def trees_in_trajectory(levels, across) do
    levels
    |> String.split("\n", trim: true)
    |> Enum.with_index()
    |> Enum.map(fn {level, level_n} -> get_cell_in_level(level, level_n * across) end)
    |> Enum.count(&tree?/1)
  end

  def test_trees_in_trajectory() do
    0 = TobogganTrajectory.trees_in_trajectory(".", 0)
    1 = TobogganTrajectory.trees_in_trajectory("#", 0)
    2 = TobogganTrajectory.trees_in_trajectory("#\n#", 0)
    1 = TobogganTrajectory.trees_in_trajectory("#\n.", 0)
    1 = TobogganTrajectory.trees_in_trajectory("#.\n#.", 1)
  end

TobogganTrajectoryTest.test_trees_in_trajectory()
File.read!("input") |> TobogganTrajectory.trees_in_trajectory(3) |> IO.inspect()