Rabin_Karp_Algorithm.cpp

// Given a text T[0. . .n-1] and a pattern P[0. . .m-1], write a function search(char P[], char T[]) that prints all occurrences of P[] present in T[] using Rabin Karp algorithm. You may assume that n > m.

// Examples: 
// Input:  T[] = “THIS IS A TEST TEXT”, P[] = “TEST”
// Output: Pattern found at index 10

/* Following program is a C++ implementation of Rabin Karp
Algorithm given in the CLRS book */
#include <bits/stdc++.h>
using namespace std;

// Search the pat string in the txt string 
void search(string pat, string txt, int q)
{
    int M = pat.size();
    int N = txt.size();
    int i, j;
    int p = 0; // hash value for pattern
    int t = 0; // hash value for txt
    int h = 1;
    int d = 256; // d is the number of characters in the input alphabet 
    // 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[i]) % q;
        t = (d * t + txt[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[i + j] != pat[j]) {
                    break;
                }
            }

            // if p == t and pat[0...M-1] = txt[i, i+1,
            // ...i+M-1]

            if (j == M)
                cout << "Pattern found at index " << i
                     << endl;
        }

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

            // We might get negative value of t, converting
            // it to positive
            if (t < 0)
                t = (t + q);
        }
    }
}

/* Driver code */
int main()
{
    string txt = "GEEKS FOR GEEKS";
    string pat = "GEEK";

    // we mod to avoid overflowing of value but we should
    // take as big q as possible to avoid the collison
    int q = INT_MAX;
    // Function Call
    search(pat, txt, q);
    return 0;
}

// This is code is contributed by rathbhupendra