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 // Copyright 2018 The Abseil Authors.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
 //
 //      https://www.apache.org/licenses/LICENSE-2.0
 //
 // Unless required by applicable law or agreed to in writing, software
 // distributed under the License is distributed on an "AS IS" BASIS,
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.
 //
 // -----------------------------------------------------------------------------
 // File: btree_map.h
 // -----------------------------------------------------------------------------
 //
 // This header file defines B-tree maps: sorted associative containers mapping
 // keys to values.
 //
 //     * `absl::btree_map<>`
 //     * `absl::btree_multimap<>`
 //
 // These B-tree types are similar to the corresponding types in the STL
 // (`std::map` and `std::multimap`) and generally conform to the STL interfaces
 // of those types. However, because they are implemented using B-trees, they
 // are more efficient in most situations.
 //
 // Unlike `std::map` and `std::multimap`, which are commonly implemented using
 // red-black tree nodes, B-tree maps use more generic B-tree nodes able to hold
 // multiple values per node. Holding multiple values per node often makes
 // B-tree maps perform better than their `std::map` counterparts, because
 // multiple entries can be checked within the same cache hit.
 //
 // However, these types should not be considered drop-in replacements for
 // `std::map` and `std::multimap` as there are some API differences, which are
 // noted in this header file. The most consequential differences with respect to
 // migrating to b-tree from the STL types are listed in the next paragraph.
 // Other API differences are minor.
 //
 // Importantly, insertions and deletions may invalidate outstanding iterators,
 // pointers, and references to elements. Such invalidations are typically only
 // an issue if insertion and deletion operations are interleaved with the use of
 // more than one iterator, pointer, or reference simultaneously.  For this
 // reason, `insert()`, `erase()`, and `extract_and_get_next()` return a valid
 // iterator at the current position. Another important difference is that
 // key-types must be copy-constructible.
 //
 // Another API difference is that btree iterators can be subtracted, and this
 // is faster than using std::distance.
 //
 // B-tree maps are not exception-safe.
 
 #ifndef ABSL_CONTAINER_BTREE_MAP_H_
 #define ABSL_CONTAINER_BTREE_MAP_H_
 
 #include "absl/base/attributes.h"
 #include "absl/container/internal/btree.h"  // IWYU pragma: export
 #include "absl/container/internal/btree_container.h"  // IWYU pragma: export
 
 namespace absl {
 ABSL_NAMESPACE_BEGIN
 
 namespace container_internal {
 
 template <typename Key, typename Data, typename Compare, typename Alloc,
           int TargetNodeSize, bool IsMulti>
 struct map_params;
 
 }  // namespace container_internal
 
 // absl::btree_map<>
 //
 // An `absl::btree_map<K, V>` is an ordered associative container of
 // unique keys and associated values designed to be a more efficient replacement
 // for `std::map` (in most cases).
 //
 // Keys are sorted using an (optional) comparison function, which defaults to
 // `std::less<K>`.
 //
 // An `absl::btree_map<K, V>` uses a default allocator of
 // `std::allocator<std::pair<const K, V>>` to allocate (and deallocate)
 // nodes, and construct and destruct values within those nodes. You may
 // instead specify a custom allocator `A` (which in turn requires specifying a
 // custom comparator `C`) as in `absl::btree_map<K, V, C, A>`.
 //
 template <typename Key, typename Value, typename Compare = std::less<Key>,
           typename Alloc = std::allocator<std::pair<const Key, Value>>>
 class ABSL_INTERNAL_ATTRIBUTE_OWNER btree_map
     : public container_internal::btree_map_container<
           container_internal::btree<container_internal::map_params<
               Key, Value, Compare, Alloc, /*TargetNodeSize=*/256,
               /*IsMulti=*/false>>> {
   using Base = typename btree_map::btree_map_container;
 
