SNAPPacket.java

package org.reprap.comms.snap;

import java.io.IOException;

public class SNAPPacket {
	
	/**
	 * 
	 */
	private final int offset_sync = 0;
	
	/**
	 * 
	 */
	private final int offset_hdb2 = 1;
	
	/**
	 * 
	 */
	private final int offset_hdb1 = 2;
	
	/**
	 * 
	 */
	private final int offset_dab = 3;
	
	/**
	 * 
	 */
	private final int offset_sab = 4;
	
	/**
	 * 
	 */
	private final int offset_payload = 5;
	
	/**
	 * 
	 */
	private final byte syncMarker = 0x54;
	
	/**
	 * 
	 */
	private final int maxSize = 64;
	
	/**
	 * Full raw packet contents including all headers 
	 */
	private byte [] buffer;

	/**
	 * 
	 */
	private int receiveLength = 0;
	
	/**
	 * 
	 */
	private boolean complete = false;
	
	/**
	 * 
	 */
	SNAPPacket() {
		buffer = new byte[maxSize];
	}
	
	/**
	 * @param srcAddress
	 * @param destAddress
	 * @param payload
	 */
	SNAPPacket(SNAPAddress srcAddress, SNAPAddress destAddress, byte [] payload) {
		buffer = new byte[payload.length + offset_payload + 1];
		buffer[offset_sync] = syncMarker;
		buffer[offset_hdb2] = 0x51;
		buffer[offset_hdb1] = 0x30;
		buffer[offset_dab] = (byte)destAddress.getAddress();
		buffer[offset_sab] = (byte)srcAddress.getAddress();
		setLength(payload.length);
		for(int i = 0; i < payload.length; i++)
			buffer[i + offset_payload] = payload[i];
		generateChecksum();
		complete = true;
	}

	/**
	 * 
	 */
	private void generateChecksum() {
		int length = getLength() + offset_payload;
		SNAPChecksum crc = new SNAPChecksum();
		for(int i = 1; i < length; i++)
			crc.addData(buffer[i]);
		buffer[length] = crc.getResult();
	}
	
	/**
	 * @return the packet type
     */
	public byte getPacketType() {
		return buffer[0]; // TODO fix offset
	}
	
    /**
     * @return the payload
     */
    public byte [] getPayload() {
    	int length = getLength();
    	byte [] payload = new byte[length];
    	for(int i = 0; i < length; i++)
    		payload[i] = buffer[i + offset_payload];
		return payload;
	}
	
	/**
	 * @return the raw data of the packet
	 */
	public byte [] getRawData() {
		return buffer;
	}
	
	/**
	 * 
	 * @param data
	 * @return true is the packet is now complete, otherwise false
	 * @throws IOException 
	 */
	public boolean receiveByte(byte data) throws IOException {
		if (complete)
			throw new IOException("Received data beyond end of packet");
		
		if (receiveLength >= maxSize)
			throw new IOException("Received too much data");
		buffer[receiveLength++] = data;
		
		if (receiveLength > 4) {
			int expectedLength = getLength() + offset_payload + 1;
			if (receiveLength >= expectedLength)
				return true;
		}
		return false;
	}
	
	/**
	 * @return true if the packet passed validation
	 */
	public boolean validate() {
		if (receiveLength < offset_payload)
			return false;
		int expectedLength = getLength() + offset_payload + 1;
		if (receiveLength != expectedLength)
			return false;

		SNAPChecksum crc = new SNAPChecksum();
		for(int i = offset_hdb2; i < receiveLength - 1; i++)
			crc.addData(buffer[i]);
		
		byte expectedCRC = buffer[receiveLength - 1];
		return crc.getResult() == expectedCRC;
	}
	
	/**
	 * @return the source address of the packet
	 */
	public SNAPAddress getSourceAddress() {
		return new SNAPAddress(buffer[offset_sab]);
	}
	
	/**
	 * @return the destination address for the packet
	 */
	public SNAPAddress getDestinationAddress() {
		return new SNAPAddress(buffer[offset_dab]);
	}

	/**
	 * @param length
	 */
	private void setLength(int length) {
		buffer[offset_hdb1] = (byte)((buffer[offset_hdb1] & 0xf0) |
				(length > 7 ? 8 : length));
	}
	
	/**
	 * @return the lenght of the packet
	 */
	public int getLength() {
		int l = buffer[offset_hdb1] & 0x0f;
		if ((l & 8) != 0)
			return 8 << (l & 7);
		return l;
	}

	/**
	 * @return NAK packet
	 */
	public SNAPPacket generateNAK() {
		SNAPPacket resp = new SNAPPacket(getDestinationAddress(), getSourceAddress(), new byte [] {});
		resp.buffer[offset_hdb2] = (byte)((resp.buffer[offset_hdb2] & 0xfc) | 3);
		resp.generateChecksum();
		return resp;
	}

	/**
	 * @return ACK packet
	 */
	public SNAPPacket generateACK() {
		SNAPPacket resp = new SNAPPacket(getDestinationAddress(), getSourceAddress(), new byte [] {});
		resp.buffer[offset_hdb2] = (byte)((resp.buffer[offset_hdb2] & 0xfc) | 2);
		resp.generateChecksum();
		return resp;
	}
	
	/**
	 * @return true if the packet represents an ACK
	 */
	public boolean isAck() {
		return ((buffer[offset_hdb2] & 3) == 2);
	}

	/**
	 * @return true if the packet represents a NAK
	 */
	public boolean isNak() {
		return ((buffer[offset_hdb2] & 3) == 3);
	}
}