defmodule Sublist do
  @type result ::
    :sublist
    | :superlist
    | :unequal
    | :equal

  @doc """
  Returns whether the first list is a sublist or a superlist of the second list
  and if not whether it is equal or unequal to the second list.
  """
  def compare(a, a), do: :equal
  def compare(a, b) when length(a) > length(b), do: (if subset?(b, a), do: :superlist, else: :unequal)
  def compare(a, b) when length(a) < length(b), do: (if subset?(a, b), do: :sublist, else: :unequal)
  def compare(_, _), do: :unequal

  defp subset?(a, b, index \\ 0)
  defp subset?([], _b, _index), do: true
  defp subset?(a, b, index) when index + length(a) > length(b), do: false
  defp subset?(a, b, index), do:
    (if Enum.slice(b, index, length(a)) === a, do: true, else: subset?(a, b, index + 1))
end