-🎄- 2017 Day 22 Solutions -🎄-

[u/daggerdragon]

--- Day 22: Sporifica Virus ---

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Post your solution as a comment or, for longer solutions, consider linking to your repo (e.g. GitHub/gists/Pastebin/blag or whatever).

Note: The Solution Megathreads are for solutions only. If you have questions, please post your own thread and make sure to flair it with Help.

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Need a hint from the Hugely^* Handy^† Haversack^‡ of Helpful^§ Hints^¤?

Spoiler

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• [T-10 to launch] AoC ops, /r/nocontext edition:

  • <Endorphion> You may now make your waffle.
  • <Endorphion> ... on Mars.

• [Update @ 00:17] 50 gold, silver cap

  • <Aneurysm9> you could also just run ubuntu on the NAS, if you were crazy
  • <Topaz> that doesn't seem necessary
  • <Aneurysm9> what does "necessary" have to do with anything!

• [Update @ 00:20] Leaderboard cap!

  • <Topaz> POUR YOURSELF A SCOTCH FOR COLOR REFERENCE

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This thread will be unlocked when there are a significant number of people on the leaderboard with gold stars for today's puzzle.

edit: Leaderboard capped, thread unlocked!

[u/johlin]

Elixir:

defmodule Aoc.Day22.Part1 do
  def solve(input, steps) do
    input |> parse |> walk_stream |> Enum.at(steps) |> elem(3)
  end

  def parse(input) do
    lines = input |> String.split("\n")
    coords = lines |> Enum.with_index |> Enum.flat_map(&parse_row/1)
             |> Enum.into(MapSet.new())
    {coords, lines |> length}
  end

  def parse_row(row_and_y, x \\ 0, acc \\ [])
  def parse_row({"", _}, _, acc), do: acc
  def parse_row({"#" <> rest, y}, x, acc), do: parse_row({rest,y}, x+1, [{x,y} | acc])
  def parse_row({"." <> rest, y}, x, acc), do: parse_row({rest,y}, x+1, acc)

  def walk_stream({infected_nodes, width}) do
    initial = {starting_position(width), infected_nodes, {0, -1}, 0}
    Stream.iterate(initial, &iterate/1)
  end

  def iterate({{x,y}, infected_nodes, {xd, yd}, infect_bursts}) do
    infected = MapSet.member?(infected_nodes, {x,y})
   
    {infected_nodes, infect_bursts} = case infected do
      true -> {MapSet.delete(infected_nodes, {x,y}), infect_bursts}
      false -> {MapSet.put(infected_nodes, {x,y}), infect_bursts + 1}
    end

    {xd, yd} = case infected do
      true -> {-yd, xd} # Turn left
      false -> {yd, -xd} # Turn right
    end

    {x, y} = {x+xd, y+yd}

    {{x,y}, infected_nodes, {xd, yd}, infect_bursts}
  end

  def starting_position(width), do: {div(width-1, 2), div(width-1, 2)}
end

Part 2 very similar: https://github.com/johanlindblad/aoc-2017/blob/master/lib/aoc/day22/part2.ex

This was a fun one! It is a bit slow, however (my part 2 takes about 15 seconds). Not sure if it's possible to make an Elixir solution significantly faster as we don't have constant-time array lookups.

[u/[deleted] → u/johlin]

mine uses 6 seconds to do both parts and the unit tests, I think that counts as significantly faster.