Add more OTP implementation of Speed Daemon - sdball/protohackers

Published: 2023-02-12

git blog


Thanks again to @whatyouhide for the example code. I added it to this repo by retyping (and somewhat tweaking) the reference implementation.

I learned a lot of tricks about using kernel functions like update_in and put_in to handle updating nested state.

The approach to have the binary protocol functions in their own module was very nice. I learned some good tricks for more idiomatically matching binary messages as well.


<<@dispatcher, roads_count::8, roads::binary-size(roads_count * 2), rest::binary>>


def client_message(<<@dispatcher, roads_count::8, rest::binary>>) do
  case rest do
    <<roads::binary-size(roads_count * 2), rest::binary>> ->"SLS.ClientMessage.dispatcher roads=#{inspect(roads)}")
      {:ok, dispatcher:, rest: rest}

    _incomplete ->

That is, using roads_count to further match within the same binary pattern instead of having a secondary pattern match like I was doing originally.

It’s always a good idea to consider (and implement!) alternatives but I do prefer some aspects of my original approach to handling the generation of tickets from observations.

  • A MapSet is a nice way to ensure uniqueness for ticketed plates by day.
  • I didn’t generate any new tickets if a ticket was already generated for the given days covered by a new violation.
  • I also didn’t regenerate all tickets for a given observation’s road and plate but only checked the observations before and after the new observation to potentially generate 0, 1, or 2 new tickets from potential violations.

Happily, like for MITM, my existing integration tests for the Speed Daemon server worked against this new implementation as well. Hooray!

dd3e8553403083e6b8b83bf53ca3b1b0de76e6fe on sdball/protohackers