The working principle and application scenarios of the Boyer-Moore algorithm in the string matching algorithm in PHP.

The Boyer-Moore algorithm is an efficient string matching algorithm that is widely used in text searches, editors, compilers and various pattern matching tools. This article will introduce how the Boyer-Moore algorithm works and give specific code examples.

1. Working Principle
Boyer-Moore algorithm starts matching from the end of the text being searched, and compares the characters of the pattern string and the text string in reverse. It utilizes two heuristic rules: the bad character rule and the good suffix rule.

Bad Character Rule:
When a character mismatch is encountered, the algorithm will slide the pattern string backwards based on the position of the bad character (the last position in the pattern string) , causing bad characters to align.

Good Suffix Rule:
When a character mismatch is encountered, the algorithm will slide the pattern string backward according to the occurrence position and length of the good suffix so that the good suffixes are aligned. A good suffix is ​​a suffix in the pattern string that matches the text string.

The Boyer-Moore algorithm continuously moves the pattern string and skips unmatched characters, thus greatly reducing the number of comparisons and improving matching efficiency.

2. Application scenarios
Boyer-Moore algorithm is suitable for large-scale text matching search, especially when the pattern string is long and the character set is large, compared with other common string matching algorithms (such as KMP, Brute-force, etc.), which has obvious advantages.

For example, in text processing, search engines, and compilers, we need to efficiently find keywords, variable names, or specific strings. The Boyer-Moore algorithm can quickly locate possible matching positions in the text, thereby speeding up the search process.

The following is a simple PHP sample code that demonstrates how to use the Boyer-Moore algorithm for string matching:

<?php

function boyerMoore($text, $pattern) {
  $textLength = strlen($text);
  $patternLength = strlen($pattern);
  $lastOccurrence = array();
  
  // 初始化坏字符的位置表
  for ($i = 0; $i < $patternLength; $i++) {
    $lastOccurrence[$pattern[$i]] = $i;
  }
  
  $offset = 0;
  while ($offset <= $textLength - $patternLength) {
    // 从末尾开始匹配
    for ($j = $patternLength - 1; $j >= 0 && $pattern[$j] == $text[$offset + $j]; $j--);
    
    if ($j < 0) {
      // 找到匹配
      return $offset;
    } else {
      // 根据坏字符规则和好后缀规则计算滑动距离
      
      // 坏字符规则
      $badCharDist = $j - $lastOccurrence[$text[$offset + $j]];
      
      // 好后缀规则
      $goodSuffixDist = 0;
      if ($j < $patternLength - 1) {
        $goodSuffixDist = $moveBy = $patternLength - $j;
        for ($k = $j + 1; $k < $patternLength - 1; $k++) {
          if ($pattern[$k] == $pattern[$k - $j - 1]) {
            $goodSuffixDist--;
          }
        }
      }
      
      // 取最大距离
      $offset += max($badCharDist, $goodSuffixDist);
    }
  }
  
  // 未找到匹配
  return -1;
}

// 示例用法

$text = "Lorem ipsum dolor sit amet, consectetur adipiscing elit.";
$pattern = "dolor";

$result = boyerMoore($text, $pattern);
if ($result == -1) {
  echo "未找到匹配的字符串";
} else {
  echo "匹配的字符串位置：".$result;
}

?>

In the above sample code, we will text string $text and pattern string $pattern are passed in to the boyerMoore function, and the function will return the matching position. If no matching string is found, the return result is -1.

Summary:
The Boyer-Moore algorithm achieves efficient string matching through the application of bad character rules and good suffix rules. It has good performance in large-scale text search, and is especially suitable for processing longer pattern strings and larger character sets. In actual application scenarios, we can use the Boyer-Moore algorithm to quickly perform string matching and improve the efficiency of search and matching.

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