package com.thealgorithms.searches;

// Following program is a Java implementation
// of Rabin Karp Algorithm given in the CLRS book

public class RabinKarpAlgorithm {

    // d is the number of characters in the input alphabet
    public static final int d = 256;

    /* pat -> pattern
        txt -> text
        q -> A prime number
    */
    public int search(String pat, String txt, int q) {
        int index = -1; // note: -1 here represent not found, it is not an index
        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 p == t and pat[0...M-1] = txt[i, i+1, ...i+M-1]
                if (j == M) {
                    System.out.println("Pattern found at index " + i);
                    index = i;
                    return index;
                }
            }

            // 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;

                // We might get negative value of t, converting it
                // to positive
                if (t < 0) t = (t + q);
            }
        }
        return index; // return -1 if pattern does not found
    }
}
// This code is contributed by nuclode