  public:
   // Constructors and Assignment Operators
   //
   // A `btree_map` supports the same overload set as `std::map`
   // for construction and assignment:
   //
   // * Default constructor
   //
   //   absl::btree_map<int, std::string> map1;
   //
   // * Initializer List constructor
   //
   //   absl::btree_map<int, std::string> map2 =
   //       {{1, "huey"}, {2, "dewey"}, {3, "louie"},};
   //
   // * Copy constructor
   //
   //   absl::btree_map<int, std::string> map3(map2);
   //
   // * Copy assignment operator
   //
   //  absl::btree_map<int, std::string> map4;
   //  map4 = map3;
   //
   // * Move constructor
   //
   //   // Move is guaranteed efficient
   //   absl::btree_map<int, std::string> map5(std::move(map4));
   //
   // * Move assignment operator
   //
   //   // May be efficient if allocators are compatible
   //   absl::btree_map<int, std::string> map6;
   //   map6 = std::move(map5);
   //
   // * Range constructor
   //
   //   std::vector<std::pair<int, std::string>> v = {{1, "a"}, {2, "b"}};
   //   absl::btree_map<int, std::string> map7(v.begin(), v.end());
   btree_map() {}
   using Base::Base;
 
   // btree_map::begin()
   //
   // Returns an iterator to the beginning of the `btree_map`.
   using Base::begin;
 
   // btree_map::cbegin()
   //
   // Returns a const iterator to the beginning of the `btree_map`.
   using Base::cbegin;
 
   // btree_map::end()
   //
   // Returns an iterator to the end of the `btree_map`.
   using Base::end;
 
   // btree_map::cend()
   //
   // Returns a const iterator to the end of the `btree_map`.
   using Base::cend;
 
   // btree_map::empty()
   //
   // Returns whether or not the `btree_map` is empty.
   using Base::empty;
 
   // btree_map::max_size()
   //
   // Returns the largest theoretical possible number of elements within a
   // `btree_map` under current memory constraints. This value can be thought
   // of as the largest value of `std::distance(begin(), end())` for a
   // `btree_map<Key, T>`.
   using Base::max_size;
 
   // btree_map::size()
   //
   // Returns the number of elements currently within the `btree_map`.
   using Base::size;
 
   // btree_map::clear()
   //
   // Removes all elements from the `btree_map`. Invalidates any references,
   // pointers, or iterators referring to contained elements.
   using Base::clear;
 
   // btree_map::erase()
   //
   // Erases elements within the `btree_map`. If an erase occurs, any references,
   // pointers, or iterators are invalidated.
   // Overloads are listed below.
   //
   // iterator erase(iterator position):
   // iterator erase(const_iterator position):
   //
   //   Erases the element at `position` of the `btree_map`, returning
   //   the iterator pointing to the element after the one that was erased
   //   (or end() if none exists).
   //
   // iterator erase(const_iterator first, const_iterator last):
   //
   //   Erases the elements in the open interval [`first`, `last`), returning
   //   the iterator pointing to the element after the interval that was erased
   //   (or end() if none exists).
   //
   // template <typename K> size_type erase(const K& key):
   //
   //   Erases the element with the matching key, if it exists, returning the
   //   number of elements erased (0 or 1).
   using Base::erase;
 
   // btree_map::insert()
   //
   // Inserts an element of the specified value into the `btree_map`,
   // returning an iterator pointing to the newly inserted element, provided that
   // an element with the given key does not already exist. If an insertion
   // occurs, any references, pointers, or iterators are invalidated.
   // Overloads are listed below.
   //
   // std::pair<iterator,bool> insert(const value_type& value):
   //
   //   Inserts a value into the `btree_map`. Returns a pair consisting of an
   //   iterator to the inserted element (or to the element that prevented the
   //   insertion) and a bool denoting whether the insertion took place.
   //
   // std::pair<iterator,bool> insert(value_type&& value):
   //
   //   Inserts a moveable value into the `btree_map`. Returns a pair
   //   consisting of an iterator to the inserted element (or to the element that
   //   prevented the insertion) and a bool denoting whether the insertion took
   //   place.
   //
   // iterator insert(const_iterator hint, const value_type& value):
   // iterator insert(const_iterator hint, value_type&& value):
   //
   //   Inserts a value, using the position of `hint` as a non-binding suggestion
   //   for where to begin the insertion search. Returns an iterator to the
   //   inserted element, or to the existing element that prevented the
   //   insertion.
   //
   // void insert(InputIterator first, InputIterator last):
   //
   //   Inserts a range of values [`first`, `last`).
   //
   // void insert(std::initializer_list<init_type> ilist):
   //
   //   Inserts the elements within the initializer list `ilist`.
   using Base::insert;
 
