JavaAlgorithms/Others/RabinKarp.java

package Others;

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

// 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 static final 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);
      }
    }
  }
}