Add Aho-Corasick string pattern matching algorithm in Java (fixes #149)

Author: CodingPrakharKapoor

Java/strings/AhoCorasick.java

@@ -0,0 +1,105 @@
+import java.util.*;
+
+class AhoCorasick {
+
+    // Maximum possible states = sum of lengths of all keywords
+    static final int MAX_STATES = 500;
+    // Number of lowercase letters
+    static final int ALPHABET_SIZE = 26;
+
+    // Output mask: bit i is 1 if word[i] ends at this state
+    static int[] output = new int[MAX_STATES];
+    // Failure links
+    static int[] failure = new int[MAX_STATES];
+    // Transition table (trie)
+    static int[][] transition = new int[MAX_STATES][ALPHABET_SIZE];
+
+    // Build the automaton from the given keywords
+    static int buildMachine(String[] words, int wordCount) {
+        Arrays.fill(output, 0);
+        for (int[] row : transition)
+            Arrays.fill(row, -1);
+
+        int stateCount = 1; // root = 0
+
+        // Build trie
+        for (int i = 0; i < wordCount; i++) {
+            String word = words[i];
+            int state = 0;
+            for (char ch : word.toCharArray()) {
+                int idx = ch - 'a';
+                if (transition[state][idx] == -1)
+                    transition[state][idx] = stateCount++;
+                state = transition[state][idx];
+            }
+            output[state] |= (1 << i);
+        }
+
+        // Fill missing transitions from root to itself
+        for (int c = 0; c < ALPHABET_SIZE; c++)
+            if (transition[0][c] == -1)
+                transition[0][c] = 0;
+
+        Arrays.fill(failure, -1);
+        Queue<Integer> queue = new LinkedList<>();
+
+        // Initialize failure links for depth-1 states
+        for (int c = 0; c < ALPHABET_SIZE; c++) {
+            if (transition[0][c] != 0) {
+                failure[transition[0][c]] = 0;
+                queue.add(transition[0][c]);
+            }
+        }
+
+        // BFS to build failure links
+        while (!queue.isEmpty()) {
+            int state = queue.remove();
+            for (int c = 0; c < ALPHABET_SIZE; c++) {
+                int next = transition[state][c];
+                if (next != -1) {
+                    int f = failure[state];
+                    while (transition[f][c] == -1)
+                        f = failure[f];
+                    f = transition[f][c];
+                    failure[next] = f;
+                    output[next] |= output[f];
+                    queue.add(next);
+                }
+            }
+        }
+        return stateCount;
+    }
+
+    // Get next state for a given input character
+    static int nextState(int current, char input) {
+        int idx = input - 'a';
+        while (transition[current][idx] == -1)
+            current = failure[current];
+        return transition[current][idx];
+    }
+
+    // Search all occurrences of patterns in text
+    static void search(String[] words, int wordCount, String text) {
+        buildMachine(words, wordCount);
+        int state = 0;
+
+        for (int i = 0; i < text.length(); i++) {
+            state = nextState(state, text.charAt(i));
+            if (output[state] == 0) continue;
+
+            for (int j = 0; j < wordCount; j++) {
+                if ((output[state] & (1 << j)) > 0) {
+                    System.out.println("Word \"" + words[j] + "\" found at [" +
+                            (i - words[j].length() + 1) + ", " + i + "]");
+                }
+            }
+        }
+    }
+
+    // Driver
+    public static void main(String[] args) {
+        String[] patterns = { "he", "she", "hers", "his" };
+        String text = "ahishers";
+        search(patterns, patterns.length, text);
+    }
+}