Day 11

Mix.install([:kino_aoc])

Section

{:ok, puzzle_input} =
  KinoAOC.download_puzzle("2024", "11", System.fetch_env!("LB_ADVENT_OF_CODE_SESSION"))

{:ok, "554735 45401 8434 0 188 7487525 77 7"}

# puzzle_input = "125 17"

stones =
  puzzle_input
  |> String.split()
  |> Enum.map(&String.to_integer/1)
  |> Enum.frequencies()

%{0 => 1, 7 => 1, 77 => 1, 188 => 1, 8434 => 1, 45401 => 1, 554735 => 1, 7487525 => 1}

defmodule Stones do
  def blink(list), do: Enum.reduce(list, %{}, &alter/2)

  defp alter({0, count}, map), do: update(map, 1, count)

  defp alter({n, count}, map) do
    len = len(n)

    if rem(len, 2) == 0 do
      half = 10 ** div(len, 2)

      map
      |> update(div(n, half), count)
      |> update(rem(n, half), count)
    else
      update(map, n * 2024, count)
    end
  end

  defp len(n) when n < 10, do: 1
  defp len(n) when n < 100, do: 2
  defp len(n) when n < 1000, do: 3
  defp len(n) when n < 10000, do: 4
  defp len(n), do: ceil(:math.log10(n))

  defp update(map, key, val), do: Map.update(map, key, val, &amp;(&amp;1 + val))
end

{:module, Stones, <<70, 79, 82, 49, 0, 0, 11, ...>>, {:update, 3}}

Part 1

1..25
|> Enum.reduce(stones, fn _, acc ->
  Stones.blink(acc)
end)
|> Map.values()
|> Enum.sum()

209412

Part 2

1..75
|> Enum.reduce(stones, fn _, acc ->
  Stones.blink(acc)
end)
|> Map.values()
|> Enum.sum()

248967696501656