Fuzzy Bitap Algorithm

This is a fuzzy 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.

									/*****Please include following header files*****/
// stdlib.h
// string.h
// limits.h
/***********************************************/

int SearchString(const char *text, const char *pattern, int k)
{
	int result = -1;
	int m = strlen(pattern);
	unsigned long *R;
	unsigned long patternMask[CHAR_MAX + 1];
	int i, d;

	if (pattern[0] == '\0') return 0;
	if (m > 31) return -1; //Error: The pattern is too long!

	R = (unsigned long*)malloc((k + 1) * sizeof *R);
	for (i = 0; i <= k; ++i)
		R[i] = ~1;

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

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

	for (i = 0; text[i] != '\0'; ++i)
	{
		unsigned long oldRd1 = R[0];

		R[0] |= patternMask[text[i]];
		R[0] <<= 1;

		for (d = 1; d <= k; ++d)
		{
			unsigned long tmp = R[d];

			R[d] = (oldRd1 & (R[d] | patternMask[text[i]])) << 1;
			oldRd1 = tmp;
		}

		if (0 == (R[k] & (1UL << m)))
		{
			result = (i - m) + 1;
			break;
		}
	}

	free(R);
	return result;
}

Example

									int index = SearchString("The quick brown foax jumps over the lazy dog", "fox", 1);

Output

									index: 16