JavaAlgorithms/Others/KMP.java

package Others;

/** Implementation of Knuth–Morris–Pratt algorithm Usage: see the main function for an example */
public class KMP {
  // a working example
  public static void main(String[] args) {
    final String haystack = "AAAAABAAABA"; // This is the full string
    final String needle = "AAAA"; // This is the substring that we want to find
    KMPmatcher(haystack, needle);
  }

  // find the starting index in string haystack[] that matches the search word P[]
  public static void KMPmatcher(final String haystack, final String needle) {
    final int m = haystack.length();
    final int n = needle.length();
    final int[] pi = computePrefixFunction(needle);
    int q = 0;
    for (int i = 0; i < m; i++) {
      while (q > 0 && haystack.charAt(i) != needle.charAt(q)) {
        q = pi[q - 1];
      }

      if (haystack.charAt(i) == needle.charAt(q)) {
        q++;
      }

      if (q == n) {
        System.out.println("Pattern starts: " + (i + 1 - n));
        q = pi[q - 1];
      }
    }
  }

  // return the prefix function
  private static int[] computePrefixFunction(final String P) {
    final int n = P.length();
    final int[] pi = new int[n];
    pi[0] = 0;
    int q = 0;
    for (int i = 1; i < n; i++) {
      while (q > 0 && P.charAt(q) != P.charAt(i)) {
        q = pi[q - 1];
      }

      if (P.charAt(q) == P.charAt(i)) {
        q++;
      }

      pi[i] = q;
    }
    return pi;
  }
}