Advent 2024 - Day 11

Mix.install([
  {:kino, "~> 0.14.2"}
])

Setup

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

input = input
  |> Kino.Input.read()
  |> String.split(" ")
  |> Enum.map(&(String.to_integer/1))

Part 1

1..25
|> Enum.reduce(input, fn _, acc ->
  for stone <- acc do
    if stone == 0 do
      [1]
    else
      stone_as_text = Integer.to_string(stone)
      stone_length = String.length(stone_as_text)

      if rem(stone_length, 2) == 0 do
        {a, b} = String.split_at(stone_as_text, div(stone_length, 2))
        [
          String.to_integer(a),
          String.to_integer(b)
        ]
      else
        stone * 2024
      end
    end
  end
  |> List.flatten()
end)
|> Enum.count()

Part 2

input = Enum.frequencies(input)

update_count = fn map, stone, count ->
  map
  |> Map.put(stone, Map.get(map, stone, 0))
  |> Map.update!(stone, fn current_count -> current_count + count end)
end

1..75
|> Enum.reduce(input, fn _, acc ->
  Enum.reduce(acc, %{}, fn {stone, count}, acc ->
    if stone == 0 do
      update_count.(acc, 1, count)
    else
      stone_as_text = Integer.to_string(stone)
      stone_length = String.length(stone_as_text)

      if rem(stone_length, 2) == 0 do
        {a, b} = String.split_at(stone_as_text, div(stone_length, 2))
        acc = update_count.(acc, String.to_integer(a), count)
        update_count.(acc, String.to_integer(b), count)
      else
        update_count.(acc, stone * 2024, count)
      end
    end
  end)
end)
|> Map.values()
|> Enum.sum()