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Boyer–Moore algorithm implementation in PHP
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<?php | |
/** | |
* Returns the index of the first occurrence of the | |
* specified substring. If it's not found return -1. | |
* | |
* @param text The string to be scanned | |
* @param pattern The target string to search | |
* @return The start index of the substring | |
*/ | |
function BoyerMoore($text, $pattern) { | |
$patlen = strlen($pattern); | |
$textlen = strlen($text); | |
$table = makeCharTable($pattern); | |
for ($i=$patlen-1; $i < $textlen;) { | |
$t = $i; | |
for ($j=$patlen-1; $pattern[$j]==$text[$i]; $j--,$i--) { | |
if($j == 0) return $i; | |
} | |
$i = $t; | |
if(array_key_exists($text[$i], $table)) | |
$i = $i + max($table[$text[$i]], 1); | |
else | |
$i += $patlen; | |
} | |
return -1; | |
} | |
function makeCharTable($string) { | |
$len = strlen($string); | |
$table = array(); | |
for ($i=0; $i < $len; $i++) { | |
$table[$string[$i]] = $len - $i - 1; | |
} | |
return $table; | |
} |
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