := make([]int, len(substr))
+	for j := range table {
+		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 && substr[j] != substr[val] {
+			val = table[val-1]
+		}
+		// is a match; go forward
+		if substr[val] == substr[j] {
+			val++
+		}
+		table[j] = val
+	}
+	return table
+}
+
+// FindAll returns the starting-position of all possibly-overlapping
+// occurances of 'substr' in the 'r' stream.
+//
+// Will panic if len(substr)==0.
+func FindAll(r io.ByteReader, substr []byte) ([]int64, error) {
+	if len(substr) == 0 {
+		panic(errors.New("kmp.FindAll: empty substring"))
+	}
+	table := buildTable(substr)
+
+	var matches []int64
+	var curMatchBeg int64
+	var curMatchLen int
+
+	pos := int64(-1) // if 'r' were a slice; define 'pos' such that 'chr=r[pos]'
+	for {
+		// I/O
+		var chr byte
+		chr, err := r.ReadByte()
+		if err != nil {
+			if errors.Is(err, io.EOF) {
+				err = nil
+			}
+			return matches, err
+		}
+		pos++
+
+		// Consider 'chr'
+		for curMatchLen > 0 && chr != substr[curMatchLen] { // shorten the match
+			overlap := table[curMatchLen-1]
+			curMatchBeg += int64(curMatchLen - overlap)
+			curMatchLen = overlap
+		}
+		if chr == substr[curMatchLen] { // lengthen the match
+			if curMatchLen == 0 {
+				curMatchBeg = pos
+			}
+			curMatchLen++
+			if curMatchLen == len(substr) {
+				matches = append(matches, curMatchBeg)
+				overlap := table[curMatchLen-1]
+				curMatchBeg += int64(curMatchLen - overlap)
+				curMatchLen = overlap
+			}
+		}
+	}
+}