A12.10 Project: a reliable link

The robot starts facing forward. The Base sends a route of three packets in the format above, one every half second, out of order. One packet arrives damaged. When the Base hears NAK <number>, it sends that packet again. 1. Receive: check messages() every 0.1 s. For each packet, recalculate its checksum. If it matches, store the command under the packet's number and send ACK <number>. If it does not, print packet <number> corrupt and send NAK <number>. Keep going until you hold a good copy of every packet, as many as the total in the header says. 2. Act: for each number from 1 to the total, in order, print run: <command>, then drive it at speed 50. A command is a direction (forward, backward, left or right, where left and right slide sideways) and a whole number of centimetres. 3. Send DONE. Do not drive until every packet is held, and read the route from the packets rather than typing it. If you act on the damaged packet, you will end up in the red zone.

# the two lines every program starts with: the commands, then the robot
from bugbot import *
connect()

def checksum(text):
    total = 0
    for ch in text:
        total = total + ord(ch)
    return format(total % 256, "02X")

wait(3)
for sender, text in messages():
    print(text)

The hint students can ask for: Only a packet whose checksum matches may be stored or acknowledged; a damaged one gets a NAK with its number and is thrown away. Keep listening until you hold every packet the header's total asks for. Do not move until then, and run the commands in sequence number order, not arrival order.

A solution

# the two lines every program starts with: the commands, then the robot
from bugbot import *
connect()

def checksum(text):
    total = 0
    for ch in text:
        total = total + ord(ch)
    return format(total % 256, "02X")

good = {}
total = None
while total is None or len(good) < total:
    wait(0.1)
    for sender, text in messages():
        if "*" not in text:
            continue
        body, check = text.rsplit("*", 1)
        header, command = body.split(":", 1)
        number, count = header.split("/")
        if checksum(body) == check:
            good[int(number)] = command
            total = int(count)
            send("ACK " + number)
        else:
            print("packet", number, "corrupt")
            send("NAK " + number)

for n in range(1, total + 1):
    word, cm = good[n].split()
    print("run:", good[n])
    if word == "forward":
        forward(50, distance=int(cm))
    elif word == "backward":
        backward(50, distance=int(cm))
    elif word == "left":
        left(50, distance=int(cm))
    elif word == "right":
        right(50, distance=int(cm))
send("DONE")

Any program that meets the task's checks is marked correct in the simulator; this is one way, not the only way.