Day 11

Mix.install([
  {:memoize, "~> 1.4"}
])

Setup

input =
  System.fetch_env!("LB_AOC_DIR")
  |> Path.join("data/day11.txt")
  |> File.read!()

nil

nil

Solve

defmodule Day11 do
  use Memoize
  def num_digits(stone) do
    1 +
      (stone
       |> :math.log10()
       |> :math.floor())
  end

  defmemo rule(stone, iteration) do
    cond do
      iteration == 0 ->
        1

      stone == 0 ->
        rule(1, iteration - 1)

      num_digits(stone) |> round() |> rem(2) == 0 ->
        half_length = div(round(num_digits(stone)), 2)
        divisor = :math.pow(10, half_length) |> trunc()

        first_half = div(stone, divisor)
        second_half = rem(stone, divisor)
        rule(first_half, iteration - 1) + rule(second_half, iteration - 1)

      true ->
        rule(stone * 2024, iteration - 1)
    end
  end

  defp run(stones, blinks) do
    for stone <- stones, reduce: 0 do
      sum ->
        sum + rule(stone, blinks )
    end
  end

  @doc ~S"""
  iex> Day11.part1("125 17")
  55312
  """
  def part1(input) do
    stones = String.split(input) |> Enum.map(&amp;String.to_integer/1)
    run(stones, 25)
  end


  def part2(input) do
    stones = String.split(input) |> Enum.map(&amp;String.to_integer/1)
    run(stones, 75)
  end
end

{:module, Day11, <<70, 79, 82, 49, 0, 0, 14, ...>>, {:part2, 1}}