Day 9

Mix.install([{:kino, "~> 0.5.0"}])

Section

input = """
R 4
U 4
L 3
D 1
R 4
D 1
L 5
R 2
"""

input = Kino.Input.textarea("paste input here:")

input = Kino.Input.read(input)

input = """
R 5
U 8
L 8
D 3
R 17
D 10
L 25
U 20
"""

defmodule Rope do
  def walk([], _, visited), do: MapSet.size(visited)
  def walk([[_, 0] | rest], positions, visited), do: walk(rest, positions, visited)

  def walk([[direction, distance] | rest], {{tail_x, tail_y}, {head_x, head_y}}, visited) do
    # IO.inspect([direction, distance])
    # IO.inspect({{tail_x, tail_y}, {head_x, head_y}})

    {new_tail, new_head} =
      case direction do
        "R" ->
          new_tail =
            if head_x > tail_x do
              {tail_x + 1, head_y}
            else
              {tail_x, tail_y}
            end

          {new_tail, {head_x + 1, head_y}}

        "L" ->
          new_tail =
            if head_x < tail_x do
              {tail_x - 1, head_y}
            else
              {tail_x, tail_y}
            end

          {new_tail, {head_x - 1, head_y}}

        "D" ->
          new_tail =
            if head_y < tail_y do
              {head_x, tail_y - 1}
            else
              {tail_x, tail_y}
            end

          {new_tail, {head_x, head_y - 1}}

        "U" ->
          new_tail =
            if head_y > tail_y do
              {head_x, tail_y + 1}
            else
              {tail_x, tail_y}
            end

          {new_tail, {head_x, head_y + 1}}
      end

    walk([[direction, distance - 1] | rest], {new_tail, new_head}, MapSet.put(visited, new_tail))
  end
end

input
|> String.split()
|> Enum.chunk_every(2)
|> Enum.map(fn [d, n] -> [d, String.to_integer(n)] end)
|> Rope.walk({{0, 0}, {0, 0}}, MapSet.new([{0, 0}]))

Part 2

defmodule Rope do
  def walk([], _, visited), do: MapSet.size(visited)
  # def walk([], rope, _), do: IO.inspect(rope)

  def walk([[_, 0] | rest], rope, visited), do: walk(rest, rope, visited)

  def walk([[direction, distance] | rest], rope, visited) do
    # IO.inspect([direction, distance])
    # IO.inspect(rope)

    [{head_x, head_y} | tail] = rope

    new_head =
      case direction do
        "R" -> {head_x + 1, head_y}
        "L" -> {head_x - 1, head_y}
        "D" -> {head_x, head_y - 1}
        "U" -> {head_x, head_y + 1}
      end

    new_tail = Enum.scan(tail, new_head, &move/2)

    walk(
      [[direction, distance - 1] | rest],
      [new_head | new_tail],
      MapSet.put(visited, List.last(new_tail))
    )
  end

  def move({tail_x, tail_y}, {head_x, head_y}) do
    new_x =
      cond do
        head_x > tail_x + 1 -> tail_x + 1
        head_x < tail_x - 1 -> tail_x - 1
        true -> :same
      end

    new_y =
      cond do
        head_y > tail_y + 1 -> tail_y + 1
        head_y < tail_y - 1 -> tail_y - 1
        true -> :same
      end

    case {new_x, new_y} do
      {:same, :same} -> {tail_x, tail_y}
      {_, :same} -> {new_x, head_y}
      {:same, _} -> {head_x, new_y}
      {_, _} -> {new_x, new_y}
    end
  end
end

input
|> String.split()
|> Enum.chunk_every(2)
|> Enum.map(fn [d, n] -> [d, String.to_integer(n)] end)
|> Rope.walk(List.duplicate({0, 0}, 10), MapSet.new([{0, 0}]))