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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_set.h
 // -----------------------------------------------------------------------------
 //
 // This header file defines B-tree sets: sorted associative containers of
 // values.
 //
 //     * `absl::btree_set<>`
 //     * `absl::btree_multiset<>`
 //
 // These B-tree types are similar to the corresponding types in the STL
 // (`std::set` and `std::multiset`) 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::set` and `std::multiset`, which are commonly implemented using
 // red-black tree nodes, B-tree sets use more generic B-tree nodes able to hold
 // multiple values per node. Holding multiple values per node often makes
 // B-tree sets perform better than their `std::set` counterparts, because
 // multiple entries can be checked within the same cache hit.
 //
 // However, these types should not be considered drop-in replacements for
 // `std::set` and `std::multiset` 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 API difference is that btree iterators can be subtracted, and this
 // is faster than using std::distance.
 //
 // B-tree sets are not exception-safe.
 
 #ifndef ABSL_CONTAINER_BTREE_SET_H_
 #define ABSL_CONTAINER_BTREE_SET_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>
 struct set_slot_policy;
 
 template <typename Key, typename Compare, typename Alloc, int TargetNodeSize,
           bool IsMulti>
 struct set_params;
 
 }  // namespace container_internal
 
 // absl::btree_set<>
 //
 // An `absl::btree_set<K>` is an ordered associative container of unique key
 // values designed to be a more efficient replacement for `std::set` (in most
 // cases).
 //
 // Keys are sorted using an (optional) comparison function, which defaults to
 // `std::less<K>`.
 //
 // An `absl::btree_set<K>` uses a default allocator of `std::allocator<K>` 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_set<K, C, A>`.
 //
 template <typename Key, typename Compare = std::less<Key>,
           typename Alloc = std::allocator<Key>>
 class ABSL_INTERNAL_ATTRIBUTE_OWNER btree_set
     : public container_internal::btree_set_container<
           container_internal::btree<container_internal::set_params<
               Key, Compare, Alloc, /*TargetNodeSize=*/256,
               /*IsMulti=*/false>>> {
   using Base = typename btree_set::btree_set_container;
 
  public:
   // Constructors and Assignment Operators
   //
   // A `btree_set` supports the same overload set as `std::set`
   // for construction and assignment:
   //
   // * Default constructor
   //
   //   absl::btree_set<std::string> set1;
   //
   // * Initializer List constructor
   //
   //   absl::btree_set<std::string> set2 =
   //       {{"huey"}, {"dewey"}, {"louie"},};
   //
   // * Copy constructor
   //
   //   absl::btree_set<std::string> set3(set2);
   //
   // * Copy assignment operator
   //
   //  absl::btree_set<std::string> set4;
   //  set4 = set3;
   //
   // * Move constructor
   //
   //   // Move is guaranteed efficient
   //   absl::btree_set<std::string> set5(std::move(set4));
   //
   // * Move assignment operator
   //
   //   // May be efficient if allocators are compatible
   //   absl::btree_set<std::string> set6;
   //   set6 = std::move(set5);
   //
   // * Range constructor
   //
   //   std::vector<std::string> v = {"a", "b"};
   //   absl::btree_set<std::string> set7(v.begin(), v.end());
   btree_set() {}
   using Base::Base;
 
   // btree_set::begin()
   //
   // Returns an iterator to the beginning of the `btree_set`.
   using Base::begin;
 
   // btree_set::cbegin()
   //
   // Returns a const iterator to the beginning of the `btree_set`.
   using Base::cbegin;
 
   // btree_set::end()
   //
   // Returns an iterator to the end of the `btree_set`.
   using Base::end;
 
   // btree_set::cend()
   //
   // Returns a const iterator to the end of the `btree_set`.
   using Base::cend;
 
   // btree_set::empty()
   //
   // Returns whether or not the `btree_set` is empty.
   using Base::empty;
 
   // btree_set::max_size()
   //
   // Returns the largest theoretical possible number of elements within a
   // `btree_set` under current memory constraints. This value can be thought
   // of as the largest value of `std::distance(begin(), end())` for a
   // `btree_set<Key>`.
   using Base::max_size;
 
   // btree_set::size()
   //
   // Returns the number of elements currently within the `btree_set`.
   using Base::size;
 
   // btree_set::clear()
   //
   // Removes all elements from the `btree_set`. Invalidates any references,
   // pointers, or iterators referring to contained elements.
   using Base::clear;
 
