How to implement Boyer-Moore algorithm using java

How to implement the Boyer-Moore algorithm using Java

Introduction:
In computer science, string matching is a common task. The string matching algorithm is the key to solving this problem. One of the efficient string matching algorithms is the Boyer-Moore algorithm. This article will introduce how to use Java language to implement this algorithm, and attach specific code examples.

The principle of Boyer-Moore algorithm:
Boyer-Moore algorithm is a multi-pattern string matching algorithm. It completes matching by preprocessing the pattern string and combining good suffix rules and bad character rules. The core idea is to skip unmatched characters as much as possible during the matching process between the pattern string and the string to be matched, thereby improving the matching efficiency.

Specific implementation steps:

1. Preprocessing pattern string:
   First, we need to preprocess the pattern string and generate two arrays: bad character array and Nice suffix array.

   • Bad character array: stores the rightmost position of each character in the pattern string.
   • Good suffix array: record the rightmost occurrence position of the suffix substring of the pattern string in the pattern string, and record whether this substring matches the prefix of the pattern string.

2. Matching process:
   During the matching process, we start matching forward from the end of the string to be matched.

   • First, align the end of the pattern string with the end of the string to be matched, and try to match.
   • If the match is successful, return the starting position of the match, otherwise move the position of the pattern string according to the bad character and good suffix rules to continue matching.

The specific code is as follows:

import java.util.Arrays;

public class BoyerMoore {

    private static final int NO_OF_CHARS = 256;

    private int[] badCharShift;
    private int[] suffixShift;
    private boolean[] goodSuffix;

    public void preProcessPattern(String pattern) {
        int m = pattern.length();
        // 初始化数组
        badCharShift = new int[NO_OF_CHARS];
        suffixShift = new int[m + 1];
        goodSuffix = new boolean[m + 1];

        Arrays.fill(badCharShift, -1);
        for (int i = 0; i < m; i++) {
            badCharShift[pattern.charAt(i)] = i;
        }

        int f = 0;
        int g = 0;
        suffixShift[m] = m + 1;

        for (int i = m - 1; i >= 0; i--) {
            if (i > f && suffixShift[i + m - f] < i - f) {
                suffixShift[i] = suffixShift[i + m - f];
            } else {
                if (i < f) {
                    f = i;
                }
                g = i;
                while (f >= 0 && pattern.charAt(f) == pattern.charAt(f + m - g)) {
                    f--;
                }
                suffixShift[i] = g - f;
            }
        }

        for (int i = 0; i < m; i++) {
            goodSuffix[i] = suffixShift[i] > m - i;
        }
    }

    public int search(String text, String pattern) {
        int n = text.length();
        int m = pattern.length();
        int i = 0;

        while (i <= n - m) {
            int j = m - 1;
            while (j >= 0 && pattern.charAt(j) == text.charAt(i + j)) {
                j--;
            }
            if (j < 0) {
                return i; // 匹配成功，返回匹配位置
            } else {
                i += Math.max(goodSuffix[j + 1], j - badCharShift[text.charAt(i + j)]);
            }
        }
        return -1; // 未匹配成功，返回-1
    }

    public static void main(String[] args) {
        BoyerMoore bm = new BoyerMoore();
        String text = "This is a test";
        String pattern = "test";
        bm.preProcessPattern(pattern);
        int index = bm.search(text, pattern);
        if (index != -1) {
            System.out.println("Pattern found at index: " + index);
        } else {
            System.out.println("Pattern not found");
        }
    }
}

Summary:
This article introduces how to use Java language to implement the Boyer-Moore algorithm, and through specific code Examples demonstrate the use of the algorithm. The Boyer-Moore algorithm has high efficiency and wide application in the field of string matching. By rationally utilizing good suffix and bad character rules, the efficiency of string matching can be greatly improved. I hope this article will help you understand and practice the Boyer-Moore algorithm.

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