Bitap Algorithm

This is a exact string matching version of bitap algorithm. The bitap algorithm (also known as the shift-or, shift-and or Baeza-Yates–Gonnet algorithm) is an approximate string matching algorithm. The algorithm tells whether a given text contains a substring which is "approximately equal" to a given pattern, where approximate equality is defined in terms of Levenshtein distance — if the substring and pattern are within a given distance k of each other, then the algorithm considers them equal. The algorithm begins by precomputing a set of bitmasks containing one bit for each element of the pattern. Then it is able to do most of the work with bitwise operations, which are extremely fast.

									function SearchString($text, $pattern)
{
	$m = strlen($pattern);
	$textLen = strlen($text);
	$patternMask = array();
	$i;

	if (empty($pattern)) return 0;
	if ($m > 31) return -1; //Error: The pattern is too long!

	$R = ~1;

	for ($i = 0; $i <= 127; ++$i)
		$patternMask[$i] = ~0;

	for ($i = 0; $i < $m; ++$i)
		$patternMask[ord($pattern[$i])] &= ~(1 << $i);

	for ($i = 0; $i < $textLen; ++$i)
	{
		$R |= $patternMask[ord($text[$i])];
		$R <<= 1;

		if (0 == ($R & (1 << $m)))
			return ($i - $m) + 1;
	}

	return -1;
}

Example

									$index = SearchString("The quick brown fox jumps over the lazy dog", "fox");

Output

									index: 16