package 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;
 }
}