102 lines
3.4 KiB
Java
102 lines
3.4 KiB
Java
/*
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* The MIT License (MIT)
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*
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* Copyright © 2022 xrv <xrv@live.com>
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*
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* Permission is hereby granted, free of charge, to any person obtaining a copy
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* of this software and associated documentation files (the "Software"), to deal
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* in the Software without restriction, including without limitation the rights
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* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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* copies of the Software, and to permit persons to whom the Software is
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* furnished to do so, subject to the following conditions:
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*
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* The above copyright notice and this permission notice shall be included in
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* all copies or substantial portions of the Software.
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*
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* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
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* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
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* THE SOFTWARE.
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*/
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package io.github.ehlxr.algorithm.match;
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import java.util.Arrays;
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/**
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* 字符串匹配算法 KPM
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*
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* @author ehlxr
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* @since 2022-03-13 10:06.
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*/
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public class Kmp {
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public static void main(String[] args) {
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String s = "bacbababaabcbab";
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String p = "abab";
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System.out.println(Arrays.toString(getNexts(p)));
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System.out.println(kmp(s, p));
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}
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public static int kmp(String s, String p) {
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char[] scs = s.toCharArray();
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char[] pcs = p.toCharArray();
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int m = s.length();
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int n = p.length();
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int[] next = getNexts(p);
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for (int i = 0; i <= m - n; i++) {
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int j = 0;
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while (j < n) {
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if (scs[i] == pcs[j]) {
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i++;
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j++;
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} else {
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// 当模式串与主串不匹配时，如果**不匹配字符**对应模式串下标大于 j > 0 (非首个模式串字符)，
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// 并且此字符前一个字符对应字符串部分匹配表中的值 next[j - 1] 也大于 0，
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// 模式串后移到 next[j - 1] 位置
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if (j > 0 && next[j - 1] > 0) {
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j = next[j - 1];
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} else {
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break;
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}
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}
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}
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if (j == n) {
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return i - n;
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}
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}
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return -1;
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}
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private static int[] getNexts(String p) {
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int m = p.length();
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char[] b = p.toCharArray();
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int[] next = new int[m];
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next[0] = 0;
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int k = 0; // 表示前后缀相匹配的最大长度
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for (int i = 1; i < m; ++i) {
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// k 为 b[0, i-1] 子串最大匹配前、后缀长度
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// b[0, k] 为 b[0, i-1] 子串最大匹配前缀子串
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while (k != 0 && b[k] != b[i]) {
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// 若：b[k] != b[i]，则求 b[0, i] 子串最大匹配前、后缀长度问题转换成了求 b[0, k] 子串最大匹配前、后缀长度问题
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k = next[k];
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}
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if (b[k] == b[i]) {
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// 若：b[k] == b[i]，则 b[0, i] 子串最大匹配前、后缀长度为 k + 1
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++k;
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}
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next[i] = k;
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}
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return next;
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}
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}
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