defmodule PascalsTriangle do
  @doc """
  Calculates the rows of a pascal triangle
  with the given height
  """
  @spec rows(integer) :: [[integer]]
  def rows(num) do
    1..num
    |> Enum.reduce([[]], fn _, triangle -> [next(hd(triangle)) | triangle] end)
    |> Enum.take(num)
    |> Enum.reverse
  end

  defp next(from, acc \\ [])
  defp next([], _acc), do: [1]
  defp next([1], _acc), do: [1, 1]
  defp next([one, two | []], acc), do: [1 | Enum.reverse([1, one + two | acc])]
  defp next([one, two | rest], acc), do: next([two | rest], [one + two | acc])
end