   // btree_map::insert_or_assign()
   //
   // Inserts an element of the specified value into the `btree_map` provided
   // that a value with the given key does not already exist, or replaces the
   // corresponding mapped type with the forwarded `obj` argument if a key for
   // that value already exists, returning an iterator pointing to the newly
   // inserted element. Overloads are listed below.
   //
   // pair<iterator, bool> insert_or_assign(const key_type& k, M&& obj):
   // pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj):
   //
   //   Inserts/Assigns (or moves) the element of the specified key into the
   //   `btree_map`. If the returned bool is true, insertion took place, and if
   //   it's false, assignment took place.
   //
   // iterator insert_or_assign(const_iterator hint,
   //                           const key_type& k, M&& obj):
   // iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj):
   //
   //   Inserts/Assigns (or moves) the element of the specified key into the
   //   `btree_map` using the position of `hint` as a non-binding suggestion
   //   for where to begin the insertion search.
   using Base::insert_or_assign;
 
   // btree_map::emplace()
   //
   // Inserts an element of the specified value by constructing it in-place
   // within the `btree_map`, provided that no element with the given key
   // already exists.
   //
   // The element may be constructed even if there already is an element with the
   // key in the container, in which case the newly constructed element will be
   // destroyed immediately. Prefer `try_emplace()` unless your key is not
   // copyable or moveable.
   //
   // If an insertion occurs, any references, pointers, or iterators are
   // invalidated.
   using Base::emplace;
 
   // btree_map::emplace_hint()
   //
   // Inserts an element of the specified value by constructing it in-place
   // within the `btree_map`, using the position of `hint` as a non-binding
   // suggestion for where to begin the insertion search, and only inserts
   // provided that no element with the given key already exists.
   //
   // The element may be constructed even if there already is an element with the
   // key in the container, in which case the newly constructed element will be
   // destroyed immediately. Prefer `try_emplace()` unless your key is not
   // copyable or moveable.
   //
   // If an insertion occurs, any references, pointers, or iterators are
   // invalidated.
   using Base::emplace_hint;
 
   // btree_map::try_emplace()
   //
   // Inserts an element of the specified value by constructing it in-place
   // within the `btree_map`, provided that no element with the given key
   // already exists. Unlike `emplace()`, if an element with the given key
   // already exists, we guarantee that no element is constructed.
   //
   // If an insertion occurs, any references, pointers, or iterators are
   // invalidated.
   //
   // Overloads are listed below.
   //
   //   std::pair<iterator, bool> try_emplace(const key_type& k, Args&&... args):
   //   std::pair<iterator, bool> try_emplace(key_type&& k, Args&&... args):
   //
   // Inserts (via copy or move) the element of the specified key into the
   // `btree_map`.
   //
   //   iterator try_emplace(const_iterator hint,
   //                        const key_type& k, Args&&... args):
   //   iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args):
   //
   // Inserts (via copy or move) the element of the specified key into the
   // `btree_map` using the position of `hint` as a non-binding suggestion
   // for where to begin the insertion search.
   using Base::try_emplace;
 
