Rabin-Karp.cpp

#include<bits/stdc++.h>
using namespace std;

#define d 256

/* pat -> pattern
    txt -> text
    q -> A prime number
*/
void Search(char pat[], char txt[], int q)
{
    int M = strlen(pat);
    int N = strlen(txt);
    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[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 on 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 program to test above function */
int main()
{
    char txt[] = "Hello Hi Hello";
    char pat[] = "Hello";
    int q = 101; // A prime number
    Search(pat, txt, q);
    return 0;
}

/*
Output
Pattern found at index 0
Pattern found at index 9
*/