Day 6

Mix.install([{:utils, path: "#{__DIR__}/utils"}])

Solution

{grid, exited?} = Utils.read_grid("day06")

start = Enum.find_value(grid, fn {k, v} -> if v == ?^, do: k end)

base_obstacles =
  grid
  |> Enum.filter(fn {_, char} -> char == ?# end)
  |> Enum.map(fn {coords, _} -> coords end)
  |> MapSet.new()

[up, down, left, right] = [{-1, 0}, {1, 0}, {0, -1}, {0, 1}]

rotate = fn
  ^up -> right
  ^right -> down
  ^down -> left
  ^left -> up
end

add = fn {a, b}, {c, d} -> {a + c, b + d} end

simulate = fn f, obstacles, pos, dir, visited ->
  cond do
    exited?.(pos) ->
      {visited, false}

    {pos, dir} in visited ->
      {visited, true}

    add.(pos, dir) in obstacles ->
      dir_ = rotate.(dir)
      f.(f, obstacles, pos, dir_, visited)

    true ->
      pos_ = add.(pos, dir)
      visited_ = MapSet.put(visited, {pos, dir})
      f.(f, obstacles, pos_, dir, visited_)
  end
end

{visited, _} = simulate.(simulate, base_obstacles, start, up, MapSet.new())
path = visited |> Enum.map(fn {pos, _} -> pos end) |> Enum.uniq()
Enum.count(path)

# this takes like 7 minutes
add_obstacle? = fn obstacle ->
  cond do
    obstacle == start ->
      false

    obstacle in base_obstacles ->
      false

    true ->
      obstacles = MapSet.put(base_obstacles, obstacle)
      {_, looped} = simulate.(simulate, obstacles, start, up, MapSet.new())
      looped
  end
end

path |> Enum.count(add_obstacle?)