RabinKarpAlgorithm.java

package String;

public class RabinKarpAlgorithm {
    static int q = 101; // A prime number.
    static int d = 256;

    static void search(String pat, String txt) {
        int M = pat.length();
        int N = txt.length();

        int i, j;
        int p = 0; // Hash value for pattern.
        int t = 0; // Hash value for txt.
        int h = 1;

        // The value of h would be "pow(d, M - 1) % q"
        for (i = 0; i < M - 1; i++)
            h = (h * d) % q;

        // Calculate the hash value of pattern and first window of text
        for (i = 0; i < M; i++) {
            p = (d * p + pat.charAt(i)) % q;
            t = (d * t + txt.charAt(i)) % q;
        }

        // Slide the pattern over text one by one.
        for (i = 0; i <= N - M; i++) {

            // Check the hash values of current window of text and pattern.
            // If the hash values match then only check for characters one by one
            if (p == t) {
                /* Check for characters one by one. */
                for (j = 0; j < M; j++) {
                    if (txt.charAt(i + j) != pat.charAt(j))
                        break;
                }

                if (j == M) System.out.println("Pattern found at index " + i);
            }

            // Calculate hash value for next window of text: Remove leading digit, add trailing digit.
            if (i < N - M) {
                t = (d * (t - txt.charAt(i) * h) + txt.charAt(i + M)) % q;
                if (t < 0) t = (t + q);
            }
        }
    }
}