Advent of Code - Day 18

sasajuric · December 18, 2018, 6:16pm

Note: This topic is to talk about Day 18 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.

sasajuric · December 18, 2018, 6:18pm

Here is my take. I solved part 2 by detecting cycles. It’s not super fast, finishes in about 7 sec. There’s some room for optimizing, but I didn’t want to uglify the code

bjorng · December 18, 2018, 7:03pm

I had fun with streams in this one. I also implemented a cycle finding algorithm.

Running the cycle finding algorithm on the infinite stream turned out to be too slow, though, so I cheated a little by pulling out 2000 values to a finite enumerable. That is probably long enough to work for most people’s data sets. The cycle in my data starts at 452 minutes.

It finishes in about 12 minutes.

@sasajuric I liked the elegant way in which you cached the data from the infinite stream and detected the cycle at the same time.

Update: Forgot the link.

JEG2 · December 19, 2018, 3:26am

I caught up just in time to go on vacation: here’s my solution to Day 18:

github.com

JEG2/advent_of_code_2018/blob/master/day_18/settlers_of_the_north_pole.exs

defmodule SettlersOfTheNorthPole do
  defstruct ~w[acres neighbors]a

  def parse(lines) do
    new_acres =
      lines
      |> Stream.with_index
      |> Enum.reduce(Map.new, fn {line, y}, acres ->
        line
        |> String.graphemes
        |> Enum.with_index
        |> Enum.reduce(acres, fn {acre, x}, acres ->
          Map.put(acres, {x, y}, acre)
        end)
      end)
    new_neighbors =
      new_acres
      |> Map.keys
      |> Enum.into(Map.new, fn {x, y} ->
        neighbors =

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marekciupak · March 26, 2019, 1:34pm

My solution: https://gist.github.com/marekciupak/9ab9a2d69a6f3dd6ca9b895642c61b2e.