/*
 * Copyright (c) 2011 Big Switch Networks, Inc.
 *
 * Licensed under the Eclipse Public License, Version 1.0 (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.eclipse.org/legal/epl-v10.html
 *
 * 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.
 *
 * This file incorporates work covered by the following copyright and
 * permission notice:
 *
 *    Originally created by David Erickson, Stanford University
 *
 *    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 org.opendaylight.controller.sal.utils;

import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
 * Maintain lists of listeners ordered by dependency.
 *
 * @author readams
 *
 */
public class ListenerDispatcher<U, T extends IListener<U>> {
    protected static Logger logger = LoggerFactory
            .getLogger(ListenerDispatcher.class);
    volatile List<T> listeners = new ArrayList<T>();

    private void visit(List<T> newlisteners, U type, HashSet<T> visited,
            List<T> ordering, T listener) {
        if (!visited.contains(listener)) {
            visited.add(listener);

            for (T i : newlisteners) {
                if (ispre(type, i, listener)) {
                    visit(newlisteners, type, visited, ordering, i);
                }
            }
            ordering.add(listener);
        }
    }

    private boolean ispre(U type, T l1, T l2) {
        return (l2.isCallbackOrderingPrereq(type, l1.getName()) || l1
                .isCallbackOrderingPostreq(type, l2.getName()));
    }

    /**
     * Add a listener to the list of listeners
     *
     * @param listener
     */
    public void addListener(U type, T listener) {
        List<T> newlisteners = new ArrayList<T>();
        if (listeners != null)
            newlisteners.addAll(listeners);

        newlisteners.add(listener);
        // Find nodes without outgoing edges
        List<T> terminals = new ArrayList<T>();
        for (T i : newlisteners) {
            boolean isterm = true;
            for (T j : newlisteners) {
                if (ispre(type, i, j)) {
                    isterm = false;
                    break;
                }
            }
            if (isterm) {
                terminals.add(i);
            }
        }

        if (terminals.size() == 0) {
            logger.error("No listener dependency solution: "
                    + "No listeners without incoming dependencies");
            listeners = newlisteners;
            return;
        }

        // visit depth-first traversing in the opposite order from
        // the dependencies. Note we will not generally detect cycles
        HashSet<T> visited = new HashSet<T>();
        List<T> ordering = new ArrayList<T>();
        for (T term : terminals) {
            visit(newlisteners, type, visited, ordering, term);
        }
        listeners = ordering;
    }

    /**
     * Remove the given listener
     *
     * @param listener
     *            the listener to remove
     */
    public void removeListener(T listener) {
        if (listeners != null) {
            List<T> newlisteners = new ArrayList<T>();
            newlisteners.addAll(listeners);
            newlisteners.remove(listener);
            listeners = newlisteners;
        }
    }

    /**
     * Clear all listeners
     */
    public void clearListeners() {
        listeners = new ArrayList<T>();
    }

    /**
     * Get the ordered list of listeners ordered by dependencies
     *
     * @return
     */
    public List<T> getOrderedListeners() {
        return listeners;
    }
}