Merge pull request #760 from Mariamawitc/Aho-Corasick

 Java file for Aho-Corasick searching algorithm

AhoCorasick.java

@@ -0,0 +1,87 @@
+import java.util.*;
+
+public class AhoCorasick {
+
+    // Trie Node Class
+    static class TrieNode {
+        Map<Character, TrieNode> children = new HashMap<>();
+        TrieNode failureLink = null;
+        List<String> output = new ArrayList<>();
+    }
+
+    private TrieNode root = new TrieNode();
+
+    // Step 1: Build the Trie
+    public void addKeyword(String keyword) {
+        TrieNode currentNode = root;
+        for (char c : keyword.toCharArray()) {
+            currentNode = currentNode.children.computeIfAbsent(c, k -> new TrieNode());
+        }
+        currentNode.output.add(keyword);
+    }
+
+    // Step 2: Build Failure Links
+    public void buildFailureLinks() {
+        Queue<TrieNode> queue = new LinkedList<>();
+        root.failureLink = root;
+        queue.add(root);
+
+        while (!queue.isEmpty()) {
+            TrieNode current = queue.poll();
+
+            for (Map.Entry<Character, TrieNode> entry : current.children.entrySet()) {
+                char c = entry.getKey();
+                TrieNode child = entry.getValue();
+
+                // Set failure link
+                TrieNode failure = current.failureLink;
+                while (failure != root && !failure.children.containsKey(c)) {
+                    failure = failure.failureLink;
+                }
+
+                if (failure.children.containsKey(c) && failure.children.get(c) != child) {
+                    child.failureLink = failure.children.get(c);
+                } else {
+                    child.failureLink = root;
+                }
+
+                // Merge output from failure link
+                child.output.addAll(child.failureLink.output);
+                queue.add(child);
+            }
+        }
+    }
+
+    // Step 3: Search the Text
+    public List<String> search(String text) {
+        List<String> results = new ArrayList<>();
+        TrieNode currentNode = root;
+
+        for (char c : text.toCharArray()) {
+            while (currentNode != root && !currentNode.children.containsKey(c)) {
+                currentNode = currentNode.failureLink;
+            }
+            if (currentNode.children.containsKey(c)) {
+                currentNode = currentNode.children.get(c);
+            }
+
+            // Add matches to results
+            results.addAll(currentNode.output);
+        }
+
+        return results;
+    }
+
+    public static void main(String[] args) {
+        AhoCorasick ac = new AhoCorasick();
+        ac.addKeyword("he");
+        ac.addKeyword("she");
+        ac.addKeyword("hers");
+        ac.addKeyword("his");
+
+        ac.buildFailureLinks();
+
+        List<String> matches = ac.search("ushers");
+        System.out.println("Matches found: " + matches);
+    }
+}