Add list complements helper

Author: dantswain

lib/beanie/helpers.ex

@@ -0,0 +1,18 @@
+defmodule Beanie.Helpers do
+  @doc """
+  Computes the relative complements of two lists - the elements that
+  are in the first list and not in the second set, and vice versa,
+  returned as the first and second elements of a tuple, respectively.
+  """
+  @spec relative_complements([term], [term]) :: {[term], [term]}
+  def relative_complements(list1, list2)
+  when is_list(list1) and is_list(list2) do
+    set1 = MapSet.new(list1)
+    set2 = MapSet.new(list2)
+
+    {
+      set1 |> MapSet.difference(set2) |> MapSet.to_list,
+      set2 |> MapSet.difference(set1) |> MapSet.to_list
+    }
+  end
+end

lib/beanie/repository/query.ex

@@ -3,13 +3,11 @@ defmodule Beanie.Repository.Query do
   alias Beanie.Repo
 
   alias Beanie.Repository
+  alias Beanie.Helpers
   alias Beanie.Tag
 
   def update_list(names) do
-    names = MapSet.new(names)
-    existing_names = MapSet.new(all_names)
-    new_names = MapSet.difference(names, existing_names)
-    deleted_names = MapSet.difference(existing_names, names) |> MapSet.to_list
+    {new_names, deleted_names} = Helpers.relative_complements(names, all_names)
 
     new_names
     |> Enum.each(fn(name) ->
@@ -28,9 +26,9 @@ defmodule Beanie.Repository.Query do
 
   def update_tag_list(repository, tags) do
     tag_names = Enum.map(tags, fn(tag) -> tag.name end) |> MapSet.new
-    existing_names = MapSet.new(all_tag_names(repository))
-    new_names = MapSet.difference(tag_names, existing_names)
-    deleted_names = MapSet.difference(existing_names, tag_names) |> MapSet.to_list
+
+    {new_names, deleted_names} =
+      Helpers.relative_complements(tag_names, all_tag_names(repository))
 
     new_names
     |> Enum.map(fn(new_name) ->

test/beanie/helpers_test.exs

@@ -0,0 +1,15 @@
+defmodule Beanie.HelpersTest do
+  use ExUnit.Case
+
+  alias Beanie.Helpers
+  
+  test "relative complements" do
+    list1 = [1, 2, 3]
+    list2 = [2, 4, 5]
+
+    in1_not2 = [1, 3]
+    in2_not1 = [4, 5]
+
+    assert {in1_not2, in2_not1} == Helpers.relative_complements(list1, list2)
+  end
+end