82 lines
1.9 KiB
Java
82 lines
1.9 KiB
Java
/** @author Prateek Kumar Oraon (https://github.com/prateekKrOraon) */
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import java.util.Scanner;
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// An implementaion of string matching using finite automata
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public class StringMatchFiniteAutomata {
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public static final int CHARS = 256;
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public static int[][] FA;
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public static Scanner scanner = null;
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public static void main(String[] args) {
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scanner = new Scanner(System.in);
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System.out.println("Enter String");
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String text = scanner.nextLine();
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System.out.println("Enter pattern");
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String pat = scanner.nextLine();
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searchPat(text, pat);
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scanner.close();
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}
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public static void searchPat(String text, String pat) {
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int m = pat.length();
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int n = text.length();
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FA = new int[m + 1][CHARS];
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computeFA(pat, m, FA);
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int state = 0;
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for (int i = 0; i < n; i++) {
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state = FA[state][text.charAt(i)];
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if (state == m) {
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System.out.println("Pattern found at index " + (i - m + 1));
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}
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}
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}
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// Computes finite automata for the partern
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public static void computeFA(String pat, int m, int[][] FA) {
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for (int state = 0; state <= m; ++state) {
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for (int x = 0; x < CHARS; ++x) {
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FA[state][x] = getNextState(pat, m, state, x);
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}
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}
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}
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public static int getNextState(String pat, int m, int state, int x) {
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// if current state is less than length of pattern
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// and input character of pattern matches the character in the alphabet
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// then automata goes to next state
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if (state < m && x == pat.charAt(state)) {
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return state + 1;
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}
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for (int ns = state; ns > 0; ns--) {
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if (pat.charAt(ns - 1) == x) {
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for (int i = 0; i < ns - 1; i++) {
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if (pat.charAt(i) != pat.charAt(state - ns + i + 1)) {
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break;
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}
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if (i == ns - 1) {
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return ns;
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}
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}
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}
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}
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return 0;
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}
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}
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