   // btree_set::erase()
   //
   // Erases elements within the `btree_set`. Overloads are listed below.
   //
   // iterator erase(iterator position):
   // iterator erase(const_iterator position):
   //
   //   Erases the element at `position` of the `btree_set`, 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_set::insert()
   //
   // Inserts an element of the specified value into the `btree_set`,
   // 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_set`. 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_set`. 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_set::emplace()
   //
   // Inserts an element of the specified value by constructing it in-place
   // within the `btree_set`, 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.
   //
   // If an insertion occurs, any references, pointers, or iterators are
   // invalidated.
   using Base::emplace;
 
   // btree_set::emplace_hint()
   //
   // Inserts an element of the specified value by constructing it in-place
   // within the `btree_set`, 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.
   //
   // If an insertion occurs, any references, pointers, or iterators are
   // invalidated.
   using Base::emplace_hint;
 
   // btree_set::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_set`
   //   does not contain an element with a matching key, this function returns an
   //   empty node handle.
   //
   // 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_set::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_set::merge()
   //
   // Extracts elements from a given `source` btree_set into this
   // `btree_set`. If the destination `btree_set` already contains an
   // element with an equivalent key, that element is not extracted.
   using Base::merge;
 
   // btree_set::swap(btree_set& other)
   //
   // Exchanges the contents of this `btree_set` with those of the `other`
   // btree_set, avoiding invocation of any move, copy, or swap operations on
   // individual elements.
   //
   // All iterators and references on the `btree_set` remain valid, excepting
   // for the past-the-end iterator, which is invalidated.
   using Base::swap;
 
   // btree_set::contains()
   //
   // template <typename K> bool contains(const K& key) const:
   //
   // Determines whether an element comparing equal to the given `key` exists
   // within the `btree_set`, returning `true` if so or `false` otherwise.
   //
   // Supports heterogeneous lookup, provided that the set has a compatible
   // heterogeneous comparator.
   using Base::contains;
 
   // btree_set::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_set`. Note that this function will return either `1` or `0`
   // since duplicate elements are not allowed within a `btree_set`.
   //
   // Supports heterogeneous lookup, provided that the set has a compatible
   // heterogeneous comparator.
   using Base::count;
 
   // btree_set::equal_range()
   //
   // Returns a closed range [first, last], defined by a `std::pair` of two
   // iterators, containing all elements with the passed key in the
   // `btree_set`.
   using Base::equal_range;
 
   // btree_set::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_set`.
   //
   // Supports heterogeneous lookup, provided that the set has a compatible
   // heterogeneous comparator.
   using Base::find;
 
   // btree_set::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 that is not less than `key` within the `btree_set`.
   //
   // Supports heterogeneous lookup, provided that the set has a compatible
   // heterogeneous comparator.
   using Base::lower_bound;
 
   // btree_set::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 that is greater than `key` within the `btree_set`.
   //
   // Supports heterogeneous lookup, provided that the set has a compatible
   // heterogeneous comparator.
   using Base::upper_bound;
 
   // btree_set::get_allocator()
   //
   // Returns the allocator function associated with this `btree_set`.
   using Base::get_allocator;
 
   // btree_set::key_comp();
   //
   // Returns the key comparator associated with this `btree_set`.
   using Base::key_comp;
 
   // btree_set::value_comp();
   //
   // Returns the value comparator associated with this `btree_set`. The keys to
   // sort the elements are the values themselves, therefore `value_comp` and its
   // sibling member function `key_comp` are equivalent.
   using Base::value_comp;
 };
 
 // absl::swap(absl::btree_set<>, absl::btree_set<>)
 //
 // Swaps the contents of two `absl::btree_set` containers.
 template <typename K, typename C, typename A>
 void swap(btree_set<K, C, A> &x, btree_set<K, C, A> &y) {
   return x.swap(y);
 }
 
 // absl::erase_if(absl::btree_set<>, Pred)
 //
 // Erases all elements that satisfy the predicate pred from the container.
 // Returns the number of erased elements.
 template <typename K, typename C, typename A, typename Pred>
 typename btree_set<K, C, A>::size_type erase_if(btree_set<K, C, A> &set,
                                                 Pred pred) {
   return container_internal::btree_access::erase_if(set, std::move(pred));
 }
 
