AC 自动机

多pattern的匹配， 比KMP更general

public class ACAutomata {
	static final int R = 256;
	private class Node {
		Node next[] = new Node[R];
		Node fail;
		String prefix;
		int count;
	}
	public Node root = new Node();

	ACAutomata(String[] patterns) {
		for (String s : patterns) insert(s);
		buildFailPointers();
	}
	private Node insert(Node root, String word, int i) {
		if (root == null) root = new Node();
		root.prefix = word.substring(0, i);
		if (i == word.length()) {
			root.count += 1;
			return root;
		}
		root.next[word.charAt(i)] = insert(root.next[word.charAt(i)], word, i + 1);
		return root;
	}
	private void insert(String word) {
		root = insert(root, word, 0);
	}
	
	private void buildFailPointers() {
		root.fail = null;
		Queue<Node> q = new LinkedList<Node>();
		for (Node x : root.next) {
			if (x != null) {
				x.fail = root;
				q.offer(x);
			}
		}	
		for (; !q.isEmpty(); ) {
			Node x = q.poll();
			for (int i = 0; i < R; ++i) {
				if (x.next[i] != null) {
					Node p = x.fail;
					for (; p != null && p.next[i] == null; p = p.fail) ;
					x.next[i].fail = p != null ? p.next[i] : root;
					q.offer(x.next[i]);
				}
			}
		}
	}
	
	public int match(String text) {
		Node x = root;
		int num = 0;
		for (int i = 0; i < text.length(); ++i) {			
			if (x.next[text.charAt(i)] != null) 
				x = x.next[text.charAt(i)]; // match case
			else if (x != root)
				x = x.fail.next[text.charAt(i)] != null ? x.fail.next[text.charAt(i)] : root;
			if (x.count > 0 ) {
				num += x.count;
				System.out.println(x.prefix + " at " + (i - x.prefix.length() + 1));
			}
		}		
		return num;
	}
}