In btree, move rightmost_ into the CompressedTuple instead of root_.

We also add accessors for rightmost()/mutable_rightmost().

PiperOrigin-RevId: 446515231
Change-Id: I4b8cb46f4bd209a0f51dcdcb96c9479e480828a3

absl/container/internal/btree.h

@@ -191,8 +191,8 @@ struct key_compare_adapter {
   // Inherit from checked_compare_base to support function pointers and also
   // keep empty-base-optimization (EBO) support for classes.
   // Note: we can't use CompressedTuple here because that would interfere
-  // with the EBO for `btree::root_`. `btree::root_` is itself a CompressedTuple
-  // and nested `CompressedTuple`s don't support EBO.
+  // with the EBO for `btree::rightmost_`. `btree::rightmost_` is itself a
+  // CompressedTuple and nested `CompressedTuple`s don't support EBO.
   // TODO(b/214288561): use CompressedTuple instead once it supports EBO for
   // nested `CompressedTuple`s.
   struct checked_compare : checked_compare_base<Compare> {
@@ -1329,7 +1329,7 @@ class btree {
 
  public:
   btree(const key_compare &comp, const allocator_type &alloc)
-      : root_(comp, alloc, EmptyNode()), rightmost_(EmptyNode()), size_(0) {}
+      : root_(EmptyNode()), rightmost_(comp, alloc, EmptyNode()), size_(0) {}
 
   btree(const btree &other) : btree(other, other.allocator()) {}
   btree(const btree &other, const allocator_type &alloc)
@@ -1337,10 +1337,10 @@ class btree {
     copy_or_move_values_in_order(other);
   }
   btree(btree &&other) noexcept
-      : root_(std::move(other.root_)),
-        rightmost_(absl::exchange(other.rightmost_, EmptyNode())),
+      : root_(absl::exchange(other.root_, EmptyNode())),
+        rightmost_(std::move(other.rightmost_)),
         size_(absl::exchange(other.size_, 0)) {
-    other.mutable_root() = EmptyNode();
+    other.mutable_rightmost() = EmptyNode();
   }
   btree(btree &&other, const allocator_type &alloc)
       : btree(other.key_comp(), alloc) {
@@ -1365,9 +1365,9 @@ class btree {
 
   iterator begin() { return iterator(leftmost()); }
   const_iterator begin() const { return const_iterator(leftmost()); }
-  iterator end() { return iterator(rightmost_, rightmost_->finish()); }
+  iterator end() { return iterator(rightmost(), rightmost()->finish()); }
   const_iterator end() const {
-    return const_iterator(rightmost_, rightmost_->finish());
+    return const_iterator(rightmost(), rightmost()->finish());
   }
   reverse_iterator rbegin() { return reverse_iterator(end()); }
   const_reverse_iterator rbegin() const {
@@ -1493,7 +1493,7 @@ class btree {
   void swap(btree &other);
 
   const key_compare &key_comp() const noexcept {
-    return root_.template get<0>();
+    return rightmost_.template get<0>();
   }
   template <typename K1, typename K2>
   bool compare_keys(const K1 &a, const K2 &b) const {
@@ -1587,10 +1587,17 @@ class btree {
   friend struct btree_access;
 
   // Internal accessor routines.
-  node_type *root() { return root_.template get<2>(); }
-  const node_type *root() const { return root_.template get<2>(); }
-  node_type *&mutable_root() noexcept { return root_.template get<2>(); }
-  key_compare *mutable_key_comp() noexcept { return &root_.template get<0>(); }
+  node_type *root() { return root_; }
+  const node_type *root() const { return root_; }
+  node_type *&mutable_root() noexcept { return root_; }
+  node_type *rightmost() { return rightmost_.template get<2>(); }
+  const node_type *rightmost() const { return rightmost_.template get<2>(); }
+  node_type *&mutable_rightmost() noexcept {
+    return rightmost_.template get<2>();
+  }
+  key_compare *mutable_key_comp() noexcept {
+    return &rightmost_.template get<0>();
+  }
 
   // The leftmost node is stored as the parent of the root node.
   node_type *leftmost() { return root()->parent(); }
@@ -1598,10 +1605,10 @@ class btree {
 
   // Allocator routines.
   allocator_type *mutable_allocator() noexcept {
-    return &root_.template get<1>();
+    return &rightmost_.template get<1>();
   }
   const allocator_type &allocator() const noexcept {
-    return root_.template get<1>();
+    return rightmost_.template get<1>();
   }
 
   // Allocates a correctly aligned node of at least size bytes using the
@@ -1709,15 +1716,14 @@ class btree {
     return res;
   }
 
-  // We use compressed tuple in order to save space because key_compare and
-  // allocator_type are usually empty.
-  absl::container_internal::CompressedTuple<key_compare, allocator_type,
-                                            node_type *>
-      root_;
+  node_type *root_;
 
   // A pointer to the rightmost node. Note that the leftmost node is stored as
-  // the root's parent.
-  node_type *rightmost_;
+  // the root's parent. We use compressed tuple in order to save space because
+  // key_compare and allocator_type are usually empty.
+  absl::container_internal::CompressedTuple<key_compare, allocator_type,
+                                            node_type *>
+      rightmost_;
 
   // Number of values.
   size_type size_;
@@ -2141,7 +2147,7 @@ template <typename K, typename... Args>
 auto btree<P>::insert_unique(const K &key, Args &&... args)
     -> std::pair<iterator, bool> {
   if (empty()) {
-    mutable_root() = rightmost_ = new_leaf_root_node(1);
+    mutable_root() = mutable_rightmost() = new_leaf_root_node(1);
   }
 
