defmodule AOC.Day21 do
  # use AOC.Day, day: 21, input: "example"
  use AOC.Day, day: 21

  def parse_input(lines) do
    map =
      lines
      |> Enum.with_index()
      |> Enum.flat_map(fn {line, y} ->
        line
        |> String.to_charlist()
        |> Enum.with_index()
        |> Enum.map(fn {tile, x} -> {{y, x}, tile} end)
      end)
      |> Enum.into(%{})

    start_position =
      map
      |> Enum.find(fn {_, tile} -> tile == ?S end)
      |> elem(0)

    {Map.put(map, start_position, ?.), start_position}
  end

  def walk(map, start_position, steps) do
    map
    |> do_walk(start_position, steps)
    |> Enum.count()
  end

  def do_walk(map, position, _) when not is_map_key(map, position), do: []
  def do_walk(map, position, _) when :erlang.map_get(position, map) == ?#, do: []
  def do_walk(_map, position, 0), do: [position]

  def do_walk(map, {y, x} = position, steps_left) do
    case :ets.lookup(:steps_memo, {position, steps_left}) do
      [] ->
        reachable =
          [{y - 1, x}, {y, x + 1}, {y + 1, x}, {y, x - 1}]
          |> Enum.flat_map(fn neighbor_position ->
            do_walk(map, neighbor_position, steps_left - 1)
          end)
          |> Enum.uniq()

        :ets.insert(:steps_memo, {{position, steps_left}, reachable})

        reachable

      [{_, reachable}] ->
        reachable
    end
  end

  def part1({map, start_position}) do
    :ets.new(:steps_memo, [:named_table])

    map
    |> walk(start_position, 64)
    |> IO.inspect()

    "TODO"
  end

  def part2(_input) do
    "TODO"
  end
end