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