Day 9

Mix.install([
  {:req, "~> 0.3.2"},
  {:vega_lite, "~> 0.1.6"},
  {:kino_vega_lite, "~> 0.1.6"}
])

alias VegaLite, as: Vl

day = 9

aoc_session = System.fetch_env!("LB_AOC_SESSION")
input_url = "https://adventofcode.com/2022/day/#{day}/input"
{:ok, %{body: input}} = Req.get(input_url, headers: [cookie: "session=#{aoc_session}"])

Input

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

Part 1

defmodule Point do
  defstruct x: 0, y: 0

  def left(p), do: %{p | x: p.x - 1}
  def right(p), do: %{p | x: p.x + 1}
  def down(p), do: %{p | y: p.y - 1}
  def up(p), do: %{p | y: p.y + 1}
end

defmodule State do
  @origin %Point{x: 0, y: 0}
  defstruct visited: MapSet.new([@origin]), t: @origin, h: @origin
end

defmodule Rope do
  def move(state, dir, steps) do
    for _ <- 0..(steps - 1), reduce: state do
      s when dir == "L" -> left(s)
      s when dir == "R" -> right(s)
      s when dir == "U" -> up(s)
      s when dir == "D" -> down(s)
    end
  end

  def left(%{t: t, h: h} = state) do
    new_t = if h.x < t.x, do: %Point{x: t.x - 1, y: h.y}, else: t
    %State{visited: MapSet.put(state.visited, new_t), t: new_t, h: Point.left(h)}
  end

  def right(%{t: t, h: h} = state) do
    new_t = if h.x > t.x, do: %Point{x: t.x + 1, y: h.y}, else: t
    %State{visited: MapSet.put(state.visited, new_t), t: new_t, h: Point.right(h)}
  end

  def down(%{t: t, h: h} = state) do
    new_t = if h.y < t.y, do: %Point{x: h.x, y: t.y - 1}, else: t
    %State{visited: MapSet.put(state.visited, new_t), t: new_t, h: Point.down(h)}
  end

  def up(%{t: t, h: h} = state) do
    new_t = if h.y > t.y, do: %Point{x: h.x, y: t.y + 1}, else: t
    %State{visited: MapSet.put(state.visited, new_t), t: new_t, h: Point.up(h)}
  end
end

%State{}
|> Rope.right()
|> Rope.right()
|> Rope.right()
|> Rope.right()
|> Rope.up()
|> Rope.up()
|> Rope.up()
|> Rope.up()
|> Rope.left()
|> Rope.left()
|> Rope.left()

%State{}
|> Rope.move("R", 4)

# input = test_input

state =
  input
  |> String.split("\n", trim: true)
  |> Enum.reduce(%State{}, fn line, state ->
    [dir, n] = String.split(line, " ")
    Rope.move(state, dir, String.to_integer(n))
  end)

MapSet.size(state.visited)

Part 2