refactor

Author: Samuel Facchinello

src/main/java/com/thealgorithms/others/StringMatchFiniteAutomata.java

@@ -1,80 +1,118 @@
 package com.thealgorithms.others;
 
-/**
- * @author Prateek Kumar Oraon (https://github.com/prateekKrOraon)
- */
 import java.util.Scanner;
+import java.util.Set;
+import java.util.TreeSet;
 
-// An implementation of string matching using finite automata
+/**
+ * A class to perform string matching using <a href="https://en.wikipedia.org/wiki/Finite-state_machine">finite automata</a>.
+ *
+ * @author <a href="https://github.com/prateekKrOraon">Prateek Kumar Oraon</a>
+ */
 public final class StringMatchFiniteAutomata {
+
+    // Constants
+    private static final int CHARS = 256; // Total number of characters in the input alphabet
+
+    // Finite automata table
+    private static int[][] finiteAutomata;
+
+    // Private constructor to prevent instantiation
     private 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();
+        // Scanner instance for user input
+        try (Scanner scanner = new Scanner(System.in);) {
+
+            System.out.println("Enter text:");
+            String text = scanner.nextLine();
 
-        searchPat(text, pat);
+            System.out.println("Enter pattern:");
+            String pattern = scanner.nextLine();
 
-        scanner.close();
+            Set<Integer> indexFound = searchPattern(text, pattern);
+            indexFound.forEach(System.out::println);
+        }
     }
 
-    public static void searchPat(String text, String pat) {
-        int m = pat.length();
-        int n = text.length();
+    /**
+     * Searches for the pattern in the given text using finite automata.
+     *
+     * @param text    The text to search within.
+     * @param pattern The pattern to search for.
+     */
+    public static Set<Integer> searchPattern(String text, String pattern) {
+        Set<Integer> indexFound = new TreeSet<>();
+        int patternLength = pattern.length();
+        int textLength = text.length();
+
+        // Initialize finite automata table
+        finiteAutomata = new int[patternLength + 1][CHARS];
 
-        fa = new int[m + 1][CHARS];
+        // Preprocess the pattern to create the finite automata table
+        computeFiniteAutomata(pattern, patternLength);
 
-        computeFA(pat, m, fa);
+        int state = 0; // Initial state
 
-        int state = 0;
-        for (int i = 0; i < n; i++) {
-            state = fa[state][text.charAt(i)];
+        // Process the text over the finite automata
+        for (int i = 0; i < textLength; i++) {
+            state = finiteAutomata[state][text.charAt(i)];
 
-            if (state == m) {
-                System.out.println("Pattern found at index " + (i - m + 1));
+            if (state == patternLength) {
+                indexFound.add((i - patternLength + 1));
             }
         }
+        return indexFound;
     }
 
-    // Computes finite automata for the pattern
-    public static void computeFA(String pat, int m, int[][] fa) {
-        for (int state = 0; state <= m; ++state) {
+    /**
+     * Computes the finite automata table for the given pattern.
+     *
+     * @param pattern       The pattern to preprocess.
+     * @param patternLength The length of the pattern.
+     */
+    private static void computeFiniteAutomata(String pattern, int patternLength) {
+        for (int state = 0; state <= patternLength; ++state) {
             for (int x = 0; x < CHARS; ++x) {
-                fa[state][x] = getNextState(pat, m, state, x);
+                finiteAutomata[state][x] = getNextState(pattern, patternLength, 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)) {
+    /**
+     * Gets the next state for the finite automata.
+     *
+     * @param pattern       The pattern being matched.
+     * @param patternLength The length of the pattern.
+     * @param state         The current state.
+     * @param x             The current character from the input alphabet.
+     * @return The next state.
+     */
+    private static int getNextState(String pattern, int patternLength, int state, int x) {
+        // If the current state is less than the length of the pattern
+        // and the character matches the pattern character, go to the next state
+        if (state < patternLength && x == pattern.charAt(state)) {
             return state + 1;
         }
 
+        // Check for the highest prefix which is also a suffix
         for (int ns = state; ns > 0; ns--) {
-            if (pat.charAt(ns - 1) == x) {
+            if (pattern.charAt(ns - 1) == x) {
+                boolean match = true;
                 for (int i = 0; i < ns - 1; i++) {
-                    if (pat.charAt(i) != pat.charAt(state - ns + i + 1)) {
+                    if (pattern.charAt(i) != pattern.charAt(state - ns + i + 1)) {
+                        match = false;
                         break;
                     }
-
-                    if (i == ns - 1) {
-                        return ns;
-                    }
+                }
+                if (match) {
+                    return ns;
                 }
             }
         }
 
+        // If no prefix which is also a suffix is found, return 0
         return 0;
     }
 }