aoc2017_day12.ex

defmodule Day12 do
  def part1(), do:
    connections() |> group("0") |> Enum.count()

  def part2(), do:
    connections() |> all_groups() |> Enum.count()

  defp connections(), do:
    "input.txt"
    |> File.stream!()
    |> Stream.map(&String.trim/1)
    |> Stream.map(&String.split(&1, " <-> "))
    |> Stream.map(fn [element, neighbours] -> {element, String.split(neighbours, ", ")} end)
    |> Map.new()

  defp all_groups(connections), do:
    Enum.reduce(
      Map.keys(connections),
      [],
      fn program, groups ->
        if Enum.any?(groups, &MapSet.member?(&1, program)),
          do: groups,
          else: [group(connections, program) | groups]
      end
    )

  defp group(connections, program) do
    {neighbours, other_connections} = Map.pop(connections, program, MapSet.new())
    Enum.reduce(neighbours, MapSet.new([program]), &MapSet.union(&2, group(other_connections, &1)))
  end
end

Day12.part1() |> IO.inspect
Day12.part2() |> IO.inspect