Java Implementation of the Knuth-Morris-Pratt (KMP) Algorithm for Pattern Search

[shriyanshi]

Hello everyone,

I wanted to share a Java implementation of the Knuth-Morris-Pratt (KMP) algorithm in java for pattern searching. The KMP algorithm is a powerful string-searching algorithm that efficiently finds occurrences of a pattern within a text.

Here's a Java code snippet that demonstrates how to implement the KMP algorithm for pattern search:

import java.util.ArrayList;
import java.util.List;

public class KMPAlgorithm {
    public static List<Integer> search(String text, String pattern) {
        List<Integer> occurrences = new ArrayList<>();
        int[] lps = computeLPSArray(pattern);
        int textIndex = 0, patternIndex = 0;

        while (textIndex < text.length()) {
            if (pattern.charAt(patternIndex) == text.charAt(textIndex)) {
                patternIndex++;
                textIndex++;
            }

            if (patternIndex == pattern.length()) {
                occurrences.add(textIndex - patternIndex);
                patternIndex = lps[patternIndex - 1];
            } else if (textIndex < text.length() && pattern.charAt(patternIndex) != text.charAt(textIndex)) {
                if (patternIndex != 0) {
                    patternIndex = lps[patternIndex - 1];
                } else {
                    textIndex++;
                }
            }
        }

        return occurrences;
    }

    private static int[] computeLPSArray(String pattern) {
        int[] lps = new int[pattern.length()];
        int len = 0;
        int i = 1;

        while (i < pattern.length()) {
            if (pattern.charAt(i) == pattern.charAt(len)) {
                len++;
                lps[i] = len;
                i++;
            } else {
                if (len != 0) {
                    len = lps[len - 1];
                } else {
                    lps[i] = 0;
                    i++;
                }
            }
        }

        return lps;
    }

    public static void main(String[] args) {
        String text = "ABABDABACDABABCABAB";
        String pattern = "ABABCABAB";
        List<Integer> occurrences = search(text, pattern);

        if (occurrences.isEmpty()) {
            System.out.println("Pattern not found in the text.");
        } else {
            System.out.println("Pattern found at positions: " + occurrences);
        }
    }
}

Feel free to use and modify this code for your pattern-searching needs.