Recursion if it’s truely while do or do while:
https://github.com/LostKobrakai/aoc2020/blob/master/lib/aoc2020/day11.ex#L182-L191

My other goto for evaluation (does not always fit) is Stream.unfold:
https://github.com/LostKobrakai/aoc2020/blob/d9e79ebd6cd6d346df1705fd7ef109bb4fa5f596/lib/aoc2020/day3.ex#L30-L39

I ended up writing a generator to create my adventofcode files for each challenge and to download the puzzle as well. You can use the code if you want to

I ended up with something nice for finding neighbours on part 2, but wow, this is veeeerryyyy slooooow (~4 seconds)

  def neighbours(current, {x, y}) do
    Enum.reduce(@dirs, 0, fn {dx, dy}, acc ->
      [v] =
        Stream.iterate(1, &(&1 + 1))
        |> Stream.map(fn mul -> {x + dx * mul, y + dy * mul} end)
        |> Stream.map(fn {xx, yy} -> Map.get(current, {xx, yy}) end)
        |> Stream.drop_while(&(&1 == @floor))
        |> Enum.take(1)

      if v == @occupied, do: acc + 1, else: acc
    end)
  end

I was thinking about the fact that the current map is RO, did someone try to add some concurrency into the mix ?