RabinKarp.java

/**
 * @author Prateek Kumar Oraon (https://github.com/prateekKrOraon)
 */
import java.util.Scanner;
import java.lang.Math;

//An implementation of Rabin-Karp string matching algorithm
//Program will simply end if there is no match
public class RabinKarp {

    public static Scanner scanner = null;
    public final static int d = 256;

    public static void main(String[] args){

        scanner = new Scanner(System.in);
        System.out.println("Enter String");
        String text = scanner.nextLine();
        System.out.println("Enter pattern");
        String pattern = scanner.nextLine();

        int q = 101;
        searchPat(text,pattern,q);

    }

    private static void searchPat(String text, String pattern, int q) {

        int m = pattern.length();
        int n = text.length();
        int t = 0;
        int p = 0;
        int h = 1;
        int j = 0;
        int i =	0;

        h = (int)Math.pow(d,m-1)%q;

        for(i =0 ; i< m; i++){
			//hash value is calculated for each character and then added with the hash value of the next character for pattern
			// as well as the text for length equal to the length of pattern
            p = (d*p + pattern.charAt(i))%q;
            t = (d*t + text.charAt(i))%q;
        }

        for(i=0; i<=n-m;i++){
			
			//if the calculated hash value of the pattern and text matches then
			//all the characters of the pattern is matched with the text of length equal to length of the pattern
			//if all matches then pattern exist in string
			//if not then the hash value of the first character of the text is subtracted and hash value of the next character after the end
			//of the evaluated characters is added 
            if(p==t){

				//if hash value matches then the individual characters are matched
                for(j=0;j<m;j++){
                
					//if not matched then break out of the loop
					if(text.charAt(i+j) != pattern.charAt(j))
						break;
                }

				//if all characters are matched then pattern exist in the string
                if (j==m){
                    System.out.println("Pattern found at index " + i);
                }

            }

			//if i<n-m then hash value of the first character of the text is subtracted and hash value of the next character after the end
			//of the evaluated characters is added to get the hash value of the next window of characters in the text
            if ( i < n-m )
            {
                t = (d*(t - text.charAt(i)*h) + text.charAt(i+m))%q;

				//if hash value becomes less than zero than q is added to make it positive
                if (t < 0)
                    t = (t + q);
            }
        }

    }

}