 // absl::btree_multiset<>
 //
 // An `absl::btree_multiset<K>` is an ordered associative container of
 // keys and associated values designed to be a more efficient replacement
 // for `std::multiset` (in most cases). Unlike `absl::btree_set`, a B-tree
 // multiset allows equivalent elements.
 //
 // Keys are sorted using an (optional) comparison function, which defaults to
 // `std::less<K>`.
 //
 // An `absl::btree_multiset<K>` uses a default allocator of `std::allocator<K>`
 // 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_multiset<K, C, A>`.
 //
 template <typename Key, typename Compare = std::less<Key>,
           typename Alloc = std::allocator<Key>>
 class ABSL_INTERNAL_ATTRIBUTE_OWNER btree_multiset
     : public container_internal::btree_multiset_container<
           container_internal::btree<container_internal::set_params<
               Key, Compare, Alloc, /*TargetNodeSize=*/256,
               /*IsMulti=*/true>>> {
   using Base = typename btree_multiset::btree_multiset_container;
 
  public:
   // Constructors and Assignment Operators
   //
   // A `btree_multiset` supports the same overload set as `std::set`
   // for construction and assignment:
   //
   // * Default constructor
   //
   //   absl::btree_multiset<std::string> set1;
   //
   // * Initializer List constructor
   //
   //   absl::btree_multiset<std::string> set2 =
   //       {{"huey"}, {"dewey"}, {"louie"},};
   //
   // * Copy constructor
   //
   //   absl::btree_multiset<std::string> set3(set2);
   //
   // * Copy assignment operator
   //
   //  absl::btree_multiset<std::string> set4;
   //  set4 = set3;
   //
   // * Move constructor
   //
   //   // Move is guaranteed efficient
   //   absl::btree_multiset<std::string> set5(std::move(set4));
   //
   // * Move assignment operator
   //
   //   // May be efficient if allocators are compatible
   //   absl::btree_multiset<std::string> set6;
   //   set6 = std::move(set5);
   //
   // * Range constructor
   //
   //   std::vector<std::string> v = {"a", "b"};
   //   absl::btree_multiset<std::string> set7(v.begin(), v.end());
   btree_multiset() {}
   using Base::Base;
 
   // btree_multiset::begin()
   //
   // Returns an iterator to the beginning of the `btree_multiset`.
   using Base::begin;
 
   // btree_multiset::cbegin()
   //
   // Returns a const iterator to the beginning of the `btree_multiset`.
   using Base::cbegin;
 
   // btree_multiset::end()
   //
   // Returns an iterator to the end of the `btree_multiset`.
   using Base::end;
 
   // btree_multiset::cend()
   //
   // Returns a const iterator to the end of the `btree_multiset`.
   using Base::cend;
 
   // btree_multiset::empty()
   //
   // Returns whether or not the `btree_multiset` is empty.
   using Base::empty;
 
   // btree_multiset::max_size()
   //
   // Returns the largest theoretical possible number of elements within a
   // `btree_multiset` under current memory constraints. This value can be
   // thought of as the largest value of `std::distance(begin(), end())` for a
   // `btree_multiset<Key>`.
   using Base::max_size;
 
   // btree_multiset::size()
   //
   // Returns the number of elements currently within the `btree_multiset`.
   using Base::size;
 
   // btree_multiset::clear()
   //
   // Removes all elements from the `btree_multiset`. Invalidates any references,
   // pointers, or iterators referring to contained elements.
   using Base::clear;
 
   // btree_multiset::erase()
   //
   // Erases elements within the `btree_multiset`. Overloads are listed below.
   //
   // iterator erase(iterator position):
   // iterator erase(const_iterator position):
   //
   //   Erases the element at `position` of the `btree_multiset`, 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_multiset::insert()
   //
   // Inserts an element of the specified value into the `btree_multiset`,
   // 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_multiset`, returning an iterator to the
   //   inserted element.
   //
   // iterator insert(value_type&& value):
   //
   //   Inserts a moveable value into the `btree_multiset`, 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_multiset::emplace()
   //
   // Inserts an element of the specified value by constructing it in-place
   // within the `btree_multiset`. Any references, pointers, or iterators are
   // invalidated.
   using Base::emplace;
 
   // btree_multiset::emplace_hint()
   //
   // Inserts an element of the specified value by constructing it in-place
   // within the `btree_multiset`, 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_multiset::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_multiset`
   //   does not contain an element with a matching key, this function returns an
   //   empty node handle.
   //
   // 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_multiset::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_multiset::merge()
   //
   // Extracts all elements from a given `source` btree_multiset into this
   // `btree_multiset`.
   using Base::merge;
 
   // btree_multiset::swap(btree_multiset& other)
   //
   // Exchanges the contents of this `btree_multiset` with those of the `other`
   // btree_multiset, avoiding invocation of any move, copy, or swap operations
   // on individual elements.
   //
   // All iterators and references on the `btree_multiset` remain valid,
   // excepting for the past-the-end iterator, which is invalidated.
   using Base::swap;
 
