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;
 }
}