Advent of Code - Day 2



Note by the Moderators: This topic is to talk about the Day 2 of the Advent of Code.

For general discussion about the Advent of Code 2018 and links to topics of the other days, see this topic.

This is my todays code:

I’m not quite happy with it though, but it runs in ~4ms/~8ms, so I leave it as is for now.

Advent Of Code 2018

I’m doing my problems in Elixir this year as well:

My solution today seems a little different from the others I’ve seen. I made use of String.myers_difference/2 for the second part:

  def common(addresses) do
    pairings = for add_a <- addresses, add_b <- addresses, add_a != add_b, do: {add_a, add_b}

    Enum.reduce_while(pairings, nil, fn {add_a, add_b}, _ ->
      case String.myers_difference(add_a, add_b) do
        [eq: _, del: << char :: size(8) >>, ins: _, eq: _] ->
          {:halt, String.replace(add_a, IO.chardata_to_string([char]), "")}

        _ -> {:cont, nil}

There are two other possible edit scripts that could have matched (first character or last character being the difference), but this worked for my inputs.


I’m pretty happy with my solutions so far, especially for Day 2 Part 2 I felt I was able to take advantage of a few things, especially a comprehension to structure it nicely:

Did I cheat at Advent of Code?

I had a lot of fun doing Day 2 Part 2.

I like how this one turned out — reasonably small and clean.

I’ve learned that finding the right functional operation (Enum.find_value in this case) really helps keep the code small (and easy to reason about). (String.myers_difference helped, of course. I wrote my first version with MapSet (and set difference and intersections), which interestingly worked for the example case, but did not work with the large input file, so I ended up using String.myers_difference)