Graph.java

/***
 * Copyright (c) 2009 Caelum - www.caelum.com.br/opensource All rights reserved.
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not
 * use this file except in compliance with the License. You may obtain a copy of
 * the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 * License for the specific language governing permissions and limitations under
 * the License.
 */
package br.com.caelum.vraptor.interceptor;

import static com.google.common.base.Preconditions.checkState;
import static com.google.common.collect.Sets.difference;
import static com.google.common.collect.Sets.newHashSet;

import java.util.ArrayList;
import java.util.List;
import java.util.Set;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

import javax.enterprise.inject.Vetoed;

import br.com.caelum.vraptor.Intercepts;

import com.google.common.collect.LinkedHashMultimap;
import com.google.common.collect.Multimap;

/**
 * A set that orders interceptors topologically based on before and after from {@link Intercepts}
 *
 * @author Lucas Cavalcanti
 * @author David Paniz
 * @author Jose Donizetti
 * @since 3.3.0
 *
 */
@Vetoed
public class Graph<E> {

	private final Multimap<E, E> graph = LinkedHashMultimap.create();
	private List<E> orderedList;

	private final Lock lock = new ReentrantLock();

	public void addEdge(E from, E to) {
		checkState(orderedList == null, "You shouldn't add more interceptors after ordering. Please notify vraptor developers.");
		graph.put(from, to);
	}

	public void addEdges(E from, E... tos) {

		if (tos.length == 0) {
			addEdge(from, null);
		} else {
			for (E to : tos) {
				addEdge(from, to);
			}
		}
	}

	public List<E> topologicalOrder() {
		if (orderedList == null) {
			lock.lock();
			try {
				if (orderedList == null) {
					this.orderedList = orderTopologically();
				}
			} finally {
				lock.unlock();
			}
		}
		return this.orderedList;
	}

	private List<E> orderTopologically() {
		List<E> list = new ArrayList<>();

		while(!graph.keySet().isEmpty()) {
			Set<E> roots = findRoots();

			if (roots.isEmpty()) {
				throw new IllegalStateException("There is a cycle on the interceptor sequence: \n" + cycle());
			}

			list.addAll(roots);
			removeRoots(roots);
		}
		return list;
	}

	private void removeRoots(Set<E> roots) {
		for (E root : roots) {
			graph.removeAll(root);
		}
	}

	private Set<E> findRoots() {
		return difference(graph.keySet(), newHashSet(graph.values())).immutableCopy();
	}

	private String cycle() {
		removeLeaves();

		return findCycle().toString();
	}

	private List<E> findCycle() {
		E node = firstElement(graph.keySet());
		List<E> cycle = new ArrayList<>();
		do {
			cycle.add(node);
		} while(!cycle.contains(node = firstElement(graph.get(node))));

		return cycle.subList(cycle.indexOf(node), cycle.size());
	}

	private E firstElement(Iterable<E> elements) {
		return elements.iterator().next();
	}

	private void removeLeaves() {
		Set<E> leaves = findLeaves();
		if (leaves.isEmpty()) {
			return;
		}

		for (E key : newHashSet(graph.keySet())) {
			for (E value : leaves) {
				graph.remove(key, value);
			}
		}
		removeLeaves();
	}

	private Set<E> findLeaves() {
		return difference(newHashSet(graph.values()), graph.keySet());
	}
}