   SearchResult<iterator, is_key_compare_to::value> res = internal_locate(key);
@@ -2208,7 +2214,7 @@ template <typename P>
 template <typename ValueType>
 auto btree<P>::insert_multi(const key_type &key, ValueType &&v) -> iterator {
   if (empty()) {
-    mutable_root() = rightmost_ = new_leaf_root_node(1);
+    mutable_root() = mutable_rightmost() = new_leaf_root_node(1);
   }
 
   iterator iter = internal_upper_bound(key);
@@ -2272,15 +2278,15 @@ auto btree<P>::operator=(btree &&other) noexcept -> btree & {
     using std::swap;
     if (absl::allocator_traits<
             allocator_type>::propagate_on_container_copy_assignment::value) {
-      // Note: `root_` also contains the allocator and the key comparator.
       swap(root_, other.root_);
+      // Note: `rightmost_` also contains the allocator and the key comparator.
       swap(rightmost_, other.rightmost_);
       swap(size_, other.size_);
     } else {
       if (allocator() == other.allocator()) {
         swap(mutable_root(), other.mutable_root());
         swap(*mutable_key_comp(), *other.mutable_key_comp());
-        swap(rightmost_, other.rightmost_);
+        swap(mutable_rightmost(), other.mutable_rightmost());
         swap(size_, other.size_);
       } else {
         // We aren't allowed to propagate the allocator and the allocator is
@@ -2422,8 +2428,7 @@ void btree<P>::clear() {
   if (!empty()) {
     node_type::clear_and_delete(root(), mutable_allocator());
   }
-  mutable_root() = EmptyNode();
-  rightmost_ = EmptyNode();
+  mutable_root() = mutable_rightmost() = EmptyNode();
   size_ = 0;
 }
 
@@ -2432,15 +2437,15 @@ void btree<P>::swap(btree &other) {
   using std::swap;
   if (absl::allocator_traits<
           allocator_type>::propagate_on_container_swap::value) {
-    // Note: `root_` also contains the allocator and the key comparator.
-    swap(root_, other.root_);
+    // Note: `rightmost_` also contains the allocator and the key comparator.
+    swap(rightmost_, other.rightmost_);
   } else {
     // It's undefined behavior if the allocators are unequal here.
     assert(allocator() == other.allocator());
-    swap(mutable_root(), other.mutable_root());
+    swap(mutable_rightmost(), other.mutable_rightmost());
     swap(*mutable_key_comp(), *other.mutable_key_comp());
   }
-  swap(rightmost_, other.rightmost_);
+  swap(mutable_root(), other.mutable_root());
   swap(size_, other.size_);
 }
 
@@ -2448,12 +2453,12 @@ template <typename P>
 void btree<P>::verify() const {
   assert(root() != nullptr);
   assert(leftmost() != nullptr);
-  assert(rightmost_ != nullptr);
+  assert(rightmost() != nullptr);
   assert(empty() || size() == internal_verify(root(), nullptr, nullptr));
   assert(leftmost() == (++const_iterator(root(), -1)).node_);
-  assert(rightmost_ == (--const_iterator(root(), root()->finish())).node_);
+  assert(rightmost() == (--const_iterator(root(), root()->finish())).node_);
   assert(leftmost()->is_leaf());
-  assert(rightmost_->is_leaf());
+  assert(rightmost()->is_leaf());
 }
 
 template <typename P>
@@ -2539,7 +2544,7 @@ void btree<P>::rebalance_or_split(iterator *iter) {
     mutable_root() = parent;
     // If the former root was a leaf node, then it's now the rightmost node.
     assert(parent->start_child()->is_internal() ||
-           parent->start_child() == rightmost_);
+           parent->start_child() == rightmost());
   }
 
   // Split the node.
@@ -2547,7 +2552,7 @@ void btree<P>::rebalance_or_split(iterator *iter) {
   if (node->is_leaf()) {
     split_node = new_leaf_node(parent);
     node->split(insert_position, split_node, mutable_allocator());
-    if (rightmost_ == node) rightmost_ = split_node;
+    if (rightmost() == node) mutable_rightmost() = split_node;
   } else {
     split_node = new_internal_node(parent);
     node->split(insert_position, split_node, mutable_allocator());
@@ -2562,7 +2567,7 @@ void btree<P>::rebalance_or_split(iterator *iter) {
 template <typename P>
 void btree<P>::merge_nodes(node_type *left, node_type *right) {
   left->merge(right, mutable_allocator());
-  if (rightmost_ == right) rightmost_ = left;
+  if (rightmost() == right) mutable_rightmost() = left;
 }
 
 template <typename P>
@@ -2627,7 +2632,7 @@ void btree<P>::try_shrink() {
   // Deleted the last item on the root node, shrink the height of the tree.
   if (orig_root->is_leaf()) {
     assert(size() == 0);
-    mutable_root() = rightmost_ = EmptyNode();
+    mutable_root() = mutable_rightmost() = EmptyNode();
   } else {
     node_type *child = orig_root->start_child();
     child->make_root();
@@ -2681,7 +2686,7 @@ inline auto btree<P>::internal_emplace(iterator iter, Args &&... args)
       old_root->set_finish(old_root->start());
       new_root->set_generation(old_root->generation());
       node_type::clear_and_delete(old_root, alloc);
-      mutable_root() = rightmost_ = new_root;
+      mutable_root() = mutable_rightmost() = new_root;
     } else {
       rebalance_or_split(&iter);
     }