Day 14: Regolith Reservoir

Mix.install([:kino])

Section

input = Kino.Input.textarea("input")

grid =
  input
  |> Kino.Input.read()
  |> String.split("\n")
  |> Enum.map(fn line ->
    line
    |> String.split([" -> ", ","])
    |> Enum.map(&String.to_integer/1)
    |> Enum.chunk_every(2)
    |> Enum.map(&List.to_tuple/1)
  end)
  |> Enum.reduce(MapSet.new(), fn line, grid ->
    line
    |> Enum.chunk_every(2, 1, :discard)
    |> Enum.reduce(grid, fn [{x0, y0}, {x1, y1}], grid ->
      MapSet.union(grid, for(x <- x0..x1, y <- y0..y1, into: MapSet.new(), do: {x, y}))
    end)
  end)

bottom = 1 + (grid |> Enum.map(&amp;elem(&amp;1, 1)) |> Enum.max())

defmodule Sand do
  def drop(grid, bottom, {x, y} \\ {500, 0}) do
    cond do
      y == bottom ->
        {x, y}

      {x, y + 1} not in grid ->
        drop(grid, bottom, {x, y + 1})

      {x - 1, y + 1} not in grid ->
        drop(grid, bottom, {x - 1, y + 1})

      {x + 1, y + 1} not in grid ->
        drop(grid, bottom, {x + 1, y + 1})

      true ->
        {x, y}
    end
  end
end

Stream.iterate(0, &(&1 + 1))
|> Enum.reduce_while(grid, fn i, grid ->
  case Sand.drop(grid, bottom) do
    {_, ^bottom} ->
      {:halt, i}

    grain ->
      {:cont, MapSet.put(grid, grain)}
  end
end)

Stream.iterate(0, &(&1 + 1))
|> Enum.reduce_while(grid, fn i, grid ->
  case Sand.drop(grid, bottom) do
    {500, 0} ->
      {:halt, i + 1}

    grain ->
      {:cont, MapSet.put(grid, grain)}
  end
end)