AK+Tests: Add IntrusiveList<T,...>::insert_before(..) method

The insert_before method on AK::InlineLinkedList is used, so in order to
achieve feature parity, we need to implement it for AK::IntrusiveList as
well.

AK/IntrusiveList.h

@@ -54,6 +54,7 @@ class IntrusiveList {
  [[nodiscard]] size_t size_slow() const;
  void append(T& n);
  void prepend(T& n);
+ void insert_before(T&, T&);
  void remove(T& n);
  [[nodiscard]] bool contains(const T&) const;
  [[nodiscard]] Container first() const;
@@ -264,6 +265,28 @@ inline void IntrusiveList<T, Container, member>::prepend(T& n)
  m_storage.m_last = &nnode;
 }
 
+template<class T, typename Container, IntrusiveListNode<T, Container> T::*member>
+inline void IntrusiveList<T, Container, member>::insert_before(T& bn, T& n)
+{
+ remove(n);
+
+ auto& new_node = n.*member;
+ auto& before_node = bn.*member;
+ new_node.m_storage = &m_storage;
+ new_node.m_next = &before_node;
+ new_node.m_prev = before_node.m_prev;
+ if (before_node.m_prev)
+ before_node.m_prev->m_next = &new_node;
+ before_node.m_prev = &new_node;
+
+ if (m_storage.m_first == &before_node) {
+ m_storage.m_first = &new_node;
+ }
+
+ if constexpr (!RemoveReference<decltype(new_node)>::IsRaw)
+ new_node.m_self.reference = &n;
+}
+
 template<class T, typename Container, IntrusiveListNode<T, Container> T::*member>
 inline void IntrusiveList<T, Container, member>::remove(T& n)
 {

Tests/AK/TestIntrusiveList.cpp

@@ -33,6 +33,32 @@ TEST_CASE(insert)
  delete list.take_last();
 }
 
+TEST_CASE(insert_before)
+{
+ IntrusiveTestList list;
+ auto two = new IntrusiveTestItem();
+ list.append(*two);
+ auto zero = new IntrusiveTestItem();
+ list.append(*zero);
+ auto one = new IntrusiveTestItem();
+ list.insert_before(*zero, *one);
+
+ EXPECT_EQ(list.first(), two);
+ EXPECT_EQ(list.last(), zero);
+ EXPECT(list.contains(*zero));
+ EXPECT(list.contains(*one));
+ EXPECT(list.contains(*two));
+
+ EXPECT(zero->m_list_node.is_in_list());
+ EXPECT(one->m_list_node.is_in_list());
+ EXPECT(two->m_list_node.is_in_list());
+ EXPECT_EQ(list.size_slow(), 3u);
+
+ while (auto elem = list.take_first()) {
+ delete elem;
+ }
+}
+
 TEST_CASE(enumeration)
 {
  constexpr size_t expected_size = 10;