/*
 * Copyright 2012-2014 Red Hat, Inc.
 *
 * This file is part of Thermostat.
 *
 * Thermostat is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published
 * by the Free Software Foundation; either version 2, or (at your
 * option) any later version.
 *
 * Thermostat is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with Thermostat; see the file COPYING.  If not see
 * <http://www.gnu.org/licenses/>.
 *
 * Linking this code with other modules is making a combined work
 * based on this code.  Thus, the terms and conditions of the GNU
 * General Public License cover the whole combination.
 *
 * As a special exception, the copyright holders of this code give
 * you permission to link this code with independent modules to
 * produce an executable, regardless of the license terms of these
 * independent modules, and to copy and distribute the resulting
 * executable under terms of your choice, provided that you also
 * meet, for each linked independent module, the terms and conditions
 * of the license of that module.  An independent module is a module
 * which is not derived from or based on this code.  If you modify
 * this code, you may extend this exception to your version of the
 * library, but you are not obligated to do so.  If you do not wish
 * to do so, delete this exception statement from your version.
 */

package com.redhat.thermostat.common.command;

import static org.jboss.netty.channel.Channels.write;

import org.jboss.netty.buffer.ChannelBuffer;
import org.jboss.netty.buffer.ChannelBuffers;
import org.jboss.netty.channel.ChannelEvent;
import org.jboss.netty.channel.ChannelHandlerContext;
import org.jboss.netty.channel.MessageEvent;
import org.jboss.netty.channel.SimpleChannelDownstreamHandler;

public abstract class MessageEncoder extends SimpleChannelDownstreamHandler {

    protected MessageEncoder() {
        super();
    }

    @Override
    public void handleDownstream(
            ChannelHandlerContext ctx, ChannelEvent evt) throws Exception {
        if (!(evt instanceof MessageEvent)) {
            ctx.sendDownstream(evt);
            return;
        }

        MessageEvent e = (MessageEvent) evt;
        Object originalMessage = e.getMessage();
        // We only know how to encode Messages. Non-message types
        // get sent downstream verbatim.
        if (!(originalMessage instanceof Message)) {
            ctx.sendDownstream(evt);
        }
        ChannelBuffer encodedMessage = encode((Message)originalMessage);
        if (encodedMessage != null) {
            write(ctx, e.getFuture(), encodedMessage, e.getRemoteAddress());
        }
    }

    /**
     * Transforms the specified message into another message and return the
     * transformed message. Note that you can not return {@code null}, unlike
     * you can in
     * {@link MessageDecoder#decode(org.jboss.netty.buffer.ChannelBuffer)}; you
     * must return something, at least {@link ChannelBuffers#EMPTY_BUFFER}.
     */
    protected abstract ChannelBuffer encode(Message originalMessage);
}