   // btree_map::extract()
   //
   // Extracts the indicated element, erasing it in the process, and returns it
   // as a C++17-compatible node handle. Any references, pointers, or iterators
   // are invalidated. Overloads are listed below.
   //
   // node_type extract(const_iterator position):
   //
   //   Extracts the element at the indicated position and returns a node handle
   //   owning that extracted data.
   //
   // template <typename K> node_type extract(const K& k):
   //
   //   Extracts the element with the key matching the passed key value and
   //   returns a node handle owning that extracted data. If the `btree_map`
   //   does not contain an element with a matching key, this function returns an
   //   empty node handle.
   //
   // NOTE: when compiled in an earlier version of C++ than C++17,
   // `node_type::key()` returns a const reference to the key instead of a
   // mutable reference. We cannot safely return a mutable reference without
   // std::launder (which is not available before C++17).
   //
   // NOTE: In this context, `node_type` refers to the C++17 concept of a
   // move-only type that owns and provides access to the elements in associative
   // containers (https://en.cppreference.com/w/cpp/container/node_handle).
   // It does NOT refer to the data layout of the underlying btree.
   using Base::extract;
 
   // btree_map::extract_and_get_next()
   //
   // Extracts the indicated element, erasing it in the process, and returns it
   // as a C++17-compatible node handle along with an iterator to the next
   // element.
   //
   // extract_and_get_next_return_type extract_and_get_next(
   //     const_iterator position):
   //
   //   Extracts the element at the indicated position, returns a struct
   //   containing a member named `node`: a node handle owning that extracted
   //   data and a member named `next`: an iterator pointing to the next element
   //   in the btree.
   using Base::extract_and_get_next;
 
   // btree_map::merge()
   //
   // Extracts elements from a given `source` btree_map into this
   // `btree_map`. If the destination `btree_map` already contains an
   // element with an equivalent key, that element is not extracted.
   using Base::merge;
 
   // btree_map::swap(btree_map& other)
   //
   // Exchanges the contents of this `btree_map` with those of the `other`
   // btree_map, avoiding invocation of any move, copy, or swap operations on
   // individual elements.
   //
   // All iterators and references on the `btree_map` remain valid, excepting
   // for the past-the-end iterator, which is invalidated.
   using Base::swap;
 
   // btree_map::at()
   //
   // Returns a reference to the mapped value of the element with key equivalent
   // to the passed key.
   using Base::at;
 
   // btree_map::contains()
   //
   // template <typename K> bool contains(const K& key) const:
   //
   // Determines whether an element comparing equal to the given `key` exists
   // within the `btree_map`, returning `true` if so or `false` otherwise.
   //
   // Supports heterogeneous lookup, provided that the map has a compatible
   // heterogeneous comparator.
   using Base::contains;
 
   // btree_map::count()
   //
   // template <typename K> size_type count(const K& key) const:
   //
   // Returns the number of elements comparing equal to the given `key` within
   // the `btree_map`. Note that this function will return either `1` or `0`
   // since duplicate elements are not allowed within a `btree_map`.
   //
   // Supports heterogeneous lookup, provided that the map has a compatible
   // heterogeneous comparator.
   using Base::count;
 
   // btree_map::equal_range()
   //
   // Returns a half-open range [first, last), defined by a `std::pair` of two
   // iterators, containing all elements with the passed key in the `btree_map`.
   using Base::equal_range;
 
   // btree_map::find()
   //
   // template <typename K> iterator find(const K& key):
   // template <typename K> const_iterator find(const K& key) const:
   //
   // Finds an element with the passed `key` within the `btree_map`.
   //
   // Supports heterogeneous lookup, provided that the map has a compatible
   // heterogeneous comparator.
   using Base::find;
 
   // btree_map::lower_bound()
   //
   // template <typename K> iterator lower_bound(const K& key):
   // template <typename K> const_iterator lower_bound(const K& key) const:
   //
   // Finds the first element with a key that is not less than `key` within the
   // `btree_map`.
   //
   // Supports heterogeneous lookup, provided that the map has a compatible
   // heterogeneous comparator.
   using Base::lower_bound;
 
