Fix infinity loop when max_size is zero

lib/deque.ex

@@ -13,12 +13,15 @@ defmodule Deque do
 
   defstruct size: 0, max_size: nil, list1: [], list2: []
 
-  @spec new(integer) :: t
+  @spec new(non_neg_integer) :: t
   def new(max_size \\ 100) do
     %Deque{max_size: max_size}
   end
 
   @spec append(t, value) :: t
+  def append(%Deque{max_size: 0}=deque, _value) do
+    deque
+  end
   def append(%Deque{size: size, max_size: max_size, list1: [], list2: list2}=deque, value) when size < max_size do
     %{deque | size: size + 1, list2: [value|list2]}
   end
@@ -33,6 +36,9 @@ defmodule Deque do
   end
 
   @spec appendleft(t, value) :: t
+    def appendleft(%Deque{max_size: 0}=deque, _value) do
+    deque
+  end
   def appendleft(%Deque{size: size, max_size: max_size, list1: list1, list2: []}=deque, value) when size < max_size do
     %{deque | size: size + 1, list1: [value|list1]}
   end

test/deque_test.exs

@@ -2,9 +2,12 @@ defmodule DequeTest do
   use ExUnit.Case
 
   test "new" do
-    deque = Deque.new(5)
-    assert deque.size == 0
-    assert deque.max_size == 5
+    0..10
+    |> Enum.each(fn max_size ->
+      deque = Deque.new(max_size)
+      assert deque.size == 0
+      assert deque.max_size == max_size
+    end)
   end
 
   test "append/appendleft/pop/popleft" do
@@ -35,14 +38,40 @@ defmodule DequeTest do
     assert deque.list2 == []
   end
 
+  test "append/appendleft/pop/popleft max_size=0" do
+    deque = Enum.reduce(1..6, Deque.new(0), &Deque.append(&2, &1))
+    assert deque.list1 == []
+    assert deque.list2 == []
+
+    deque = Enum.reduce(1..3, deque, &Deque.appendleft(&2, &1))
+    assert deque.list1 == []
+    assert deque.list2 == []
+
+    {value, deque} = Deque.pop(deque)
+    assert value == nil
+    assert deque.list1 == []
+    assert deque.list2 == []
+
+    {value, deque} = Deque.popleft(deque)
+    assert value == nil
+    assert deque.list1 == []
+    assert deque.list2 == []
+  end
+
   test "enumerable" do
     deque = Enum.reduce(1..6, Deque.new(5), &Deque.append(&2, &1))
     assert Enum.to_list(deque) == [2, 3, 4, 5, 6]
+
+    deque = Enum.reduce(1..6, Deque.new(0), &Deque.append(&2, &1))
+    assert Enum.to_list(deque) == []
   end
 
   test "collectable/inspect" do
     deque = Enum.into(1..6, Deque.new(5))
     assert inspect(deque) == "#Deque<[2, 3, 4, 5, 6]>"
+
+    deque = Enum.into(1..6, Deque.new(0))
+    assert inspect(deque) == "#Deque<[]>"
   end
 
   test "take_while" do
@@ -51,6 +80,9 @@ defmodule DequeTest do
 
     deque = gen_take_while(400..500, 10, 492)
     assert Enum.to_list(deque) == [493, 494, 495, 496, 497, 498, 499, 500]
+
+    deque = gen_take_while(400..500, 0, 492)
+    assert Enum.to_list(deque) == []
   end
 
   defp gen_take_while(range, max_size, max_value) do