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