   // btree_map::upper_bound()
   //
   // template <typename K> iterator upper_bound(const K& key):
   // template <typename K> const_iterator upper_bound(const K& key) const:
   //
   // Finds the first element with a key that is greater than `key` within the
   // `btree_map`.
   //
   // Supports heterogeneous lookup, provided that the map has a compatible
   // heterogeneous comparator.
   using Base::upper_bound;
 
   // btree_map::operator[]()
   //
   // Returns a reference to the value mapped to the passed key within the
   // `btree_map`, performing an `insert()` if the key does not already
   // exist.
   //
   // If an insertion occurs, any references, pointers, or iterators are
   // invalidated. Otherwise iterators are not affected and references are not
   // invalidated. Overloads are listed below.
   //
   // T& operator[](key_type&& key):
   // T& operator[](const key_type& key):
   //
   //   Inserts a value_type object constructed in-place if the element with the
   //   given key does not exist.
   using Base::operator[];
 
   // btree_map::get_allocator()
   //
   // Returns the allocator function associated with this `btree_map`.
   using Base::get_allocator;
 
   // btree_map::key_comp();
   //
   // Returns the key comparator associated with this `btree_map`.
   using Base::key_comp;
 
   // btree_map::value_comp();
   //
   // Returns the value comparator associated with this `btree_map`.
   using Base::value_comp;
 };
 
 // absl::swap(absl::btree_map<>, absl::btree_map<>)
 //
 // Swaps the contents of two `absl::btree_map` containers.
 template <typename K, typename V, typename C, typename A>
 void swap(btree_map<K, V, C, A> &x, btree_map<K, V, C, A> &y) {
   return x.swap(y);
 }
 
 // absl::erase_if(absl::btree_map<>, Pred)
 //
 // Erases all elements that satisfy the predicate pred from the container.
 // Returns the number of erased elements.
 template <typename K, typename V, typename C, typename A, typename Pred>
 typename btree_map<K, V, C, A>::size_type erase_if(
     btree_map<K, V, C, A> &map, Pred pred) {
   return container_internal::btree_access::erase_if(map, std::move(pred));
 }
 
 // absl::btree_multimap
 //
 // An `absl::btree_multimap<K, V>` is an ordered associative container of
 // keys and associated values designed to be a more efficient replacement for
 // `std::multimap` (in most cases). Unlike `absl::btree_map`, a B-tree multimap
 // allows multiple elements with equivalent keys.
 //
 // Keys are sorted using an (optional) comparison function, which defaults to
 // `std::less<K>`.
 //
 // An `absl::btree_multimap<K, V>` uses a default allocator of
 // `std::allocator<std::pair<const K, V>>` to allocate (and deallocate)
 // nodes, and construct and destruct values within those nodes. You may
 // instead specify a custom allocator `A` (which in turn requires specifying a
 // custom comparator `C`) as in `absl::btree_multimap<K, V, C, A>`.
 //
 template <typename Key, typename Value, typename Compare = std::less<Key>,
           typename Alloc = std::allocator<std::pair<const Key, Value>>>
 class ABSL_INTERNAL_ATTRIBUTE_OWNER btree_multimap
     : public container_internal::btree_multimap_container<
           container_internal::btree<container_internal::map_params<
               Key, Value, Compare, Alloc, /*TargetNodeSize=*/256,
               /*IsMulti=*/true>>> {
   using Base = typename btree_multimap::btree_multimap_container;
 
