Day 10

Mix.install([
  {:req, "~> 0.4.5"},
  {:kino, "~> 0.11.3"}
])

Input

input =
  Req.get!(
    "https://adventofcode.com/2023/day/10/input",
    headers: [{"Cookie", ~s"session=#{System.fetch_env!("LB_AOC_SESSION")}"}]
  ).body

Kino.Text.new(input, terminal: true)

Part 1

defmodule Part1 do
  def parse(input) do
    lines = String.split(input, "\n", trim: true)
    height = length(lines)
    width = String.length(hd(lines))
    padding = String.duplicate(".", width)
    lines = ([padding] ++ lines ++ [padding]) |> Enum.map(&amp;("." <> &amp;1 <> "."))

    map =
      for {line, y} <- Enum.with_index(lines),
          {char, x} <- line |> String.graphemes() |> Enum.with_index(),
          into: %{} do
        {{y, x}, char}
      end

    {map, {height + 2, width + 2}}
  end

  @connections %{
    "S" => [{-1, 0}, {1, 0}, {0, -1}, {0, 1}],
    "|" => [{-1, 0}, {1, 0}],
    "-" => [{0, -1}, {0, 1}],
    "L" => [{-1, 0}, {0, 1}],
    "J" => [{-1, 0}, {0, -1}],
    "7" => [{1, 0}, {0, -1}],
    "F" => [{1, 0}, {0, 1}],
    "." => []
  }

  def loop(pipes) do
    {{y, x} = start, _} = Enum.find(pipes, fn {_, char} -> char == "S" end)

    {dy, dx} =
      delta =
      Enum.find(@connections[pipes[start]], fn {dy, dx} ->
        {-dy, -dx} in @connections[pipes[{y + dy, x + dx}]]
      end)

    loop(pipes, start, {y + dy, x + dx}, delta, [start])
  end

  def loop(_, start, curr, _, path) when start == curr, do: Enum.reverse(path)

  def loop(pipes, start, {y, x} = curr, {dy0, dx0}, path) do
    [{dy1, dx1} = delta] = @connections[pipes[curr]] -- [{-dy0, -dx0}]
    loop(pipes, start, {y + dy1, x + dx1}, delta, [curr | path])
  end
end

{map, _} = Part1.parse(input)
map |> Part1.loop() |> length() |> div(2)

Part 2

defmodule Part2 do
  def enhance(map, {height, width}, path) do
    map =
      map
      |> Enum.flat_map(fn {{y0, x0}, c} ->
        {y1, x1} = {2 * y0, 2 * x0}

        [
          {{y1, x1}, c},
          {{y1 - 1, x1}, "."},
          {{y1 - 1, x1 - 1}, "."},
          {{y1, x1 - 1}, "."}
        ]
        |> Enum.filter(fn {{y1, x1}, _} -> y1 >= 0 and x1 >= 0 end)
      end)
      |> Map.new()

    {path, map} =
      path
      |> Enum.chunk_every(2, 1, Enum.take(path, 1))
      |> Enum.flat_map_reduce(map, fn [{y0, x0}, {y1, x1}], acc ->
        {dy, dx} = {y1 - y0, x1 - x0}
        {y2, x2} = {2 * y0, 2 * x0}
        gap = {y2 + dy, x2 + dx}
        char = if abs(dy) == 1, do: "|", else: "-"
        acc = %{acc | gap => char}

        {[{y2, x2}, gap], acc}
      end)

    {map, {2 * height - 1, 2 * width - 1}, path}
  end

  def fill(map, size, path) do
    path = MapSet.new(path)
    frontier = [{0, 0}]
    explored = MapSet.new(frontier) |> MapSet.union(path)
    fill(map, size, frontier, explored)
  end

  defp fill(_, _, [], explored), do: explored

  defp fill(map, {height, width} = size, [{y0, x0} | rest], explored) do
    neighbors =
      [{y0 - 1, x0}, {y0, x0 - 1}, {y0 + 1, x0}, {y0, x0 + 1}]
      |> Enum.filter(fn {y1, x1} = coord ->
        y1 >= 0 and
          y1 < height and
          x1 >= 0 and
          x1 < width and
          not MapSet.member?(explored, coord)
      end)

    frontier = neighbors ++ rest
    explored = MapSet.new(neighbors) |> MapSet.union(explored)
    fill(map, size, frontier, explored)
  end
end

{map, size} = Part1.parse(input)
path = Part1.loop(map)
{map, size, path} = Part2.enhance(map, size, path)

outside = Part2.fill(map, size, path)

map
|> Map.keys()
|> MapSet.new()
|> MapSet.difference(outside)
|> Enum.filter(fn {y, x} -> rem(y, 2) == 0 and rem(x, 2) == 0 end)
|> length()