A collection of my solutions for the server programming challenge at https://protohackers.com.

• Smoke Test: https://protohackers.com/problem/0
• Prime Time: https://protohackers.com/problem/1
• Means to an End: https://protohackers.com/problem/2
• Budget Chat: https://protohackers.com/problem/3
• Unusual Database Program: https://protohackers.com/problem/4
• Mob in the Middle: https://protohackers.com/problem/5
• Speed Daemon: https://protohackers.com/problem/6
• Line Reversal: https://protohackers.com/problem/7

Building a solution's Docker image (smoke-test in this example) and testing locally ...

$ docker build -f Dockerfile.smoke-test --tag smoke-test .
$ docker run -p 5000:5000 smoke-test

Interact with solutions locally using Netcat after running an image or with, for example, go run ./cmd/smoke-test ...

$ nc localhost 5000

To deploy to fly.io (requires account) ...

$ flyctl deploy --dockerfile Dockerfile.smoke-test

To test on fly.io ...

$ nc [FLY_HOSTNAME] 5000

For submission at https://protohackers.com, find the public IP address of your fly.io app with flyctl ips list.

Tail logs with ...

flyctl logs --app [APP_NAME]

The VS Code Hex Editor extension might be useful to you. Sending binary data might then look like cat data.dat | nc localhost 5000 or the following (by converting a hexdump into binary) ...

echo '490000303900000065490000303a00000066' | xxd -r -p | nc localhost 5000

To run the Docker image locally, override the -host flag (it defaults to a value required by fly.io otherwise) ...

docker run -p 5000:5000/udp unusual-database-program -host=0.0.0.0

You can now test UDP with Netcat using the -u flag ...

nc -u localhost 5000

Note that ^D / CTRL+D will send EOF.

This could be tested locally by first creating a couple of named pipes (multiple pairs to test multiple simultaneous clients) ...

mkfifo out
mkfifo in

Making use of the pipes after starting the server ...

nc localhost 5000 <out >in &; cat > out

Sending commands from another terminal window ...

echo -e -n '\x80\x00\x7b\x00\x08\x00\x3c' > out # IAmCamera
echo -e -n '\x81\x03\x00\x42\x01\x70\x13\x88' > out # IAmDispatcher
echo -e -n '\x20\x04\x55\x4e\x31\x58\x00\x00\x03\xe8' > out # Plate
echo -e -n '\x40\x00\x00\x00\x0a' > out # WantHeartbeat

The -n flag will prevent trailing newlines from being sent, and the -e flag allows us to use hex notation.

And watching for the responses in yet another terminal window ...

cat in

A short example scenario with 3 connected clients ...

# Client 1: camera at mile 8
echo -e -n '\x80\x00\x7b\x00\x08\x00\x3c' > out # IAmCamera{road: 123, mile: 8, limit: 60}
echo -e -n '\x20\x04\x55\x4e\x31\x58\x00\x00\x00\x00' > out # Plate{plate: "UN1X", timestamp: 0}

# Client 2: camera at mile 9
echo -e -n '\x80\x00\x7b\x00\x09\x00\x3c' > out2 # IAmCamera{road: 123, mile: 9, limit: 60}
echo -e -n '\x20\x04\x55\x4e\x31\x58\x00\x00\x00\x2d' > out2 # Plate{plate: "UN1X", timestamp: 45}

# Client 3: ticket dispatcher (should receive a ticket after identifying)
echo -e -n '\x81\x01\x00\x7b' > out3 # IAmDispatcher{roads: [123]}