aoc/23/elixir/lib/days/day8.ex

defmodule AOC.Day8 do
  use AOC.Day, day: 8

  @start "AAA"
  @stop "ZZZ"

  def parse_input([instructions, _ | nodes]) do
    instructions = instructions |> String.to_charlist() |> Stream.cycle()

    nodes =
      Enum.map(nodes, fn s ->
        %{"id" => id, "left" => left, "right" => right} =
          Regex.named_captures(
            ~r/^(?<id>[[:alnum:]]{3}) = \((?<left>[[:alnum:]]{3}), (?<right>[[:alnum:]]{3})\)$/,
            s
          )

        {id, {left, right}}
      end)
      |> Enum.into(%{})

    {instructions, nodes}
  end

  def part1({instructions, nodes}) do
    reach_end(instructions, nodes)
  end

  def part2({instructions, nodes}) do
    IO.puts("Use LCM calculator for the following numbers :)")

    Enum.filter(nodes, fn {k, _v} -> String.ends_with?(k, "A") end)
    |> Enum.map(fn {k, _v} -> find_cycle(instructions, nodes, k) end)
    |> Enum.map(&elem(&1, 0))
    |> Enum.map(&Integer.to_string/1)
    |> Enum.join(" ")
  end

  def reach_end(instructions, nodes) do
    Enum.reduce_while(instructions, {@start, 0}, fn instruction, {current_node, step_count} ->
      if current_node == @stop do
        {:halt, step_count}
      else
        {left, right} = Map.fetch!(nodes, current_node)

        next_node =
          case instruction do
            ?L -> left
            ?R -> right
          end

        {:cont, {next_node, step_count + 1}}
      end
    end)
  end

  def find_cycle(instructions, nodes, start) do
    Enum.reduce_while(instructions, {start, 0, nil}, fn instruction,
                                                        {current_node, step_count, last_end_count} ->
      if String.ends_with?(current_node, "Z") and last_end_count != nil do
        {:halt, {last_end_count, step_count - last_end_count}}
      else
        last_end_count =
          if String.ends_with?(current_node, "Z"), do: step_count, else: last_end_count

        {left, right} = Map.fetch!(nodes, current_node)

        next_node =
          case instruction do
            ?L -> left
            ?R -> right
          end

        {:cont, {next_node, step_count + 1, last_end_count}}
      end
    end)
  end
end