sublist

elixir/sublist/lib/sublist.ex

@@ -0,0 +1,22 @@
+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