Packet.java

import java.io.*;
import java.net.*;
import java.util.*;
import java.nio.*;

/** Class for working with studio3 packets. */
public class Packet {
	// packet fields - note: all are public
	public byte					type;				// packet type
	public short				seqNum;			// sequence number in
													// [0,2^15)
	public String				payload;			// application payload
	public static final byte	DATA_TYPE	= 0;

	/** Constructor, initializes fields to default values. */
	public Packet() {
		clear();
	}

	/** Initialize all packet fields.
	 *  Initializes all fields to an undefined value.
	 */
	public void clear() {
		type = 0;
		seqNum = 0;
		payload = "";
	}

	/** Pack attributes defining packet fields into buffer.
	 *  Fails if the packet type is undefined or if the resulting
	 *  buffer exceeds the allowed length of 1400 bytes.
	 *  @return null on failure, otherwise a byte array
	 *  containing the packet payload.
	 */
	public byte[] pack() {
		byte[] pbuf;
		try {
			pbuf = payload.getBytes("US-ASCII");
		} catch (Exception e) {
			return null;
		}
		if (pbuf.length > 1400 - 3)
			return null;
		ByteBuffer bbuf = ByteBuffer.allocate(3 + pbuf.length);
		bbuf.order(ByteOrder.BIG_ENDIAN);
		bbuf.put(type);
		bbuf.putShort(seqNum);
		bbuf.put(pbuf);
		return bbuf.array();
	}

	/** Unpack attributes defining packet fields from buffer.
	 *  @param buf is a byte array containing the packet
	 *  (or if you like, the payload of a UDP packet).
	 *  @param bufLen is the number of valid bytes in buf
	 */
	public boolean unpack(byte[] buf, int bufLen) {
		if (bufLen < 3)
			return false;
		ByteBuffer bbuf = ByteBuffer.wrap(buf);
		bbuf.order(ByteOrder.BIG_ENDIAN);
		type = bbuf.get();
		seqNum = bbuf.getShort();
		try {
			payload = new String(buf, 3, bufLen - 3, "US-ASCII");
		} catch (Exception e) {
			return false;
		}
		return true;
	}

	/** Create String representation of packet.
	 *  The resulting String is produced using the defined
	 *  attributes and is formatted with one field per line,
	 *  allowing it to be used as the actual buffer contents.
	 */
	public String toString() {
		if (type == DATA_TYPE)
			return "data[" + seqNum + "] " + payload;
		else
			return "ack[" + seqNum + "]";
	}
}