THUAI6/CAPI/cpp/grpc/include/absl/container/btree_map.h

// 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()` and `erase()` return a valid iterator at the current
// position. Another important difference is that key-types must be
// copy-constructible.

#ifndef ABSL_CONTAINER_BTREE_MAP_H_
#define ABSL_CONTAINER_BTREE_MAP_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 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. 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::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 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::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) -> 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_