Levenshtein Distance calculation in a category on NSString.

NSString+Levenshtein.m

@implementation NSString (Levenshtein)

- (NSUInteger)lev_distanceToString:(NSString *)comparisonString {
    NSString *referenceString = [self copy];
    NSUInteger referenceLength = referenceString.length;
    NSUInteger comparisonLength = comparisonString.length;
    unsigned long m, n;
    unsigned int distanceMatrix[comparisonLength + 1][referenceLength + 1];

    distanceMatrix[0][0] = 0;

    for (m = 1; m <= comparisonLength; m++) {
        distanceMatrix[m][0] = distanceMatrix[m - 1][0] + 1;
    }

    for (n = 1; n <= referenceLength; n++) {
        distanceMatrix[0][n] = distanceMatrix[0][n - 1] + 1;
    }

    for (m = 1; m <= comparisonLength; m++) {
        for (n = 1; n <= referenceLength; n++) {
            BOOL equalChars = [referenceString characterAtIndex:n - 1] == [comparisonString characterAtIndex:m - 1];
            int penalty = equalChars ? 0 : 1;
            distanceMatrix[m][n] = lev_min(distanceMatrix[m - 1][n] + 1,
                                           distanceMatrix[m][n - 1] + 1,
                                           distanceMatrix[m - 1][n - 1] + penalty);
        }
    }

    unsigned int distance = distanceMatrix[comparisonLength][referenceLength];
    return distance;
}

unsigned int lev_min(unsigned int a, unsigned int b, unsigned int c) {
    return MIN(MIN(MIN(a, b), MIN(a, c)), MIN(b, c));
}

@end