The Algorithms logo
The Algorithms
AboutDonate

String Match Finite Automata

A
package com.thealgorithms.others;

/**
 * @author Prateek Kumar Oraon (https://github.com/prateekKrOraon)
 */
import java.util.Scanner;

// An implementaion of string matching using finite automata
public class StringMatchFiniteAutomata {

    public static final int CHARS = 256;
    public static int[][] FA;
    public static Scanner scanner = null;

    public static void main(String[] args) {

        scanner = new Scanner(System.in);
        System.out.println("Enter String");
        String text = scanner.nextLine();
        System.out.println("Enter pattern");
        String pat = scanner.nextLine();

        searchPat(text, pat);

        scanner.close();
    }

    public static void searchPat(String text, String pat) {

        int m = pat.length();
        int n = text.length();

        FA = new int[m + 1][CHARS];

        computeFA(pat, m, FA);

        int state = 0;
        for (int i = 0; i < n; i++) {
            state = FA[state][text.charAt(i)];

            if (state == m) {
                System.out.println("Pattern found at index " + (i - m + 1));
            }
        }
    }

    // Computes finite automata for the partern
    public static void computeFA(String pat, int m, int[][] FA) {

        for (int state = 0; state <= m; ++state) {
            for (int x = 0; x < CHARS; ++x) {
                FA[state][x] = getNextState(pat, m, state, x);
            }
        }
    }

    public static int getNextState(String pat, int m, int state, int x) {

        // if current state is less than length of pattern
        // and input character of pattern matches the character in the alphabet
        // then automata goes to next state
        if (state < m && x == pat.charAt(state)) {
            return state + 1;
        }

        for (int ns = state; ns > 0; ns--) {

            if (pat.charAt(ns - 1) == x) {

                for (int i = 0; i < ns - 1; i++) {

                    if (pat.charAt(i) != pat.charAt(state - ns + i + 1)) {
                        break;
                    }

                    if (i == ns - 1) {
                        return ns;
                    }
                }
            }
        }

        return 0;
    }
}