  public:
   // Constructors and Assignment Operators
   //
   // A `btree_multimap` supports the same overload set as `std::multimap`
   // for construction and assignment:
   //
   // * Default constructor
   //
   //   absl::btree_multimap<int, std::string> map1;
   //
   // * Initializer List constructor
   //
   //   absl::btree_multimap<int, std::string> map2 =
   //       {{1, "huey"}, {2, "dewey"}, {3, "louie"},};
   //
   // * Copy constructor
   //
   //   absl::btree_multimap<int, std::string> map3(map2);
   //
   // * Copy assignment operator
   //
   //  absl::btree_multimap<int, std::string> map4;
   //  map4 = map3;
   //
   // * Move constructor
   //
   //   // Move is guaranteed efficient
   //   absl::btree_multimap<int, std::string> map5(std::move(map4));
   //
   // * Move assignment operator
   //
   //   // May be efficient if allocators are compatible
   //   absl::btree_multimap<int, std::string> map6;
   //   map6 = std::move(map5);
   //
   // * Range constructor
   //
   //   std::vector<std::pair<int, std::string>> v = {{1, "a"}, {2, "b"}};
   //   absl::btree_multimap<int, std::string> map7(v.begin(), v.end());
   btree_multimap() {}
   using Base::Base;
 
   // btree_multimap::begin()
   //
   // Returns an iterator to the beginning of the `btree_multimap`.
   using Base::begin;
 
   // btree_multimap::cbegin()
   //
   // Returns a const iterator to the beginning of the `btree_multimap`.
   using Base::cbegin;
 
   // btree_multimap::end()
   //
   // Returns an iterator to the end of the `btree_multimap`.
   using Base::end;
 
   // btree_multimap::cend()
   //
   // Returns a const iterator to the end of the `btree_multimap`.
   using Base::cend;
 
   // btree_multimap::empty()
   //
   // Returns whether or not the `btree_multimap` is empty.
   using Base::empty;
 
   // btree_multimap::max_size()
   //
   // Returns the largest theoretical possible number of elements within a
   // `btree_multimap` under current memory constraints. This value can be
   // thought of as the largest value of `std::distance(begin(), end())` for a
   // `btree_multimap<Key, T>`.
   using Base::max_size;
 
   // btree_multimap::size()
   //
   // Returns the number of elements currently within the `btree_multimap`.
   using Base::size;
 
   // btree_multimap::clear()
   //
   // Removes all elements from the `btree_multimap`. Invalidates any references,
   // pointers, or iterators referring to contained elements.
   using Base::clear;
 
   // btree_multimap::erase()
   //
   // Erases elements within the `btree_multimap`. If an erase occurs, any
   // references, pointers, or iterators are invalidated.
   // Overloads are listed below.
   //
   // iterator erase(iterator position):
   // iterator erase(const_iterator position):
   //
   //   Erases the element at `position` of the `btree_multimap`, returning
   //   the iterator pointing to the element after the one that was erased
   //   (or end() if none exists).
   //
   // iterator erase(const_iterator first, const_iterator last):
   //
   //   Erases the elements in the open interval [`first`, `last`), returning
   //   the iterator pointing to the element after the interval that was erased
   //   (or end() if none exists).
   //
   // template <typename K> size_type erase(const K& key):
   //
   //   Erases the elements matching the key, if any exist, returning the
   //   number of elements erased.
   using Base::erase;
 
   // btree_multimap::insert()
   //
   // Inserts an element of the specified value into the `btree_multimap`,
   // returning an iterator pointing to the newly inserted element.
   // Any references, pointers, or iterators are invalidated.  Overloads are
   // listed below.
   //
   // iterator insert(const value_type& value):
   //
   //   Inserts a value into the `btree_multimap`, returning an iterator to the
   //   inserted element.
   //
   // iterator insert(value_type&& value):
   //
   //   Inserts a moveable value into the `btree_multimap`, returning an iterator
   //   to the inserted element.
   //
   // iterator insert(const_iterator hint, const value_type& value):
   // iterator insert(const_iterator hint, value_type&& value):
   //
   //   Inserts a value, using the position of `hint` as a non-binding suggestion
   //   for where to begin the insertion search. Returns an iterator to the
   //   inserted element.
   //
   // void insert(InputIterator first, InputIterator last):
   //
   //   Inserts a range of values [`first`, `last`).
   //
   // void insert(std::initializer_list<init_type> ilist):
   //
   //   Inserts the elements within the initializer list `ilist`.
   using Base::insert;
 
