RabinKarpAlgorithm.java

// 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
	*/
	static void search(String pat, String txt, int q)
	{
		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 on 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);
			}
	
			// 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);
			}
		}
	}
	
	/* Driver Code */
	public static void main(String[] args)
	{
		String txt = "GEEKS FOR GEEKS";
		String pat = "GEEK";
		
		// A prime number
		int q = 101; 
	
		// Function Call
		search(pat, txt, q);
	}
}

// This code is contributed by nuclode