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

  @doc """
  Return the number of years a person that has lived for 'seconds' seconds is
  aged on 'planet'.
  """
  @spec age_on(planet, pos_integer) :: float
  def age_on(planet, seconds) do
      cond do
        planet == :mercury ->
            earth_age(seconds) / 0.2408467
        planet == :venus ->
            earth_age(seconds) / 0.61519726
        planet == :earth ->
            earth_age(seconds)
        planet == :mars ->
            earth_age(seconds) / 1.8808158
        planet == :jupiter ->
            earth_age(seconds) / 11.862615
        planet == :saturn ->
            earth_age(seconds) / 29.447498
        planet == :uranus ->
            earth_age(seconds) / 84.016846
        planet == :neptune ->
            earth_age(seconds) / 164.79132
      end
  end

  def earth_age(seconds) do
      seconds / 31557600
  end

end