/*
 * Author: Wenbing Zhao
 * Last Modified: 9/18/2006
 * For EEC484/584 Project 1
 */

// This class describes the frame structure used by the par protocol
public class Frame {
    public static final byte[] PREAMBLE = 
       {'E','E','C','4','8','4','/','5','8','4'};
    public static final int MAX_FRAME_PAYLOAD = 1024;
    public static final int MAX_FRAME_SIZE = MAX_FRAME_PAYLOAD + 6 +
	PREAMBLE.length; // payload length + header fields + preamble len

    public int seq; // sequence number for duplicate detection, actually 1 bit
    public int ack; // acked sequence number, actually 1 bit
    public int length; // payload length, really need 4 bytes
    public byte[] payload = new byte[MAX_FRAME_PAYLOAD];

    private boolean m_isValid = true;

    public Frame() {
	seq = -1;
	ack = -1;
	length = 0;
    }

    public Frame(byte[] receivedData) {
	// Does the frame carries the right preamble?
	m_isValid = verifyFrame(receivedData);
	int index = PREAMBLE.length;
	if(m_isValid) {
	    //System.out.println("Valid frame");

	    // set seq number
	    seq = receivedData[index++];

	    // set ack sequence number
	    ack = receivedData[index++];
	    
	    //System.out.println("b1: "+receivedData[index]);
	    //System.out.println("b2: "+receivedData[index+1]);
	    //System.out.println("b3: "+receivedData[index+2]);
	    //System.out.println("b4: "+receivedData[index+3]);

	    // payload length
	    byte[] intArray = {receivedData[index], receivedData[index+1],
		      receivedData[index+2], receivedData[index+3]};
	    index += 4;
	    length = ByteArrayUtils.readInt(intArray);

	    // the rest is the application payload
	    //System.out.println("payload len: "+length);

	    for(int i=0; i<length; i++) {
		payload[i] = receivedData[index+i];
	    }
	    //String recvd = new String(payload);
	    //System.out.println("Frame::Received: "+recvd);
	    //System.out.println("Got ack, try get another message to send");
	}
    }

    public byte[] toBytes() {
	// we want to construct a byte array consisting
	// a preamble, a sequence number, an ack, 
	// a length field (4 bytes), and the payload
	int totalLen = length + 2 + 4 + PREAMBLE.length;
	byte[] data = new byte[totalLen];
	//System.out.println("Send total length: "+totalLen);

	int i = 0;
	for(i=0; i<PREAMBLE.length; i++)
	    data[i] = PREAMBLE[i];
	// note that our sequence number is 1-bit, so it fits one byte
	data[i++] = (byte)seq; 
	data[i++] = (byte)ack; 

	byte[] intArray = new byte[4];
	ByteArrayUtils.writeInt(intArray, length);
	int k = 0;
	data[i++] = intArray[k++];
	data[i++] = intArray[k++];
	data[i++] = intArray[k++];
	data[i++] = intArray[k++];

	//System.out.println("Frame::toBytes: payload len="+length);

	for(int j=0; j<length; j++,i++)
	    data[i] = payload[j];

	// self test on marshalling
	//System.out.println("----");
	//Frame test = new Frame(data);
	//System.out.println("----");

	return data;
    }

    private boolean verifyFrame(byte[] data) {
	boolean verified = true;
	for(int i=0; i<PREAMBLE.length; i++)
	    if(data[i] != PREAMBLE[i]) {
		verified = false;
		break;
	    }
	return verified;
    }

    public boolean isValid() {
	return m_isValid;
    }
}