Day 11

Solution

defmodule Cache do
  use Agent

  def start do
    Agent.start_link(fn -> %{} end, name: __MODULE__)
    Agent.update(__MODULE__, fn _ -> %{} end)
  end

  def get(k) do
    Agent.get(__MODULE__, &Map.get(&1, k))
  end

  def put(k, v) do
    Agent.update(__MODULE__, &Map.put(&1, k, v))
  end
end

defmodule Day11 do
  defp transform(0), do: [1]

  defp transform(stone) do
    len = trunc(:math.log10(stone) + 1)

    if rem(len, 2) == 0 do
      n = 10 |> :math.pow(div(len, 2)) |> round()
      [div(stone, n), rem(stone, n)]
    else
      [2024 * stone]
    end
  end

  def blink(_, 0), do: 1

  def blink(stone, n) do
    cache = Cache.get({stone, n})

    if is_nil(cache) do
      result = stone |> transform() |> Enum.map(&blink(&1, n - 1)) |> Enum.sum()
      Cache.put({stone, n}, result)
      result
    else
      cache
    end
  end
end

{:ok, contents} = File.read("#{__DIR__}/inputs/day11.txt")

Cache.start()

contents
|> String.split()
|> Enum.map(&String.to_integer/1)
|> Enum.map(&Day11.blink(&1, 75))
|> Enum.sum()