// bslstl_unorderedmultiset.h                                         -*-C++-*-
#ifndef INCLUDED_BSLSTL_UNORDEREDMULTISET
#define INCLUDED_BSLSTL_UNORDEREDMULTISET

#include <bsls_ident.h>
BSLS_IDENT("$Id: $")

//@PURPOSE: Provide an STL-compliant `unordered_multiset` container.
//
//@CLASSES:
//   bsl::unordered_multiset : STL-compliant `unordered_multiset` container
//
//@CANONICAL_HEADER: bsl_unordered_set.h
//
//@SEE_ALSO: package bos+stdhdrs in the bos package group
//
//@DESCRIPTION: This component defines a single class template,
// `bsl::unordered_multiset`, implementing the standard container holding a
// collection of possibly duplicate keys with no guarantees on ordering (unless
// keys have the same value).
//
// An instantiation of `unordered_multiset` is an allocator-aware,
// value-semantic type whose salient attributes are its size (number of keys)
// and the set of keys the `unordered_multiset` contains, without regard to
// their order.  If `unordered_multiset` is instantiated with a key type that
// is not itself value-semantic, then it will not retain all of its
// value-semantic qualities.  It is possible to instantiate
// `unordered_multiset` with a key type that does not have an accessible
// copy-constructor, in which case the `unordered_multiset` will not be
// copyable.  Note that the equality operator for each element is used to
// determine when two `unordered_multiset` objects have the same value, and not
// the equality comparator supplied at construction.
//
// An `unordered_multiset` meets the requirements of an unordered associative
// container with forward iterators in the C++11 standard [unord].  The
// `unordered_multiset` implemented here adheres to the C++11 standard, except
// that it may rehash when setting the `max_load_factor` in order to preserve
// the property that the value is always respected (which is a potentially
// throwing operation).
//
///Requirements on `KEY`
///---------------------
// An `unordered_multiset` instantiation is a fully "Value-Semantic Type" (see
// {`bsldoc_glossary`}) only if the supplied `KEY` template parameter is fully
// value-semantic.  It is possible to instantiate an `unordered_multiset` with
// a `KEY` parameter argument that does not provide a full set of
// value-semantic operations, but then some methods of the container may not be
// instantiable.  The following terminology, adopted from the C++11 standard,
// is used in the function documentation of `unordered_multiset` to describe a
// function's requirements for the `KEY` template parameter.  These terms are
// also defined in section [utility.arg.requirements] of the C++11 standard.
// Note that, in the context of an `unordered_multiset` instantiation, the
// requirements apply specifically to the `unordered_multiset`s element type,
// `value_type`, which is an alias for `KEY`.
//
// Legend
// ------
// `X`    - denotes an allocator-aware container type (`unordered_multiset`)
// `T`    - `value_type` associated with `X`
// `A`    - type of the allocator used by `X`
// `m`    - lvalue of type `A` (allocator)
// `p`    - address (`T *`) of uninitialized storage for a `T` within an `X`
// `rv`   - rvalue of type (non-`const`) `T`
// `v`    - rvalue or lvalue of type (possibly `const`) `T`
// `args` - 0 or more arguments
//
// The following terms are used to more precisely specify the requirements on
// template parameter types in function-level documentation.
//:
//: *default-insertable*: `T` has a default constructor.  More precisely, `T`
//:     is `default-insertable` into `X` means that the following expression is
//:     well-formed:
//:
//:      `allocator_traits<A>::construct(m, p)`
//:
//: *move-insertable*: `T` provides a constructor that takes an rvalue of type
//:     (non-`const`) `T`.  More precisely, `T` is `move-insertable` into `X`
//:     means that the following expression is well-formed:
//:
//:      `allocator_traits<A>::construct(m, p, rv)`
//:
//: *copy-insertable*: `T` provides a constructor that takes an lvalue or
//:     rvalue of type (possibly `const`) `T`.  More precisely, `T` is
//:     `copy-insertable` into `X` means that the following expression is
//:     well-formed:
//:
//:      `allocator_traits<A>::construct(m, p, v)`
//:
//: *move-assignable*: `T` provides an assignment operator that takes an rvalue
//:     of type (non-`const`) `T`.
//:
//: *copy-assignable*: `T` provides an assignment operator that takes an lvalue
//:     or rvalue of type (possibly `const`) `T`.
