Day 12

Setup

• Stream recording

Mix.install([
  {:kino, "~> 0.5.0"}
])

input = Kino.Input.textarea("Input")

edges =
  input
  |> Kino.Input.read()
  |> String.split("\n", trim: true)
  |> Enum.reduce(%{}, fn line, acc ->
    [left, right] = String.split(line, "-")
    acc = Map.update(acc, left, [right], &[right | &1])

    if left == "start" or right == "end" do
      acc
    else
      Map.update(acc, right, [left], &[left | &1])
    end
  end)

Part 1

defmodule Once do
  def recur(edges) do
    recur(edges["start"], edges, MapSet.new(), ["start"], 0)
  end

  defp recur(["end" | caves], edges, seen, path, count) do
    recur(caves, edges, seen, path, count + 1)
  end

  defp recur([cave | caves], edges, seen, path, count) do
    count =
      cond do
        cave == "start" or cave in seen ->
          count

        lowercased?(cave) ->
          recur(edges[cave], edges, MapSet.put(seen, cave), [cave | path], count)

        true ->
          recur(edges[cave], edges, seen, [cave | path], count)
      end

    recur(caves, edges, seen, path, count)
  end

  defp recur([], _edges, _seen, _path, count) do
    count
  end

  defp lowercased?(cave), do: String.downcase(cave, :ascii) == cave
end

Once.recur(edges)

Part 2

defmodule Twice do
  def recur(edges) do
    recur(edges["start"], edges, MapSet.new(), false, ["start"], 0)
  end

  defp recur(["end" | caves], edges, seen, once?, path, count) do
    recur(caves, edges, seen, once?, path, count + 1)
  end

  defp recur([cave | caves], edges, seen, once?, path, count) do
    count =
      cond do
        cave == "start" or (cave in seen and once?) ->
          count

        cave in seen ->
          recur(edges[cave], edges, MapSet.put(seen, cave), true, [cave | path], count)

        lowercased?(cave) ->
          recur(edges[cave], edges, MapSet.put(seen, cave), once?, [cave | path], count)

        true ->
          recur(edges[cave], edges, seen, once?, [cave | path], count)
      end

    recur(caves, edges, seen, once?, path, count)
  end

  defp recur([], _edges, _seen, _path, _once?, count) do
    count
  end

  defp lowercased?(cave), do: String.downcase(cave, :ascii) == cave
end

Twice.recur(edges)