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Network communication library for solver-aggregator connections.
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/*
 * Owl Platform Solver-Aggregator Library for Java
 * Copyright (C) 2012 Robert Moore and the Owl Platform
 * 
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU Lesser General Public License as published by
 * the Free Software Foundation; either version 2.1 of the License, or
 * (at your option) any later version.
 *  
 * This program 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 Lesser General Public License for more details.
 *  
 * You should have received a copy of the GNU Lesser General Public License along
 * with this program; if not, write to the Free Software Foundation, Inc.,
 * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 */

package com.owlplatform.solver.protocol.codec;

import java.nio.charset.Charset;

import org.apache.mina.core.buffer.IoBuffer;
import org.apache.mina.core.session.IoSession;
import org.apache.mina.filter.codec.ProtocolDecoderOutput;
import org.apache.mina.filter.codec.demux.MessageDecoder;
import org.apache.mina.filter.codec.demux.MessageDecoderResult;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import com.owlplatform.solver.protocol.messages.HandshakeMessage;

/**
 * Decoder for {@code HandshakeMessage} according to the Solver-Aggregator protocol.
 * @author Robert Moore
 *
 */
public class HandshakeDecoder implements MessageDecoder {

  /**
   * Logger for this class.
   */
	private static final Logger log = LoggerFactory
			.getLogger(HandshakeDecoder.class);

	/**
	 * State key for tracking connection state.
	 */
	public static final String CONN_STATE_KEY = HandshakeDecoder.class
			.getName()
			+ ".STATE";

	/**
	 * Simple class for tracking whether a handshake was received on a session.
	 * @author Robert Moore
	 *
	 */
	static final class HubConnectionState {
	  /**
	   * Flag to indicate if a handshake was previously received.
	   */
		boolean handshakeReceived = false;
		
		
		/**
		 * Default constructor with the handshake received flag set to false.
		 */
		public HubConnectionState(){}
	}

	/**
	 * Charset for encoding the protocol string.
	 */
	private static final Charset charsetASCII = Charset.forName("US-ASCII");

	@Override
	public MessageDecoderResult decodable(IoSession arg0, IoBuffer arg1) {
		HubConnectionState connState = (HubConnectionState) arg0
				.getAttribute(CONN_STATE_KEY);

		if (connState == null) {
			log.debug("Creating new handshake connection state for {}.", arg0);
			connState = new HubConnectionState();
			connState.handshakeReceived = false;
			arg0.setAttribute(CONN_STATE_KEY, connState);
		}

		if (connState.handshakeReceived) {
			return MessageDecoderResult.NOT_OK;
		}

		if (!arg1.prefixedDataAvailable(4,
				HandshakeMessage.PROTOCOL_STRING_LENGTH)) {
		  return MessageDecoderResult.NEED_DATA;
		}

		// TODO: Need better logic to determine decodability
		return MessageDecoderResult.OK;
	}

	@Override
	public MessageDecoderResult decode(IoSession arg0, IoBuffer arg1,
			ProtocolDecoderOutput arg2) throws Exception {
		HubConnectionState connState = (HubConnectionState) arg0
				.getAttribute(CONN_STATE_KEY);
		if (connState == null) {
			log.debug("Creating new handshake connection state for {}.", arg0);
			connState = new HubConnectionState();
			connState.handshakeReceived = false;
			arg0.setAttribute(CONN_STATE_KEY, connState);
		}

		// If handshake already received, skip this decoder
		if (connState.handshakeReceived) {
			log.warn("Handshake already received.");
			return MessageDecoderResult.NOT_OK;
		}

		if (arg1.prefixedDataAvailable(4,
				HandshakeMessage.PROTOCOL_STRING_LENGTH)) {
			HandshakeMessage message = new HandshakeMessage();
			message.setStringLength(arg1.getInt());
			if (message.getStringLength() != HandshakeMessage.PROTOCOL_STRING_LENGTH) {
				throw new RuntimeException(String.format(
						"Handshake protocol string length is incorrect: %d",Integer.valueOf(
						message.getStringLength())));
			}

			message.setProtocolString(String.valueOf(arg1.getString(message
					.getStringLength(), HandshakeDecoder.charsetASCII
					.newDecoder())));
			message.setVersionNumber(arg1.get());
			message.setReservedBits(arg1.get());

			arg2.write(message);
			log.debug("Wrote {}.", message);
			connState.handshakeReceived = true;
			return MessageDecoderResult.OK;
		}
		
		return MessageDecoderResult.NEED_DATA;
	}

	@Override
	public void finishDecode(IoSession arg0, ProtocolDecoderOutput arg1)
			throws Exception {
		// Nothing to do
	}

}