Commit 44dff63

@@ -108,7 +108,8 @@ static void swap(Node *x, Node *y) {
     if (xp)
       y->prev->next = y;
     y->next = xn;
-    y->next->prev = y;
+    if (xn)
+      y->next->prev = y;
   }
 }
 
@@ -421,6 +422,33 @@ Ensure(DoublyLinkedList, when_swapping_tail_with_non_adjacent_node) {
   free(head);
 }
 
+Ensure(DoublyLinkedList, when_swapping_tail_with_non_adjacent_node_inverted) {
+  Node *head = initialize(100);
+  Node *mid1 = add(head, 200);
+  Node *mid2 = add(head, 300);
+  Node *tail = add(head, 400);
+
+  swap(tail, mid1);
+
+  assert_that(head->prev, is_equal_to(NULL));
+  assert_that(head->data, is_equal_to(100));
+  assert_that(head->next, is_equal_to(tail));
+
+  assert_that(mid1->prev, is_equal_to(mid2));
+  assert_that(mid1->data, is_equal_to(200));
+  assert_that(mid1->next, is_equal_to(NULL));
+
+  assert_that(mid2->prev, is_equal_to(tail));
+  assert_that(mid2->data, is_equal_to(300));
+  assert_that(mid2->next, is_equal_to(mid1));
+
+  assert_that(tail->prev, is_equal_to(head));
+  assert_that(tail->data, is_equal_to(400));
+  assert_that(tail->next, is_equal_to(mid2));
+
+  free(head);
+}
+
 Ensure(DoublyLinkedList, when_swapping_with_NULL) {
   Node *head = initialize(100);
   Node *mid = add(head, 200);
@@ -476,6 +504,7 @@ TestSuite *swap_doubly_linked_list_tests() {
   add_test_with_context(suite, DoublyLinkedList, when_swapping_tail_adjacent);
   add_test_with_context(suite, DoublyLinkedList, when_swapping_tail_adjacent_inverted);
   add_test_with_context(suite, DoublyLinkedList, when_swapping_tail_with_non_adjacent_node);
+  add_test_with_context(suite, DoublyLinkedList, when_swapping_tail_with_non_adjacent_node_inverted);
   add_test_with_context(suite, DoublyLinkedList, when_swapping_with_NULL);
   add_test_with_context(suite, DoublyLinkedList, when_swapping_self);