NTW/hw2/src/impl/GBNTReceiver.java

/*
 * GBNTReceiver - Claudio Maggioni (2020)
 * NTW2020 HW2
 *
 * No external source was used to create this file, other than regular course material.
 *
 * Note on implementation: change value of constant RECEIVER_TIMEOUT_MS to change the
 * value of the receiver disconnection timeout.
 */

import transport.Receiver;
import transport.TimeoutAction;

public class GBNTReceiver extends Receiver implements TimeoutAction {

    private char sequence;
    private boolean firstPacket = true;
    private static final int RECEIVER_TIMEOUT_MS = 15000;
    private State state = State.SETUP;

    enum State {
        SETUP,
        RECEIVING,
        CLOSED,
    }

    private static byte[] generateACK(char sequence) {
        byte[] dest = new byte[2];
        dest[0] = (byte) ((sequence >> 8) & 0xFF);
        dest[1] = (byte) (sequence & 0xFF);
        return dest;
    }

    private static char readBigEndianChar(byte[] src, int offset) {
        return (char) (((src[offset] << 8) & 0xFF00) | (src[offset + 1] & 0xFF));
    }

    private static char incrementAndRollover(char c) {
        return (char) ((c + 1) % 65536);
    }

    @Override
    public void timeoutAction() {
        System.out.println("Disconnect timeout triggered");
        try {
            disconnect();
            deliver(null, 0, END_OF_STREAM);
            state = State.CLOSED;
        } catch (java.lang.InterruptedException ex) {
            System.out.println("Thread interrupted.  Exiting directly.");
            System.exit(0);
        }
    }

    @Override
    protected void unreliableReceive(byte[] bytes, int offset, int length) {
        if (state == State.CLOSED) return;
        cancelTimeout(this);
        setTimeout(RECEIVER_TIMEOUT_MS, this);
        switch(this.state) {
            case SETUP:
                if (length != 2) {
                    if (!firstPacket) {
                        state = State.RECEIVING;
                        this.unreliableReceive(bytes, offset, length);
                        return;
                    }
                    return;
                }
                sequence = readBigEndianChar(bytes, offset);
                System.out.println("Synchronization ACK: " + (int) sequence);
                unreliableSend(bytes, offset, 2);
                firstPacket = false;
                break;
            case RECEIVING:
                if (length == 2) { // if packet is a close packet
                    char a = readBigEndianChar(bytes, offset);
                    if (incrementAndRollover(this.sequence) != a) return; // ignore if not synchronized
                    this.unreliableSend(bytes, offset, 2);
                    System.out.println("Received valid FIN packet");
                    timeoutAction();
                }
                if (length < 3) return;
                char seq = readBigEndianChar(bytes, offset);
                System.out.print("Read sequence: " + (int) seq + " Exp: " + (int) incrementAndRollover(sequence)+ " -> ");

                if (seq == incrementAndRollover(sequence)) {
                    this.deliver(bytes, offset + 2, length - 2);
                    sequence = incrementAndRollover(sequence);
                    System.out.print("OK! ");
                } else {
                    System.out.print("Drop ");
                }

                System.out.println("Sending ack: "+ (int) sequence);
                this.unreliableSend(generateACK(sequence), 0, 2);
                break;
        }
    }
}