Rabin Karp string matching algorithm

Question

I\'ve seen this Rabin Karp string matching algorithm in the forums on the website and I\'m interested in trying to implement it but I was wondering If anyone could tell me w

Answer 1

About the magic numbers Paul's answer is pretty clear.

As far as the code is concerned, Rabin Karp's principal idea is to perform an hash comparison between a sliding portion of the string and the pattern.

The hash cannot be computed each time on the whole substrings, otherwise the computation complexity would be quadratic O(n^2) instead of linear O(n).

Therefore, a rolling hash function is applied, such as at each iteration only one character is needed to update the hash value of the substring.

So, let's comment your code:

for (int i = 0; i < B.Length; i++)
{
    siga = (siga * D + (ulong)A[i]) % Q;
    sigb = (sigb * D + (ulong)B[i]) % Q;
}
if (siga == sigb)
{
    Console.WriteLine(string.Format(">>{0}<<{1}", A.Substring(0, B.Length), A.Substring(B.Length)));
    return;
}

^ This piece computes the hash of pattern B (sigb), and the hashcode of the initial substring of A of the same length of B. Actually it's not completely correct because hash can collide¹ and so, it is necessary to modify the if statement : if (siga == sigb && A.Substring(0, B.Length) == B).

ulong pow = 1;
for (int k = 1; k <= B.Length - 1; k++)
    pow = (pow * D) % Q;

^ Here's computed pow that is necessary to perform the rolling hash.

for (int j = 1; j <= A.Length - B.Length; j++)
{
    siga = (siga + Q - pow * (ulong)A[j - 1] % Q) % Q;
    siga = (siga * D + (ulong)A[j + B.Length - 1]) % Q;
    if (siga == sigb)
    {
        if (A.Substring(j, B.Length) == B)
        {
            Console.WriteLine(string.Format("{0}>>{1}<<{2}", A.Substring(0, j),
                                                                A.Substring(j, B.Length),
                                                                A.Substring(j + B.Length)));
            return;
        }
    }
}

^ Finally, the remaining string (i.e. from the second character to end), is scanned updating the hash value of the A substring and compared with the hash of B (computed at the beginning).

If the two hashes are equal, the substring and the pattern are compared¹ and if they're actually equal a message is returned.

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¹ Hash values can collide; hence, if two strings have different hash values they're definitely different, but if the two hashes are equal they can be equal or not.