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/*

Implementation of Knuth–Morris–Pratt algorithm

Usage:

final String T = "AAAAABAAABA";

final String P = "AAAA";

KMPmatcher(T, P);

*/

public class KMP {

// find the starting index in string T[] that matches the search word P[]

public void KMPmatcher(final String T, final String P) {

final int m = T.length();

final int n = P.length();

final int[] pi = computePrefixFunction(P);

int q = 0;

for (int i = 0; i < m; i++) {

while (q > 0 && T.charAt(i) != P.charAt(q)) {

q = pi[q - 1];

}

if (T.charAt(i) == P.charAt(q)) {

q++;

}

if (q == n) {

System.out.println("Pattern starts: " + (i + 1 - n));

q = pi[q - 1];

}

}

}

// return the prefix function

private 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;

}

}