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package com.thealgorithms.strings;

import java.util.ArrayList;

import java.util.List;

/**

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

* for an example

*/

public final class KMP {

private KMP() {

}

/**

* find the starting index in string haystack[] that matches the search word P[]

*

* @param haystack The text to be searched

* @param needle The pattern to be searched for

* @return A list of starting indices where the pattern is found

*/

public static List<Integer> kmpMatcher(final String haystack, final String needle) {

List<Integer> occurrences = new ArrayList<>();

if (haystack == null || needle == null || needle.isEmpty()) {

return occurrences;

}

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) {

occurrences.add(i + 1 - n);

q = pi[q - 1];

}

}

return occurrences;

}

// 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;

}

}