space_age

elixir/space-age/lib/space_age.ex

@@ -0,0 +1,35 @@
+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