linkpacket.java

基于Java的地图数据管理软件。使用MySQL数据库管理系统。

package net.aetherial.gis.garmin;

import java.io.*;
import java.util.*;

public class LinkPacket
{
	/**
	 * This method takes a byte array and converts it into a LinkPacket object.
	 */

	public LinkPacket (byte[] contents)
	{
		packetID = contents[1];
		packetSize = contents[2];

		if (packetSize > 0)
		{
			ByteArrayOutputStream packetContents = new ByteArrayOutputStream ();

			for (int i = 0; i < (int) packetSize; i++)
			{
                          //System.out.println("" + contents.length + "/" + i + "/" +
                          //                   ( (int) packetSize));
                          packetContents.write(contents[i + 3]);
                        }

			packetData = packetContents.toByteArray ();
		}

		checksum = contents[contents.length  - 3];
	}

	/**
	 * This constructs a new LinkPacket with a Packet ID of packetType and Packet Data composed of payload.
	 */

	public LinkPacket (int packetType, byte[] payload)
	{
		packetID = (byte) packetType;
		if (payload != null)
		{
			packetSize = (byte) payload.length;
			packetData = payload;
		}
		else
		{
			packetSize = 0;
			packetData = null;
		}
		checksum = computeChecksum ();
	}

	public byte computeChecksum ()
	{
		byte[] contents;

		if (packetData == null)
			contents = new byte[2];
		else
			contents = new byte[2 + packetData.length];

		contents[0] = packetID;
		contents[1] = packetSize;

		if (packetData != null)
		{
			for (int i = 2; i < packetData.length + 2; i++)
				contents[i] = packetData[i - 2];
		}

		byte sum = 0;

		for (int i = 0; i < contents.length; i++)
			sum = (byte) ((byte) sum + (byte) contents[i]);

		return (byte) -sum;
	}


	public boolean goodChecksum ()
	{

		return true;
/*
		if (checksum == 0 || checksum == -1)
			return true;

		// System.out.println (computeChecksum()+" == "+checksum+"?");
		if (computeChecksum () != checksum)
			return false;
		else
			return true;
*/
	}

	/**
	 * This method converts a LinkPacket into a byte array that is suitable for sending through serial ports.
	 */

	public byte[] toByteArray ()
	{
		int i;
		byte[] packetBytes = new byte[packetSize + 6];
		packetBytes[0] = dle;
		packetBytes[1] = packetID;
		packetBytes[2] = packetSize;

		if (packetData != null)
		{
			for (i = 0; i < packetSize; i++)
				packetBytes[i + 3] = packetData[i];
		}

		packetBytes[packetBytes.length - 3] = checksum;
		packetBytes[packetBytes.length - 2] = dle;
		packetBytes[packetBytes.length - 1] = eot;

		return packetBytes;
	}

	/**
	 * This method returns the Packet ID of the LinkPacket.
	 */

	public int getPacketType ()
	{
		return (int) packetID;
	}

	/**
	 * This method returns the LinkPacket's data payload.
	 */

	public byte[] getPacketData ()
	{
		return packetData;
	}


	public byte getChecksum ()
	{
		return checksum;
	}

	public void printPacket ()
	{

		System.out.println ("Packet Data");
		System.out.println ("Packet ID: "+packetID);
		System.out.println ("Packet Size: "+packetSize);
		System.out.println ("Checksum: "+checksum);
		System.out.println ("Raw Packet Bytes:");

		byte[] packetBytes = this.toByteArray ();

		for (int i = 0; i < packetBytes.length; i++)
		{
			System.out.print ("["+i+"] "+packetBytes[i]+"  ");
		}

		System.out.println ("");
	}

	private final byte dle = (byte) 16;
	private byte packetID;
	private byte packetSize;
	private byte[] packetData;
	private byte checksum;
	private final byte eot = (byte) 3;
}