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Source file src/strings/search.go

Documentation: strings

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

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