Message.java

/**
 * Copyright (c) 2010-2016 by the respective copyright holders.
 *
 * All rights reserved. This program and the accompanying materials
 * are made available under the terms of the Eclipse Public License v1.0
 * which accompanies this distribution, and is available at
 * http://www.eclipse.org/legal/epl-v10.html
 */
package org.openhab.binding.plugwise.protocol;

import java.io.UnsupportedEncodingException;

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

/**
 * Base class to represent Plugwise protocol data units
 *
 * In general a message consists of a hex string containing the following parts:
 * a type indicator - many types are yet to be reverse engineered
 * a sequence number - messages in PW are numbered so that we can keep track of them in an application
 * a PW MAC address - basically, the destination of the message
 * a payload
 * a CRC checksum that is calculated using the previously mentioned segments of the message
 *
 * Before sending off a message in the PW network they are prepended with a protocol header and trailer is
 * added at the end
 *
 *
 * @author Karel Goderis
 * @since 1.1.0
 */
public abstract class Message {

    protected static final Logger logger = LoggerFactory.getLogger(Message.class);

    protected MessageType type;
    protected int sequenceNumber = 0;
    protected String payLoad;
    protected String MAC;

    // to keep track how many attempts we have made to send this message
    protected int retryCount = 0;

    // for messages where the MAC address is part of the payload
    public Message(int sequenceNumber, String payLoad) {
        this.sequenceNumber = sequenceNumber;
        this.payLoad = payLoad;

        parsePayLoad();
    }

    @Override
    public String toString() {

        return "Plugwise Message [type=" + type.toString() + ", MAC=" + MAC + ", SeqNr=" + sequenceNumber + ", payload="
                + payLoad + ", retrycount=" + retryCount + "]";

    }

    // for messages where we do not care about the MAC address
    public Message(String payLoad) {
        this.payLoad = payLoad;

        if (payLoad != null) {
            parsePayLoad();
        }
    }

    public Message(int sequenceNumber, String MAC, String payLoad) {
        this.payLoad = payLoad;
        this.sequenceNumber = sequenceNumber;
        this.MAC = MAC;

        if (payLoad != null) {
            parsePayLoad();
        }
    }

    // for messages where we do not care about the sequence number
    public Message(String MAC, String payLoad) {
        this.payLoad = payLoad;
        this.MAC = MAC;

        if (payLoad != null) {
            parsePayLoad();
        }
    }

    public void increaseAttempts() {
        retryCount++;
    }

    public int getAttempts() {
        return retryCount;
    }

    public MessageType getType() {
        return type;
    }

    public int getSequenceNumber() {
        return sequenceNumber;
    }

    public void setSequenceNumber(int sequenceNumber) {
        this.sequenceNumber = sequenceNumber;
    }

    public String getPayLoad() {
        return payLoad;
    }

    public String getMAC() {
        return MAC;
    }

    protected String sequenceNumberToHexString() {
        return String.format("%04X", sequenceNumber);
    }

    abstract protected String payLoadToHexString();

    private String MACToHexString() {
        return getMAC();
    }

    private String typeToHexString() {
        return String.format("%04X", type.toInt());
    }

    // Method that implementation classes have to override, and that is responsible for parsing the payload into
    // meaningful fields
    abstract protected void parsePayLoad();

    public String toHexString() {
        StringBuilder sb = new StringBuilder();
        sb.append(typeToHexString());
        sb.append(sequenceNumberToHexString());
        sb.append(MACToHexString());
        sb.append(payLoadToHexString());

        String result = sb.toString();
        String CRC = getCRC(result);
        result = result + CRC;

        return result;
    }

    protected String getCRC(String buffer) {

        int crc = 0x0000;
        int polynomial = 0x1021; // 0001 0000 0010 0001 (0, 5, 12)

        byte[] bytes = new byte[0];
        try {
            bytes = buffer.getBytes("ASCII");
        } catch (UnsupportedEncodingException e) {
            return "";
        }

        for (byte b : bytes) {
            for (int i = 0; i < 8; i++) {
                boolean bit = ((b >> (7 - i) & 1) == 1);
                boolean c15 = ((crc >> 15 & 1) == 1);
                crc <<= 1;
                if (c15 ^ bit) {
                    crc ^= polynomial;
                }
            }
        }

        crc &= 0xffff;

        return (String.format("%04X", crc).toUpperCase());
    }

}