"""Go-Back-N. Your group writes Sender.send(); the receiver is given.

Read stopwait.py first. Go-Back-N keeps up to `window` packets in flight
instead of one, with a single timer for the oldest packet not yet ACKed. ACKs
are cumulative, exactly as in stop-and-wait: ACK n means the receiver has
every packet before n. That is why stopwait.Receiver already works as the
Go-Back-N receiver, with no changes (P&D 2.5.2 explains why).
"""

import socket
import time

import packet
import stopwait

Receiver = stopwait.Receiver


class Sender(stopwait.Sender):
    def __init__(self, sock, peer, window=8, timeout=0.1, max_fin_tries=20):
        super().__init__(sock, peer, window, timeout, max_fin_tries)
        self.window = window

    def send(self, data):
        """Deliver `data` reliably with a window of self.window, then close.

        YOUR TASK. Keep sending while fewer than self.window packets are
        unACKed. When an ACK moves the start of the window, restart the timer
        if anything is still in flight. When the timer runs out, send every
        unACKed packet again. Count transmissions and retransmissions in
        self.stats the way stop-and-wait does; experiments.py reports them.
        Finish with self._close(number of data packets), as stop-and-wait does.
        """
        raise NotImplementedError("gbn.Sender.send")