public class SlidingWindowReceiver {

    /* Buffer for packet data. */
    Object[] buf;

    /* Range of sequence numbers: Sequence numbers s 
     * with 0 <= s < seqNrRange are valid. */
    int seqNrRange;

    /* Sequence number of the next frame expected. */
    int nfe = 0;

    /* Buffer index at which the nfe-Paket is (or will be) stored. */
    int nfeIndex = 0;

    /* Create a sliding window receiver with the specified window size
     * and sequence number range. */
    public SlidingWindowReceiver(int windowSize, int snr) {
        buf = new Object[windowSize];
        seqNrRange = snr;
    }

    private int nextBufferIndex(int index) {

        // ... implement this method

        return 0; // ... change this as necessary
    }

    private int nextSeqNr(int seqNr) {

        // ... implement this method

        return 0; // ... change this as necessary 
    }

    /* Return the next packet in the sequence. If no packet is
     * available, null will be returned. This method is called by the
     * layer above. */
    public Object getNextPacket() {

        // ... implement this method

        return null; // ... change this as necessary
    }

    /* Handle a packet delivered by the layer below. Return the
     * sequence number of the acknowledgement to be sent back. If no
     * acknowledgement is to be sent, return -1. */
    public int handlePacket(int seqNr, Object data) {
        if (seqNr < 0 || seqNr >= seqNrRange) {
            // Sequence number out of range. Do not send an ACK.
            return -1;
        }

        // ... implement this method

        return -1; // change this as necessary
    }

    private int step = 0;
    public String show() {

        // ... implement this method

        return "<not implemented yet>";
    }


}