defmodule SpaceAge do
  @type planet ::
          :mercury
          | :venus
          | :earth
          | :mars
          | :jupiter
          | :saturn
          | :uranus
          | :neptune

  @planets ~w(mercury venus earth mars jupiter saturn uranus neptune)a

  defguardp is_planet(planet) when planet in @planets

  defp factor(:mercury), do: 0.2408467
  defp factor(:venus), do: 0.61519726
  defp factor(:earth), do: 1.0
  defp factor(:mars), do: 1.8808158
  defp factor(:jupiter), do: 11.862615
  defp factor(:saturn), do: 29.447498
  defp factor(:uranus), do: 84.016846
  defp factor(:neptune), do: 164.79132

  @doc """
  Return the number of years a person that has lived for 'seconds' seconds is
  aged on 'planet', or an error if 'planet' is not a planet.
  """
  @spec age_on(planet, pos_integer) :: {:ok, float} | {:error, String.t()}
  def age_on(planet, seconds) when is_planet(planet),
    do: {:ok, seconds / (factor(planet) * 31_557_600)}

  def age_on(_planet, _seconds),
    do: {:error, "not a planet"}
end