🎄 Year 2023 🔔 Day 16

Setup

map =
  File.read!("#{__DIR__}/../../../inputs/2023/day16.txt")
  |> String.split("\n", trim: true)
  |> Enum.map(&String.split(&1, "", trim: true))
  |> Enum.with_index(fn line, y ->
    line
    |> Enum.with_index(fn char, x ->
      {{x, y}, char}
    end)
  end)
  |> Enum.concat()
  |> Map.new()

{{max_x, max_y}, _} = Enum.max_by(map, fn {{x, y}, _} -> x + y end)

Part 1

Stream.iterate([{{-1, 0}, {1, 0}}], fn es ->
  Enum.flat_map(es, fn {{cx, cy}, {dx, dy}} ->
    new_coords = {cx + dx, cy + dy}
    tile = Map.get(map, new_coords)

    case tile do
      nil -> []
      "." -> [{new_coords, {dx, dy}}]
      "-" when dy == 0 -> [{new_coords, {dx, dy}}]
      "-" -> [{new_coords, {1, 0}}, {new_coords, {-1, 0}}]
      "|" when dx == 0 -> [{new_coords, {dx, dy}}]
      "|" -> [{new_coords, {0, 1}}, {new_coords, {0, -1}}]
      "\\" -> [{new_coords, {dy, dx}}]
      "/" -> [{new_coords, {-dy, -dx}}]
    end
  end)
  |> Enum.filter(fn {coords, dir} ->
    if Process.get({coords, dir}) == nil do
      Process.put({coords, dir}, true)
      true
    else
      false
    end
  end)
end)
|> Enum.take_while(fn es -> es != [] end)
|> Enum.concat()
|> Enum.map(&elem(&1, 0))
|> MapSet.new()
|> Enum.count()
|> then(&(&1 - 1))

Part 2

Enum.concat([
  for(x <- [-1], y <- 0..max_y, dir <- [{1, 0}], do: {{x, y}, dir}),
  for(x <- [max_x + 1], y <- 0..max_y, dir <- [{-1, 0}], do: {{x, y}, dir}),
  for(x <- 0..max_x, y <- [-1], dir <- [{0, 1}], do: {{x, y}, dir}),
  for(x <- 0..max_x, y <- [max_y + 1], dir <- [{0, -1}], do: {{x, y}, dir})
])
|> Task.async_stream(fn {start_coord, start_dir} ->
  Stream.iterate([{start_coord, start_dir}], fn es ->
    Enum.flat_map(es, fn {{cx, cy}, {dx, dy}} ->
      new_coords = {cx + dx, cy + dy}
      tile = Map.get(map, new_coords)

      case tile do
        nil -> []
        "." -> [{new_coords, {dx, dy}}]
        "-" when dy == 0 -> [{new_coords, {dx, dy}}]
        "-" -> [{new_coords, {1, 0}}, {new_coords, {-1, 0}}]
        "|" when dx == 0 -> [{new_coords, {dx, dy}}]
        "|" -> [{new_coords, {0, 1}}, {new_coords, {0, -1}}]
        "\\" -> [{new_coords, {dy, dx}}]
        "/" -> [{new_coords, {-dy, -dx}}]
      end
    end)
    |> Enum.filter(fn {coords, dir} ->
      if Process.get({coords, dir}) == nil do
        Process.put({coords, dir}, true)
        true
      else
        false
      end
    end)
  end)
  |> Enum.take_while(fn es -> es != [] end)
  |> Enum.concat()
  |> Enum.map(&amp;elem(&amp;1, 0))
  |> MapSet.new()
  |> Enum.count()
  |> then(&amp;(&amp;1 - 1))
end)
|> Enum.max()
|> elem(1)