   // btree_multimap::emplace()
   //
   // Inserts an element of the specified value by constructing it in-place
   // within the `btree_multimap`. Any references, pointers, or iterators are
   // invalidated.
   using Base::emplace;
 
   // btree_multimap::emplace_hint()
   //
   // Inserts an element of the specified value by constructing it in-place
   // within the `btree_multimap`, using the position of `hint` as a non-binding
   // suggestion for where to begin the insertion search.
   //
   // Any references, pointers, or iterators are invalidated.
   using Base::emplace_hint;
 
   // btree_multimap::extract()
   //
   // Extracts the indicated element, erasing it in the process, and returns it
   // as a C++17-compatible node handle. Overloads are listed below.
   //
   // node_type extract(const_iterator position):
   //
   //   Extracts the element at the indicated position and returns a node handle
   //   owning that extracted data.
   //
   // template <typename K> node_type extract(const K& k):
   //
   //   Extracts the element with the key matching the passed key value and
   //   returns a node handle owning that extracted data. If the `btree_multimap`
   //   does not contain an element with a matching key, this function returns an
   //   empty node handle.
   //
   // NOTE: when compiled in an earlier version of C++ than C++17,
   // `node_type::key()` returns a const reference to the key instead of a
   // mutable reference. We cannot safely return a mutable reference without
   // std::launder (which is not available before C++17).
   //
   // NOTE: In this context, `node_type` refers to the C++17 concept of a
   // move-only type that owns and provides access to the elements in associative
   // containers (https://en.cppreference.com/w/cpp/container/node_handle).
   // It does NOT refer to the data layout of the underlying btree.
   using Base::extract;
 
   // btree_multimap::extract_and_get_next()
   //
   // Extracts the indicated element, erasing it in the process, and returns it
   // as a C++17-compatible node handle along with an iterator to the next
   // element.
   //
   // extract_and_get_next_return_type extract_and_get_next(
   //     const_iterator position):
   //
   //   Extracts the element at the indicated position, returns a struct
   //   containing a member named `node`: a node handle owning that extracted
   //   data and a member named `next`: an iterator pointing to the next element
   //   in the btree.
   using Base::extract_and_get_next;
 
   // btree_multimap::merge()
   //
   // Extracts all elements from a given `source` btree_multimap into this
   // `btree_multimap`.
   using Base::merge;
 
   // btree_multimap::swap(btree_multimap& other)
   //
   // Exchanges the contents of this `btree_multimap` with those of the `other`
   // btree_multimap, avoiding invocation of any move, copy, or swap operations
   // on individual elements.
   //
   // All iterators and references on the `btree_multimap` remain valid,
   // excepting for the past-the-end iterator, which is invalidated.
   using Base::swap;
 
   // btree_multimap::contains()
   //
   // template <typename K> bool contains(const K& key) const:
   //
   // Determines whether an element comparing equal to the given `key` exists
   // within the `btree_multimap`, returning `true` if so or `false` otherwise.
   //
   // Supports heterogeneous lookup, provided that the map has a compatible
   // heterogeneous comparator.
   using Base::contains;
 
   // btree_multimap::count()
   //
   // template <typename K> size_type count(const K& key) const:
   //
   // Returns the number of elements comparing equal to the given `key` within
   // the `btree_multimap`.
   //
   // Supports heterogeneous lookup, provided that the map has a compatible
   // heterogeneous comparator.
   using Base::count;
 
   // btree_multimap::equal_range()
   //
   // Returns a half-open range [first, last), defined by a `std::pair` of two
   // iterators, containing all elements with the passed key in the
   // `btree_multimap`.
   using Base::equal_range;
 
