Sublist Exercise

sublist.exs

defmodule Sublist2 do
  @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, b) when a === b, do: :equal
  # Passing the smaller list twice to is_sublist?/3
  def compare(a, b) when length(a) < length(b), do: if is_sublist?(a, b, a), do: :sublist, else: :unequal
  def compare(a, b) when length(a) > length(b), do: if is_sublist?(b, a, b), do: :superlist, else: :unequal
  def compare(_, _), do: :unequal

  # When the smaller list is empty, all comparisons have been equal
  defp is_sublist?([], _, _), do: true
  # When the smaller list is NOT empty, and the larger list is empty, the last comparison was unequal
  defp is_sublist?(_, [], _), do: false
  # When both heads are equal, advance both lists. NOTE: use of the same variable name `h` for the pattern match
  defp is_sublist?([h | t1], [h | t2], sublist_candidate) do
    is_sublist?(t1, t2, sublist_candidate)
  end
  # When heads are not equal, advance larger list, restart comparison from the beginning of the sublist
  defp is_sublist?(_l1, [_h|t2], sublist_candidate) do
    is_sublist?(sublist_candidate, t2, sublist_candidate)
  end


end