54 lines
1.4 KiB
Java
54 lines
1.4 KiB
Java
package Others;
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/** Implementation of Knuth–Morris–Pratt algorithm Usage: see the main function for an example */
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public class KMP {
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// a working example
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public static void main(String[] args) {
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final String haystack = "AAAAABAAABA"; // This is the full string
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final String needle = "AAAA"; // This is the substring that we want to find
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KMPmatcher(haystack, needle);
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}
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// find the starting index in string haystack[] that matches the search word P[]
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public static void KMPmatcher(final String haystack, final String needle) {
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final int m = haystack.length();
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final int n = needle.length();
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final int[] pi = computePrefixFunction(needle);
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int q = 0;
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for (int i = 0; i < m; i++) {
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while (q > 0 && haystack.charAt(i) != needle.charAt(q)) {
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q = pi[q - 1];
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}
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if (haystack.charAt(i) == needle.charAt(q)) {
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q++;
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}
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if (q == n) {
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System.out.println("Pattern starts: " + (i + 1 - n));
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q = pi[q - 1];
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}
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}
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}
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// return the prefix function
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private static int[] computePrefixFunction(final String P) {
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final int n = P.length();
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final int[] pi = new int[n];
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pi[0] = 0;
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int q = 0;
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for (int i = 1; i < n; i++) {
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while (q > 0 && P.charAt(q) != P.charAt(i)) {
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q = pi[q - 1];
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}
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if (P.charAt(q) == P.charAt(i)) {
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q++;
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}
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pi[i] = q;
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}
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return pi;
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}
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}
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