   // btree_multiset::contains()
   //
   // template <typename K> bool contains(const K& key) const:
   //
   // Determines whether an element comparing equal to the given `key` exists
   // within the `btree_multiset`, returning `true` if so or `false` otherwise.
   //
   // Supports heterogeneous lookup, provided that the set has a compatible
   // heterogeneous comparator.
   using Base::contains;
 
   // btree_multiset::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_multiset`.
   //
   // Supports heterogeneous lookup, provided that the set has a compatible
   // heterogeneous comparator.
   using Base::count;
 
   // btree_multiset::equal_range()
   //
   // Returns a closed range [first, last], defined by a `std::pair` of two
   // iterators, containing all elements with the passed key in the
   // `btree_multiset`.
   using Base::equal_range;
 
   // btree_multiset::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_multiset`.
   //
   // Supports heterogeneous lookup, provided that the set has a compatible
   // heterogeneous comparator.
   using Base::find;
 
   // btree_multiset::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 that is not less than `key` within the
   // `btree_multiset`.
   //
   // Supports heterogeneous lookup, provided that the set has a compatible
   // heterogeneous comparator.
   using Base::lower_bound;
 
   // btree_multiset::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 that is greater than `key` within the
   // `btree_multiset`.
   //
   // Supports heterogeneous lookup, provided that the set has a compatible
   // heterogeneous comparator.
   using Base::upper_bound;
 
   // btree_multiset::get_allocator()
   //
   // Returns the allocator function associated with this `btree_multiset`.
   using Base::get_allocator;
 
   // btree_multiset::key_comp();
   //
   // Returns the key comparator associated with this `btree_multiset`.
   using Base::key_comp;
 
   // btree_multiset::value_comp();
   //
   // Returns the value comparator associated with this `btree_multiset`. The
   // keys to sort the elements are the values themselves, therefore `value_comp`
   // and its sibling member function `key_comp` are equivalent.
   using Base::value_comp;
 };
 
 // absl::swap(absl::btree_multiset<>, absl::btree_multiset<>)
 //
 // Swaps the contents of two `absl::btree_multiset` containers.
 template <typename K, typename C, typename A>
 void swap(btree_multiset<K, C, A> &x, btree_multiset<K, C, A> &y) {
   return x.swap(y);
 }
 
 // absl::erase_if(absl::btree_multiset<>, Pred)
 //
 // Erases all elements that satisfy the predicate pred from the container.
 // Returns the number of erased elements.
 template <typename K, typename C, typename A, typename Pred>
 typename btree_multiset<K, C, A>::size_type erase_if(
    btree_multiset<K, C, A> & set, Pred pred) {
   return container_internal::btree_access::erase_if(set, std::move(pred));
 }
 
 namespace container_internal {
 
 // This type implements the necessary functions from the
 // absl::container_internal::slot_type interface for btree_(multi)set.
 template <typename Key>
 struct set_slot_policy {
   using slot_type = Key;
   using value_type = Key;
   using mutable_value_type = Key;
 
   static value_type &element(slot_type *slot) { return *slot; }
   static const value_type &element(const slot_type *slot) { return *slot; }
 
   template <typename Alloc, class... Args>
   static void construct(Alloc *alloc, slot_type *slot, Args &&...args) {
     absl::allocator_traits<Alloc>::construct(*alloc, slot,
                                              std::forward<Args>(args)...);
   }
 
   template <typename Alloc>
   static void construct(Alloc *alloc, slot_type *slot, slot_type *other) {
     absl::allocator_traits<Alloc>::construct(*alloc, slot, std::move(*other));
   }
 
   template <typename Alloc>
   static void construct(Alloc *alloc, slot_type *slot, const slot_type *other) {
     absl::allocator_traits<Alloc>::construct(*alloc, slot, *other);
   }
 
   template <typename Alloc>
   static void destroy(Alloc *alloc, slot_type *slot) {
     absl::allocator_traits<Alloc>::destroy(*alloc, slot);
   }
 };
 
 // A parameters structure for holding the type parameters for a btree_set.
 // Compare and Alloc should be nothrow copy-constructible.
 template <typename Key, typename Compare, typename Alloc, int TargetNodeSize,
           bool IsMulti>
 struct set_params : common_params<Key, Compare, Alloc, TargetNodeSize, IsMulti,
                                   /*IsMap=*/false, set_slot_policy<Key>> {
   using value_type = Key;
   using slot_type = typename set_params::common_params::slot_type;
 
   template <typename V>
   static const V &key(const V &value) {
     return value;
   }
   static const Key &key(const slot_type *slot) { return *slot; }
   static const Key &key(slot_type *slot) { return *slot; }
 };
 
 }  // namespace container_internal
 
 ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_BTREE_SET_H_

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