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import java.util.Random;
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// https://en.wikipedia.org/wiki/Knuth–Morris–Pratt_algorithm
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public class Kmp {
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public static int[] prefixFunction(String s) {
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int[] p = new int[s.length()];
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int k = 0;
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for (int i = 1; i < s.length(); i++) {
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while (k > 0 && s.charAt(k) != s.charAt(i))
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k = p[k - 1];
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if (s.charAt(k) == s.charAt(i))
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++k;
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p[i] = k;
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}
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return p;
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}
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public static int kmpMatcher(String s, String pattern) {
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int m = pattern.length();
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if (m == 0)
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return 0;
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int[] p = prefixFunction(pattern);
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for (int i = 0, k = 0; i < s.length(); i++) {
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while (k > 0 && pattern.charAt(k) != s.charAt(i))
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k = p[k - 1];
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if (pattern.charAt(k) == s.charAt(i))
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++k;
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if (k == m)
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return i + 1 - m;
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}
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return -1;
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}
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// random tests
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public static void main(String[] args) {
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Random rnd = new Random(1);
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for (int step = 0; step < 10_000; step++) {
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String s = getRandomString(rnd, 100);
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String pattern = getRandomString(rnd, 5);
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int pos1 = kmpMatcher(s, pattern);
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int pos2 = s.indexOf(pattern);
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if (pos1 != pos2)
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throw new RuntimeException();
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}
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}
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static String getRandomString(Random rnd, int maxlen) {
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int n = rnd.nextInt(maxlen);
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char[] s = new char[n];
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for (int i = 0; i < n; i++)
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s[i] = (char) ('a' + rnd.nextInt(3));
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return new String(s);
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}
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}
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