Day Eighteen

Mix.install([
  {:kino, "~> 0.7.0"}
])

Section

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

defmodule DayEighteen do
  def solve1(input) do
    pts =
      input
      |> String.split("\n", trim: true)
      |> Enum.map(&parse/1)
      |> MapSet.new()

    pts
    |> Enum.map(&exposed_sides(&1, pts))
    |> Enum.sum()
  end

  def solve2(input) do
    pts =
      input
      |> String.split("\n", trim: true)
      |> Enum.map(&parse/1)
      |> MapSet.new()

    minx = Enum.map(pts, &elem(&1, 0)) |> Enum.min()
    miny = Enum.map(pts, &elem(&1, 1)) |> Enum.min()
    minz = Enum.map(pts, &elem(&1, 2)) |> Enum.min()

    maxx = Enum.map(pts, &elem(&1, 0)) |> Enum.max()
    maxy = Enum.map(pts, &elem(&1, 1)) |> Enum.max()
    maxz = Enum.map(pts, &elem(&1, 2)) |> Enum.max()

    pts
    |> Enum.flat_map(&exposed_sides2(&1, pts))
    |> Enum.filter(fn pt ->
      case exposed?(pt, pts, MapSet.new(), {minx, miny, minz}, {maxx, maxy, maxz}) do
        true -> true
        _ -> false
      end
    end)
    |> length()
  end

  defp exposed_sides({x, y, z}, pts) do
    [{x - 1, y, z}, {x + 1, y, z}, {x, y - 1, z}, {x, y + 1, z}, {x, y, z - 1}, {x, y, z + 1}]
    |> Enum.count(fn pt -> !MapSet.member?(pts, pt) end)
  end

  defp exposed_sides2({x, y, z}, pts) do
    [{x - 1, y, z}, {x + 1, y, z}, {x, y - 1, z}, {x, y + 1, z}, {x, y, z - 1}, {x, y, z + 1}]
    |> Enum.reject(fn pt -> MapSet.member?(pts, pt) end)
  end

  defp exposed?({x, y, z}, pts, checked, {minx, miny, minz}, {maxx, maxy, maxz}) do
    if x < minx || x > maxx || y < miny || y > maxy || z < minz || z > maxz do
      true
    else
      new_pts = [
        {x - 1, y, z},
        {x + 1, y, z},
        {x, y - 1, z},
        {x, y + 1, z},
        {x, y, z - 1},
        {x, y, z + 1}
      ]

      new_checked = MapSet.union(checked, MapSet.new(new_pts))

      new_pts
      |> Enum.reject(&amp;MapSet.member?(checked, &amp;1))
      |> Enum.reject(&amp;MapSet.member?(pts, &amp;1))
      |> Enum.reduce_while(new_checked, fn pt, checked ->
        case exposed?(pt, pts, checked, {minx, miny, minz}, {maxx, maxy, maxz}) do
          true -> {:halt, true}
          checked -> {:cont, checked}
        end
      end)
    end
  end

  defp parse(line) do
    line
    |> String.split(",")
    |> Enum.map(&amp;String.to_integer/1)
    |> then(fn [x, y, z] -> {x, y, z} end)
  end
end

DayEighteen.solve2(Kino.Input.read(input))