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package com.thealgorithms.strings;

/**

* Implementation of Knuth–Morris–Pratt algorithm Usage: see the main function

* for an example

*/

public final class KMP {

private 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;

}

}