libgo/go/strings/search.go

   1 // Copyright 2012 The Go Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style
   3 // license that can be found in the LICENSE file.
   4
   5 package strings
   6
   7 // stringFinder efficiently finds strings in a source text. It's implemented
   8 // using the Boyer-Moore string search algorithm:
   9 // https://en.wikipedia.org/wiki/Boyer-Moore_string_search_algorithm
  10 // https://www.cs.utexas.edu/~moore/publications/fstrpos.pdf (note: this aged
  11 // document uses 1-based indexing)
  12 type stringFinder struct {
  13         // pattern is the string that we are searching for in the text.
  14         pattern string
  15
  16         // badCharSkip[b] contains the distance between the last byte of pattern
  17         // and the rightmost occurrence of b in pattern. If b is not in pattern,
  18         // badCharSkip[b] is len(pattern).
  19         //
  20         // Whenever a mismatch is found with byte b in the text, we can safely
  21         // shift the matching frame at least badCharSkip[b] until the next time
  22         // the matching char could be in alignment.
  23         badCharSkip [256]int
  24
  25         // goodSuffixSkip[i] defines how far we can shift the matching frame given
  26         // that the suffix pattern[i+1:] matches, but the byte pattern[i] does
  27         // not. There are two cases to consider:
  28         //
  29         // 1. The matched suffix occurs elsewhere in pattern (with a different
  30         // byte preceding it that we might possibly match). In this case, we can
  31         // shift the matching frame to align with the next suffix chunk. For
  32         // example, the pattern "mississi" has the suffix "issi" next occurring
  33         // (in right-to-left order) at index 1, so goodSuffixSkip[3] ==
  34         // shift+len(suffix) == 3+4 == 7.
  35         //
  36         // 2. If the matched suffix does not occur elsewhere in pattern, then the
  37         // matching frame may share part of its prefix with the end of the
  38         // matching suffix. In this case, goodSuffixSkip[i] will contain how far
  39         // to shift the frame to align this portion of the prefix to the
  40         // suffix. For example, in the pattern "abcxxxabc", when the first
  41         // mismatch from the back is found to be in position 3, the matching
  42         // suffix "xxabc" is not found elsewhere in the pattern. However, its
  43         // rightmost "abc" (at position 6) is a prefix of the whole pattern, so
  44         // goodSuffixSkip[3] == shift+len(suffix) == 6+5 == 11.
  45         goodSuffixSkip []int
  46 }
  47
  48 func makeStringFinder(pattern string) *stringFinder {
  49         f := &stringFinder{
  50                 pattern:        pattern,
  51                 goodSuffixSkip: make([]int, len(pattern)),
  52         }
  53         // last is the index of the last character in the pattern.
  54         last := len(pattern) - 1
  55
  56         // Build bad character table.
  57         // Bytes not in the pattern can skip one pattern's length.
  58         for i := range f.badCharSkip {
  59                 f.badCharSkip[i] = len(pattern)
  60         }
  61         // The loop condition is < instead of <= so that the last byte does not
  62         // have a zero distance to itself. Finding this byte out of place implies
  63         // that it is not in the last position.
  64         for i := 0; i < last; i++ {
  65                 f.badCharSkip[pattern[i]] = last - i
  66         }
  67
  68         // Build good suffix table.
  69         // First pass: set each value to the next index which starts a prefix of
  70         // pattern.
  71         lastPrefix := last
  72         for i := last; i >= 0; i-- {
  73                 if HasPrefix(pattern, pattern[i+1:]) {
  74                         lastPrefix = i + 1
  75                 }
  76                 // lastPrefix is the shift, and (last-i) is len(suffix).
  77                 f.goodSuffixSkip[i] = lastPrefix + last - i
  78         }
  79         // Second pass: find repeats of pattern's suffix starting from the front.
  80         for i := 0; i < last; i++ {
  81                 lenSuffix := longestCommonSuffix(pattern, pattern[1:i+1])
  82                 if pattern[i-lenSuffix] != pattern[last-lenSuffix] {
  83                         // (last-i) is the shift, and lenSuffix is len(suffix).
  84                         f.goodSuffixSkip[last-lenSuffix] = lenSuffix + last - i
  85                 }
  86         }
  87
  88         return f
  89 }
  90
  91 func longestCommonSuffix(a, b string) (i int) {
  92         for ; i < len(a) && i < len(b); i++ {
  93                 if a[len(a)-1-i] != b[len(b)-1-i] {
  94                         break
  95                 }
  96         }
  97         return
  98 }
  99
 100 // next returns the index in text of the first occurrence of the pattern. If
 101 // the pattern is not found, it returns -1.
 102 func (f *stringFinder) next(text string) int {
 103         i := len(f.pattern) - 1
 104         for i < len(text) {
 105                 // Compare backwards from the end until the first unmatching character.
 106                 j := len(f.pattern) - 1
 107                 for j >= 0 && text[i] == f.pattern[j] {
 108                         i--
 109                         j--
 110                 }
 111                 if j < 0 {
 112                         return i + 1 // match
 113                 }
 114                 i += max(f.badCharSkip[text[i]], f.goodSuffixSkip[j])
 115         }
 116         return -1
 117 }
 118
 119 func max(a, b int) int {
 120         if a > b {
 121                 return a
 122         }
 123         return b
 124 }