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KMP.java
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57 lines (49 loc) · 1.62 KB
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package Others;
/**
* Implementation of Knuth–Morris–Pratt algorithm
* Usage: see the main function for an example
*/
public class 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;
}
}