91 lines
1.7 KiB
Java
91 lines
1.7 KiB
Java
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/**
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* @author Prateek Kumar Oraon (https://github.com/prateekKrOraon)
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*/
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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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