Added Unlink, erase, pop_back and pop_front to VectorBackedLinkedList

Implemented Unlink, erase, pop_back and pop_front.
The first one unlink the given node from prev/next nodes.
|erase| removes the node with the given iterator from the used list and
prepends it to the free list as a free node.
At this time, its value is set to be empty.

Change-Id: Ic8a64c89f429037ef5628c25d89a58ce16081e8c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2094481
Commit-Queue: Kei Nakashima <keinakashima@google.com>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748169}

third_party/blink/renderer/platform/wtf/vector_backed_linked_list.h

@@ -129,6 +129,17 @@ class VectorBackedLinkedList {
   // is executed by just updating indices of related nodes.
   void MoveTo(const_iterator target, const_iterator new_position);
 
+  iterator erase(const_iterator);
+
+  void pop_front() {
+    DCHECK(!empty());
+    erase(cbegin());
+  }
+  void pop_back() {
+    DCHECK(!empty());
+    erase(--cend());
+  }
+
  private:
   bool IsFreeListEmpty() const { return free_head_index_ == anchor_index_; }
 
@@ -424,6 +435,24 @@ void VectorBackedLinkedList<T>::MoveTo(const_iterator target,
   target_node.next_index_ = new_position_index;
 }
 
+template <typename T>
+typename VectorBackedLinkedList<T>::iterator VectorBackedLinkedList<T>::erase(
+    const_iterator position) {
+  DCHECK(position != end());
+  wtf_size_t position_index = position.GetIndex();
+  Node& node = nodes_[position_index];
+  wtf_size_t next_index = node.next_index_;
+
+  Unlink(node);
+  node.value_ = HashTraits<T>::EmptyValue();
+
+  node.next_index_ = free_head_index_;
+  free_head_index_ = position_index;
+
+  size_--;
+  return iterator(next_index, this);
+}
+
 template <typename T>
 void VectorBackedLinkedList<T>::Unlink(const Node& node) {
   wtf_size_t prev_index = node.prev_index_;

third_party/blink/renderer/platform/wtf/vector_backed_linked_list_test.cc

@@ -110,6 +110,89 @@ TEST(VectorBackedLinkedList, MoveTo) {
   EXPECT_EQ(*it, 2);
 }
 
+TEST(VectorBackedLinkedList, Erase) {
+  using List = VectorBackedLinkedList<int>;
+  List list;
+
+  List::iterator it = list.insert(list.end(), 1);
+  EXPECT_EQ(*it, 1);
+  list.push_back(2);
+  list.push_back(3);
+  list.push_back(4);
+  list.push_back(5);
+  EXPECT_EQ(list.size(), 5u);
+
+  List::iterator target = list.begin();
+  ++target;
+  it = list.erase(target);  // list = {1, 3, 4, 5}
+  EXPECT_EQ(*it, 3);
+  EXPECT_EQ(list.size(), 4u);
+  it = list.erase(++it);  // list = {1, 3, 5}
+  EXPECT_EQ(*it, 5);
+  EXPECT_EQ(list.size(), 3u);
+
+  it = list.erase(list.begin());  // list = {3, 5}
+  EXPECT_EQ(*it, 3);
+  EXPECT_EQ(list.size(), 2u);
+
+  it = list.begin();
+  EXPECT_EQ(*it, 3);
+  ++it;
+  EXPECT_EQ(*it, 5);
+  ++it;
+  EXPECT_TRUE(it == list.end());
+}
+
+TEST(VectorBackedLinkedList, PopFront) {
+  using List = VectorBackedLinkedList<int>;
+  List list;
+
+  list.push_back(1);
+  list.push_back(2);
+  list.push_back(3);
+
+  int i = 1;
+  for (auto element : list) {
+    EXPECT_EQ(element, i);
+    i++;
+  }
+
+  list.pop_front();
+  EXPECT_EQ(list.front(), 2);
+  EXPECT_EQ(list.back(), 3);
+  EXPECT_EQ(list.size(), 2u);
+
+  list.pop_front();
+  EXPECT_EQ(list.front(), 3);
+  EXPECT_EQ(list.back(), 3);
+  EXPECT_EQ(list.size(), 1u);
+
+  list.pop_front();
+  EXPECT_TRUE(list.empty());
+}
+
+TEST(VectorBackedLinkedList, PopBack) {
+  using List = VectorBackedLinkedList<int>;
+  List list;
+
+  list.push_back(1);
+  list.push_back(2);
+  list.push_back(3);
+
+  list.pop_back();
+  EXPECT_EQ(list.front(), 1);
+  EXPECT_EQ(list.back(), 2);
+  EXPECT_EQ(list.size(), 2u);
+
+  list.pop_back();
+  EXPECT_EQ(list.front(), 1);
+  EXPECT_EQ(list.back(), 1);
+  EXPECT_EQ(list.size(), 1u);
+
+  list.pop_back();
+  EXPECT_TRUE(list.empty());
+}
+
 TEST(VectorBackedLinkedList, Iterator) {
   using List = VectorBackedLinkedList<int>;
   List list;