allergies

elixir/allergies/lib/allergies.ex

@@ -0,0 +1,27 @@
+defmodule Allergies do
+  @allergens %{
+    "eggs" => 1,
+    "peanuts" => 2,
+    "shellfish" => 4,
+    "strawberries" => 8,
+    "tomatoes" => 16,
+    "chocolate" => 32,
+    "pollen" => 64,
+    "cats" => 128
+  }
+
+  @doc """
+  List the allergies for which the corresponding flag bit is true.
+  """
+  @spec list(non_neg_integer) :: [String.t()]
+  def list(flags) do
+    for {allergen, mask} <- @allergens, allergic_to?(mask, flags), do: allergen
+  end
+
+  @doc """
+  Returns whether the corresponding flag bit in 'flags' is set for the item.
+  """
+  @spec allergic_to?(non_neg_integer, String.t() | non_neg_integer) :: boolean
+  def allergic_to?(flags, mask) when is_integer(mask), do: Bitwise.band(mask, flags) > 0
+  def allergic_to?(flags, allergen) when is_binary(allergen), do: allergic_to?(@allergens[allergen], flags)
+end