   // btree_multimap::find()
   //
   // template <typename K> iterator find(const K& key):
   // template <typename K> const_iterator find(const K& key) const:
   //
   // Finds an element with the passed `key` within the `btree_multimap`.
   //
   // Supports heterogeneous lookup, provided that the map has a compatible
   // heterogeneous comparator.
   using Base::find;
 
   // btree_multimap::lower_bound()
   //
   // template <typename K> iterator lower_bound(const K& key):
   // template <typename K> const_iterator lower_bound(const K& key) const:
   //
   // Finds the first element with a key that is not less than `key` within the
   // `btree_multimap`.
   //
   // Supports heterogeneous lookup, provided that the map has a compatible
   // heterogeneous comparator.
   using Base::lower_bound;
 
   // btree_multimap::upper_bound()
   //
   // template <typename K> iterator upper_bound(const K& key):
   // template <typename K> const_iterator upper_bound(const K& key) const:
   //
   // Finds the first element with a key that is greater than `key` within the
   // `btree_multimap`.
   //
   // Supports heterogeneous lookup, provided that the map has a compatible
   // heterogeneous comparator.
   using Base::upper_bound;
 
   // btree_multimap::get_allocator()
   //
   // Returns the allocator function associated with this `btree_multimap`.
   using Base::get_allocator;
 
   // btree_multimap::key_comp();
   //
   // Returns the key comparator associated with this `btree_multimap`.
   using Base::key_comp;
 
   // btree_multimap::value_comp();
   //
   // Returns the value comparator associated with this `btree_multimap`.
   using Base::value_comp;
 };
 
 // absl::swap(absl::btree_multimap<>, absl::btree_multimap<>)
 //
 // Swaps the contents of two `absl::btree_multimap` containers.
 template <typename K, typename V, typename C, typename A>
 void swap(btree_multimap<K, V, C, A> &x, btree_multimap<K, V, C, A> &y) {
   return x.swap(y);
 }
 
 // absl::erase_if(absl::btree_multimap<>, Pred)
 //
 // Erases all elements that satisfy the predicate pred from the container.
 // Returns the number of erased elements.
 template <typename K, typename V, typename C, typename A, typename Pred>
 typename btree_multimap<K, V, C, A>::size_type erase_if(
     btree_multimap<K, V, C, A> &map, Pred pred) {
   return container_internal::btree_access::erase_if(map, std::move(pred));
 }
 
 namespace container_internal {
 
 // A parameters structure for holding the type parameters for a btree_map.
 // Compare and Alloc should be nothrow copy-constructible.
 template <typename Key, typename Data, typename Compare, typename Alloc,
           int TargetNodeSize, bool IsMulti>
 struct map_params : common_params<Key, Compare, Alloc, TargetNodeSize, IsMulti,
                                   /*IsMap=*/true, map_slot_policy<Key, Data>> {
   using super_type = typename map_params::common_params;
   using mapped_type = Data;
   // This type allows us to move keys when it is safe to do so. It is safe
   // for maps in which value_type and mutable_value_type are layout compatible.
   using slot_policy = typename super_type::slot_policy;
   using slot_type = typename super_type::slot_type;
   using value_type = typename super_type::value_type;
   using init_type = typename super_type::init_type;
 
   template <typename V>
   static auto key(const V &value ABSL_ATTRIBUTE_LIFETIME_BOUND)
       -> decltype((value.first)) {
     return value.first;
   }
   static const Key &key(const slot_type *s) { return slot_policy::key(s); }
   static const Key &key(slot_type *s) { return slot_policy::key(s); }
   // For use in node handle.
   static auto mutable_key(slot_type *s)
       -> decltype(slot_policy::mutable_key(s)) {
     return slot_policy::mutable_key(s);
   }
   static mapped_type &value(value_type *value) { return value->second; }
 };
 
 }  // namespace container_internal
 
 ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_BTREE_MAP_H_

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