57 lines
2.0 KiB
Java
57 lines
2.0 KiB
Java
package com.jwetherell.algorithms.strings;
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import java.util.ArrayList;
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import java.util.List;
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/**
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* This class implements KMP algorithm for finding length of maximal prefix-suffix for each prefix of the string.
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* Prefix-suffix of string S is a substring which occurs at the beginning and at the end of S.
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* <p>
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* Time complexity: O(n) <br>
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* <br>
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* https://en.wikipedia.org/wiki/Knuth%E2%80%93Morris%E2%80%93Pratt_algorithm
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* <br>
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* @author Szymon Stankiewicz <mail@stankiewicz.me>
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* @author Justin Wetherell <phishman3579@gmail.com>
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*/
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public class KnuthMorrisPratt {
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private KnuthMorrisPratt() {}
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/**
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* This function implements KMP algorithm for finding length of maximal prefix-suffix for each prefix of the string.
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* Prefix-suffix of string S is a substring which occurs at the beginning and at the end of S.
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* <p>
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* Time complexity: O(n)<p>
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* <p>
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* https://en.wikipedia.org/wiki/Knuth%E2%80%93Morris%E2%80%93Pratt_algorithm
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* <br>
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* @param text Text
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* @return maximal length of prefix-suffix for each prefix of the string text
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*/
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public static List<Integer> getPrefSufTable(String text) {
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final List<Integer> prefSufTable = new ArrayList<Integer>();
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final char[] chars = text.toCharArray();
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if (text.length() == 0)
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return prefSufTable;
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prefSufTable.add(0);
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for (int i = 1; i<chars.length; i++) {
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int sizeOfPrefSuf = prefSufTable.get(i-1);
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while (sizeOfPrefSuf > 0 && (chars[i] != chars[sizeOfPrefSuf]))
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sizeOfPrefSuf = prefSufTable.get(sizeOfPrefSuf-1); // because string is 0-indexed
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// if characters at this positions are different then sizeOfPrefSuf is equal to zero,
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// so there is no proper prefix-suffix
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if (chars[i] == chars[sizeOfPrefSuf]) {
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prefSufTable.add(sizeOfPrefSuf+1);
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} else {
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prefSufTable.add(0);
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}
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}
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return prefSufTable;
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}
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}
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