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 final static 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