Day 11: Reactor

Day 11: Reactor

Day 11: Reactor

Part 1

defmodule AdventOfCode2025Day11Part1 do
  def run(input) do
    input
    |> parse()
    |> then(& solve(:you, &1))
  end

  def solve(:out, _map) do
    1
  end

  def solve(nil, _map) do
    0
  end

  def solve(node, map) do
    Map.get(map, node)
    |> Enum.reduce(0, fn node, acc ->
      acc + solve(node, map)
    end)
  end

  defp parse(input) do
    input
    |> String.split("\n", trim: true)
    |> Enum.reduce(%{}, fn line, acc ->
      [key, values] = String.split(line, ":", trim: true)
      Map.put(acc, String.to_atom(key), String.split(values, " ", trim: true) |> Enum.map(&String.to_atom/1))
    end)
  end
end

example = """
aaa: you hhh
you: bbb ccc
bbb: ddd eee
ccc: ddd eee fff
ddd: ggg
eee: out
fff: out
ggg: out
hhh: ccc fff iii
iii: out
"""

AdventOfCode2025Day11Part1.run(example)

Part 2

defmodule AdventOfCode2025Day11Part2 do
  def run(input) do
    map = parse(input)
    {ans, _memo} = solve(:svr, map, false, false, %{})
    ans
  end

  # ---- core ----

  defp solve(:out, _map, true, true, memo), do: {1, memo}
  defp solve(:out, _map, _sd, _sf, memo), do: {0, memo}

  defp solve(node, map, seen_dac, seen_fft, memo) do
    {sd, sf} = update_flags(node, seen_dac, seen_fft)
    key = {node, sd, sf}

    case Map.fetch(memo, key) do
      {:ok, cached} ->
        {cached, memo}

      :error ->
        {sum, memo2} = sum_children(node, map, sd, sf, memo)
        {sum, Map.put(memo2, key, sum)}
    end
  end

  defp sum_children(node, map, sd, sf, memo) do
    children = Map.get(map, node, [])

    Enum.reduce(children, {0, memo}, fn child, {acc, memo_acc} ->
      {v, memo_next} = solve(child, map, sd, sf, memo_acc)
      {acc + v, memo_next}
    end)
  end

  defp update_flags(node, seen_dac, seen_fft) do
    {
      seen_dac or node == :dac,
      seen_fft or node == :fft
    }
  end

  # ---- parse ----

  defp parse(input) do
    input
    |> String.split("\n", trim: true)
    |> Enum.reduce(%{}, fn line, acc ->
      [key, values] = String.split(line, ":", trim: true)

      Map.put(
        acc,
        String.to_atom(key),
        values |> String.split(" ", trim: true) |> Enum.map(&String.to_atom/1)
      )
    end)
  end
end

example = """
svr: aaa bbb
aaa: fft
fft: ccc
bbb: tty
tty: ccc
ccc: ddd eee
ddd: hub
hub: fff
eee: dac
dac: fff
fff: ggg hhh
ggg: out
hhh: out
"""

AdventOfCode2025Day11Part2.run(example)