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mqtt-client provides an ASL 2.0 licensed API to MQTT. It takes care of automatically reconnecting to your MQTT server and restoring your client session if any network failures occur. Applications can use a blocking API style, a futures based API, or a callback/continuations passing API style.
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/**
 * Copyright (C) 2010-2012, FuseSource Corp.  All rights reserved.
 *
 *     http://fusesource.com
 *
 * 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.fusesource.mqtt.codec;

import java.io.IOException;
import java.net.ProtocolException;

import org.fusesource.hawtbuf.Buffer;
import org.fusesource.hawtbuf.DataByteArrayInputStream;
import org.fusesource.hawtbuf.DataByteArrayOutputStream;
import org.fusesource.hawtbuf.UTF8Buffer;
import org.fusesource.mqtt.client.QoS;

/**
 * 
 * 
 *
 * @author Hiram Chirino
 */
public final class MessageSupport {

    private MessageSupport() throws InstantiationException {
        throw new InstantiationException("This class is not for instantiation");
    }

    /**
     * All command objects implement this interface.
     */
    static public interface Message {
        public byte messageType();
        public Message decode(MQTTFrame frame) throws ProtocolException;
        public MQTTFrame encode();
    }


    /**
     * All command objects that can get acked implement this interface.
     */
    static public interface Acked extends Message {
        public boolean dup();
        public Acked dup(boolean dup);
        public QoS qos();
        public short messageId();
        public Acked messageId(short messageId);
    }

    static protected UTF8Buffer readUTF(DataByteArrayInputStream is) throws ProtocolException {
        int size = is.readUnsignedShort();
        if (size < 0) {
            throw new ProtocolException("Invalid message encoding");
        }
        Buffer buffer = is.readBuffer(size);
        if (buffer == null || buffer.length != size) {
            throw new ProtocolException("Invalid message encoding");
        }
        return buffer.utf8();
    }

    static protected void writeUTF(DataByteArrayOutputStream os, Buffer buffer) throws IOException {
        os.writeShort(buffer.length);
        os.write(buffer);
    }

    static abstract public class AckBase {

        private short messageId;

        abstract byte messageType();

        protected AckBase decode(MQTTFrame frame) throws ProtocolException {
            assert(frame.buffers.length == 1);
            DataByteArrayInputStream is = new DataByteArrayInputStream(frame.buffers[0]);
            messageId = is.readShort();
            return this;
        }

        public MQTTFrame encode() {
            try {
                DataByteArrayOutputStream os = new DataByteArrayOutputStream(2);
                os.writeShort(messageId);

                MQTTFrame frame = new MQTTFrame();
                frame.commandType(messageType());
                return frame.buffer(os.toBuffer());
            } catch (IOException e) {
                throw new RuntimeException("The impossible happened");
            }
        }

        public short messageId() {
            return messageId;
        }

        protected AckBase messageId(short messageId) {
            this.messageId = messageId;
            return this;
        }

        @Override
        public String toString() {
            return getClass().getSimpleName()+"{" +
                "messageId=" + messageId +
                '}';
        }
    }

    static abstract public class EmptyBase {
        abstract  byte messageType();

        protected EmptyBase decode(MQTTFrame frame) throws ProtocolException {
            return this;
        }
        public MQTTFrame encode() {
            return new MQTTFrame().commandType(messageType());
        }
    }


   /**
    * 
    * 
    *
    * @author Hiram Chirino
    */
   static public class HeaderBase {

       protected byte header;

       protected byte header() {
           return header;
       }
       protected HeaderBase header(byte header) {
           this.header = header;
           return this;
       }

       protected byte messageType() {
           return (byte) ((header & 0xF0) >>> 4);
       }
       protected HeaderBase commandType(int type) {
           this.header &= 0x0F;
           this.header |= (type << 4) & 0xF0;
           return this;
       }

       protected QoS qos() {
           return QoS.values()[((header & 0x06) >>> 1)];
       }
       protected HeaderBase qos(QoS qos) {
           this.header &= 0xF9;
           this.header |= (qos.ordinal() << 1) & 0x06;
           return this;
       }

       protected boolean dup() {
           return (header & 0x08) > 0;
       }
       protected HeaderBase dup(boolean dup) {
           if(dup) {
               this.header |= 0x08;
           } else {
               this.header &= 0xF7;
           }
           return this;
       }

       protected boolean retain() {
           return (header & 0x01) > 0;
       }
       protected HeaderBase retain(boolean retain) {
           if(retain) {
               this.header |= 0x01;
           } else {
               this.header &= 0xFE;
           }
           return this;
       }

   }


}