Day 11

Mix.install([:kino_aoc])

Parse

{:ok, puzzle_input} =
  KinoAOC.download_puzzle("2025", "11", System.fetch_env!("LB_ADVENT_OF_CODE_SESSION"))

graph =
  puzzle_input
  |> String.split("\n", trim: true)
  |> Map.new(fn <> <> ": " <> rest ->
    {from, String.split(rest)}
  end)

Implementation

defmodule Servers do
  def paths(graph, from, to), do: paths(graph, from, to, [from])

  defp paths(_graph, to, to, acc), do: [Enum.reverse(acc)]

  defp paths(graph, from, to, acc) do
    if next = graph[from] do
      Stream.flat_map(next, &paths(graph, &1, to, [&1 | acc]))
    else
      []
    end
  end

  def paths_through(graph, from, to, required),
    do: path_through(graph, from, to, MapSet.new(required), %{})

  defp path_through(_graph, to, to, required, memo),
    do: {if(Enum.empty?(required), do: 1, else: 0), memo}

  defp path_through(graph, from, to, required, memo) do
    state = MapSet.delete(required, from)

    with :error <- Map.fetch(memo, {from, state}),
         {:ok, next} <- Map.fetch(graph, from) do
      {sum, memo} =
      Enum.reduce(next, {0, memo}, fn n, {sum, acc} ->
        {c, next_acc} = path_through(graph, n, to, state, acc)

        {c + sum, next_acc}
      end)

      {sum, Map.put(memo, {from, state}, sum)}
    else
      :error -> {0, memo}
      {:ok, val} -> {val, memo}
    end
  end
end

Part 1

Servers.paths(graph, "you", "out") |> Enum.count()

Part 2

Servers.paths_through(graph, "svr", "out", ["dac", "fft"])
|> elem(0)