SatelMessage.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.satel.internal.protocol;

import java.util.Arrays;

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

/**
 * Represents a message sent to (a command) or received from (a response)
 * {@link SatelModule}. It consists of one byte that specifies command type or
 * response status and certain number of payload bytes. Number of payload byte
 * depends on command type. The class allows to serialize a command to bytes and
 * deserialize a response from given bytes. It also computes and validates
 * message checksum.
 *
 * @author Krzysztof Goworek
 * @since 1.7.0
 */
public class SatelMessage {
    private static final Logger logger = LoggerFactory.getLogger(SatelMessage.class);

    private byte command;
    private byte[] payload;

    protected static final byte[] EMPTY_PAYLOAD = new byte[0];

    /**
     * Creates new instance with specified command code and payload.
     *
     * @param command
     *            command code
     * @param payload
     *            command payload
     */
    public SatelMessage(byte command, byte[] payload) {
        this.command = command;
        this.payload = payload;
    }

    /**
     * Creates new instance with specified command code and empty payload.
     *
     * @param command
     *            command code
     */
    public SatelMessage(byte command) {
        this(command, EMPTY_PAYLOAD);
    }

    /**
     * Deserializes new message instance from specified byte buffer.
     *
     * @param buffer
     *            bytes to deserialize a message from
     * @return deserialized message instance
     */
    public static SatelMessage fromBytes(byte[] buffer) {
        // we need at least command and checksum
        if (buffer.length < 3) {
            logger.error("Invalid message length: {}", buffer.length);
            return null;
        }

        // check crc
        int receivedCrc = 0xffff & ((buffer[buffer.length - 2] << 8) | (buffer[buffer.length - 1] & 0xff));
        int expectedCrc = calculateChecksum(buffer, buffer.length - 2);
        if (receivedCrc != expectedCrc) {
            logger.error("Invalid message checksum: received = {}, expected = {}", receivedCrc, expectedCrc);
            return null;
        }

        SatelMessage message = new SatelMessage(buffer[0], new byte[buffer.length - 3]);
        if (message.payload.length > 0) {
            System.arraycopy(buffer, 1, message.payload, 0, buffer.length - 3);
        }
        return message;
    }

    /**
     * Returns command byte.
     *
     * @return the command
     */
    public byte getCommand() {
        return this.command;
    }

    /**
     * Returns the payload bytes.
     *
     * @return payload as byte array
     */
    public byte[] getPayload() {
        return this.payload;
    }

    /**
     * Returns the message serialized as array of bytes with checksum calculated
     * at last two bytes.
     *
     * @return the message as array of bytes
     */
    public byte[] getBytes() {
        byte buffer[] = new byte[this.payload.length + 3];
        buffer[0] = this.command;
        if (this.payload.length > 0) {
            System.arraycopy(this.payload, 0, buffer, 1, this.payload.length);
        }
        int checksum = calculateChecksum(buffer, buffer.length - 2);
        buffer[buffer.length - 2] = (byte) ((checksum >> 8) & 0xff);
        buffer[buffer.length - 1] = (byte) (checksum & 0xff);
        return buffer;
    }

    private String getPayloadAsHex() {
        StringBuilder result = new StringBuilder();
        for (int i = 0; i < this.payload.length; ++i) {
            if (i > 0) {
                result.append(" ");
            }
            result.append(String.format("%02X", this.payload[i]));
        }
        return result.toString();
    }

    /**
     * Calculates a checksum for the specified buffer.
     *
     * @param buffer
     *            the buffer to calculate.
     * @return the checksum value.
     */
    private static int calculateChecksum(byte[] buffer, int length) {
        int checkSum = 0x147a;
        for (int i = 0; i < length; i++) {
            checkSum = ((checkSum << 1) | ((checkSum >> 15) & 1));
            checkSum ^= 0xffff;
            checkSum += ((checkSum >> 8) & 0xff) + (buffer[i] & 0xff);
        }
        checkSum &= 0xffff;
        logger.trace("Calculated checksum = {}", String.format("%04X", checkSum));
        return checkSum;
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public String toString() {
        return String.format("Message: command = %02X, payload = %s", this.command, getPayloadAsHex());
    };

    /**
     * {@inheritDoc}
     */
    @Override
    public boolean equals(Object obj) {
        if (this == obj) {
            return true;
        }

        if (obj == null) {
            return false;
        }

        if (!obj.getClass().equals(this.getClass())) {
            return false;
        }

        SatelMessage other = (SatelMessage) obj;

        if (other.command != this.command) {
            return false;
        }

        if (!Arrays.equals(other.payload, this.payload)) {
            return false;
        }

        return true;
    }

    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + command;
        result = prime * result + Arrays.hashCode(payload);
        return result;
    }
}