Day 12

Setup

Mix.install([
  {:kino, "~> 0.4.1"}
])

Input

input = Kino.Input.textarea("Please paste your input:")

Parse

map =
  input
  |> Kino.Input.read()
  |> String.split("\n", trim: true)
  |> Enum.reduce(%{}, fn line, acc ->
    [a, b] = String.split(line, "-")

    acc
    |> then(fn acc ->
      if b != "start", do: Map.update(acc, a, [b], &[b | &1]), else: acc
    end)
    |> then(fn acc ->
      if a != "start", do: Map.update(acc, b, [a], &[a | &1]), else: acc
    end)
  end)

Submarine

defmodule Submarine do
  def travel(map, start_node) do
    travel(map, start_node, map[start_node], [], [])
  end

  defp travel(_map, "end", _, path, all_paths) do
    end_path = ["end" | path] |> Enum.reverse()
    [end_path | all_paths]
  end

  defp travel(_map, _node, [], _path, all_paths) do
    all_paths
  end

  defp travel(_map, _node, _nodes, ["end" | _], all_paths) do
    all_paths
  end

  defp travel(map, node, [next_node | rest], path, all_paths) do
    if can_visit?(path, node) do
      travel(map, node, rest, path, all_paths) ++
        travel(map, next_node, map[next_node], [node | path], all_paths)
    else
      all_paths
    end
  end

  defp can_visit?(path, node) do
    case String.upcase(node) do
      ^node -> true
      _ -> node not in path
    end
  end
end

BetterSubmarine

defmodule BetterSubmarine do
  def travel(map, start_node) do
    travel(map, start_node, map[start_node], [], [])
  end

  defp travel(_map, "end", _, path, all_paths) do
    [["end" | path] | all_paths] |> Enum.reverse()
  end

  defp travel(_map, _node, [], _path, all_paths) do
    all_paths
  end

  defp travel(_map, _node, _nodes, ["end" | _], all_paths) do
    all_paths
  end

  defp travel(map, node, [next_node | rest], path, all_paths) do
    if can_visit?(path, node) do
      travel(map, node, rest, path, all_paths) ++
        travel(map, next_node, map[next_node], [node | path], all_paths)
    else
      all_paths
    end
  end

  defp can_visit?(path, node) do
    case String.upcase(node) do
      ^node ->
        true

      _ ->
        path
        |> Enum.reject(fn node -> String.upcase(node) == node end)
        |> Enum.frequencies()
        |> then(fn fqs ->
          any_node_visited_twice? = Enum.any?(fqs, fn {_n, fq} -> fq > 1 end)
          this_node_times_visited = Map.get(fqs, node, 0)

          this_node_times_visited == 0 or !any_node_visited_twice?
        end)
    end
  end
end

Part 1

Submarine.travel(map, "start")
|> Enum.count()

Part 2

BetterSubmarine.travel(map, "start")
|> Enum.count()