// Copyright 2012 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package strings
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// stringFinder efficiently finds strings in a source text. It's implemented
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// using the Boyer-Moore string search algorithm:
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// https://en.wikipedia.org/wiki/Boyer-Moore_string_search_algorithm
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// https://www.cs.utexas.edu/~moore/publications/fstrpos.pdf (note: this aged
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// document uses 1-based indexing)
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type stringFinder struct {
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// pattern is the string that we are searching for in the text.
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pattern string
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// badCharSkip[b] contains the distance between the last byte of pattern
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// and the rightmost occurrence of b in pattern. If b is not in pattern,
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// badCharSkip[b] is len(pattern).
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//
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// Whenever a mismatch is found with byte b in the text, we can safely
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// shift the matching frame at least badCharSkip[b] until the next time
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// the matching char could be in alignment.
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badCharSkip [256]int
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// goodSuffixSkip[i] defines how far we can shift the matching frame given
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// that the suffix pattern[i+1:] matches, but the byte pattern[i] does
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// not. There are two cases to consider:
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//
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// 1. The matched suffix occurs elsewhere in pattern (with a different
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// byte preceding it that we might possibly match). In this case, we can
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// shift the matching frame to align with the next suffix chunk. For
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// example, the pattern "mississi" has the suffix "issi" next occurring
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// (in right-to-left order) at index 1, so goodSuffixSkip[3] ==
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// shift+len(suffix) == 3+4 == 7.
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//
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// 2. If the matched suffix does not occur elsewhere in pattern, then the
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// matching frame may share part of its prefix with the end of the
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// matching suffix. In this case, goodSuffixSkip[i] will contain how far
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// to shift the frame to align this portion of the prefix to the
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// suffix. For example, in the pattern "abcxxxabc", when the first
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// mismatch from the back is found to be in position 3, the matching
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// suffix "xxabc" is not found elsewhere in the pattern. However, its
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// rightmost "abc" (at position 6) is a prefix of the whole pattern, so
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// goodSuffixSkip[3] == shift+len(suffix) == 6+5 == 11.
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goodSuffixSkip []int
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}
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func makeStringFinder(pattern string) *stringFinder {
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f := &stringFinder{
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pattern: pattern,
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goodSuffixSkip: make([]int, len(pattern)),
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}
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// last is the index of the last character in the pattern.
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last := len(pattern) - 1
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// Build bad character table.
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// Bytes not in the pattern can skip one pattern's length.
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for i := range f.badCharSkip {
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f.badCharSkip[i] = len(pattern)
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}
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// The loop condition is < instead of <= so that the last byte does not
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// have a zero distance to itself. Finding this byte out of place implies
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// that it is not in the last position.
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for i := 0; i < last; i++ {
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f.badCharSkip[pattern[i]] = last - i
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}
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// Build good suffix table.
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// First pass: set each value to the next index which starts a prefix of
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// pattern.
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lastPrefix := last
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for i := last; i >= 0; i-- {
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if HasPrefix(pattern, pattern[i+1:]) {
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lastPrefix = i + 1
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}
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// lastPrefix is the shift, and (last-i) is len(suffix).
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f.goodSuffixSkip[i] = lastPrefix + last - i
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}
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// Second pass: find repeats of pattern's suffix starting from the front.
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for i := 0; i < last; i++ {
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lenSuffix := longestCommonSuffix(pattern, pattern[1:i+1])
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if pattern[i-lenSuffix] != pattern[last-lenSuffix] {
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// (last-i) is the shift, and lenSuffix is len(suffix).
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f.goodSuffixSkip[last-lenSuffix] = lenSuffix + last - i
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}
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}
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return f
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}
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func longestCommonSuffix(a, b string) (i int) {
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for ; i < len(a) && i < len(b); i++ {
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if a[len(a)-1-i] != b[len(b)-1-i] {
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break
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}
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}
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return
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}
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// next returns the index in text of the first occurrence of the pattern. If
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// the pattern is not found, it returns -1.
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func (f *stringFinder) next(text string) int {
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i := len(f.pattern) - 1
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for i < len(text) {
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// Compare backwards from the end until the first unmatching character.
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j := len(f.pattern) - 1
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for j >= 0 && text[i] == f.pattern[j] {
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i--
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j--
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}
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if j < 0 {
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return i + 1 // match
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}
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i += max(f.badCharSkip[text[i]], f.goodSuffixSkip[j])
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}
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return -1
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}
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func max(a, b int) int {
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if a > b {
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return a
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}
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return b
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}
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