//:
//: *emplace-constructible*: `T` is `emplace-constructible` into `X` from
//:     `args` means that the following expression is well-formed:
//:
//:      `allocator_traits<A>::construct(m, p, args)`
//:
//: *erasable*: `T` provides a destructor.  More precisely, `T` is `erasable`
//:     from `X` means that the following expression is well-formed:
//:
//:      `allocator_traits<A>::destroy(m, p)`
//:
//: *equality-comparable*: The type provides an equality-comparison operator
//:     that defines an equivalence relationship and is both reflexive and
//:     transitive.
//
///Requirements on `HASH` and `EQUAL`
///----------------------------------
// The (template parameter) types `HASH` and `EQUAL` must be copy-constructible
// function-objects.  Note that this requirement is somewhat stronger than the
// requirement currently in the standard; see the discussion for Issue 2215
// (http://cplusplus.github.com/LWG/lwg-active.html#2215);
//
// `HASH` shall support a function call operator compatible with the following
// statements:
// ```
// HASH        hash;
// KEY         key;
// std::size_t result = hash(key);
// ```
// where the definition of the called function meets the requirements of a
// hash function, as specified in {`bslstl_hash`|Standard Hash Function}.
//
// `EQUAL` shall support the a function call operator compatible with the
//  following statements:
// ```
// EQUAL equal;
// KEY   key1, key2;
// bool  result = equal(key1, key2);
// ```
// where the definition of the called function defines an equivalence
// relationship on keys that is both reflexive and transitive.
//
// `HASH` and `EQUAL` function-objects are further constrained, such for any
// two objects whose keys compare equal by the comparator, shall produce the
// same value from the hasher.
//
///Memory Allocation
///-----------------
// The type supplied as an unordered multiset's `ALLOCATOR` template parameter
// determines how that unordered multiset will allocate memory.  The
// `unordered_multiset` template supports allocators meeting the requirements
// of the C++11 standard [allocator.requirements], and in addition it supports
// scoped-allocators derived from the `bslma::Allocator` memory allocation
// protocol.  Clients intending to use `bslma`-style allocators should use the
// template's default `ALLOCATOR` type.  The default type for the `ALLOCATOR`
// template parameter, `bsl::allocator`, provides a C++11 standard-compatible
// adapter for a `bslma::Allocator` object.
//
///`bslma`-Style Allocators
/// - - - - - - - - - - - -
// If the parameterized `ALLOCATOR` type of an `unordered_multiset`
// instantiation is `bsl::allocator`, then objects of that unordered multiset
// type will conform to the standard behavior of a `bslma`-allocator-enabled
// type.  Such an unordered multiset accepts an optional `bslma::Allocator`
// argument at construction.  If the address of a `bslma::Allocator` object is
// explicitly supplied at construction, it will be used to supply memory for
// the `unordered_multiset` throughout its lifetime; otherwise, the
// `unordered_multiset` will use the default allocator installed at the time of
// the `unordered_multiset`s construction (see `bslma_default`).  In addition
// to directly allocating memory from the indicated `bslma::Allocator`, an
// `unordered_multiset` supplies that allocator's address to the constructors
// of contained objects of the (template parameter) type `KEY` with the
// `bslalg::TypeTraitUsesBslmaAllocator` trait.
//
///Operations
///----------
// This section describes the run-time complexity of operations on instances
// of `unordered_multiset`:
// ```
// Legend
// ------
// 'K'             - (template parameter) type 'KEY' of the unordered multiset
// 'a', 'b'        - two distinct objects of type 'unordered_multiset<K>'
// 'rv'            - modifiable rvalue of type 'unordered_multiset<K>'
// 'n', 'm'        - number of elements in 'a' and 'b' respectively
// 'w'             - number of buckets of 'a'
// 'value_type'    - unordered_multiset<K>::value_type
// 'hf'            - hash function for objects of type 'K'
// 'eq'            - equality comparator for objects of type 'K'
// 'al'            - STL-style memory allocator
// 'i1', 'i2'      - two iterators defining a sequence of 'value_type' objects
// 'rg'            - range of objects convertable to 'value_type'
// 'li'            - object of type 'initializer_list<K>'
// 'k'             - object of type 'K'
// 'rk'            - modifiable rvalue of type 'K'
// 'v'             - object of type 'value_type'
// 'p1', 'p2'      - two 'const_iterator's belonging to 'a'
// distance(i1,i2) - number of elements in the range '[i1 .. i2)'
// distance(p1,p2) - number of elements in the range '[p1 .. p2)'
//
// +----------------------------------------------------+--------------------+
// | Operation                                          | Complexity         |
// +====================================================+====================+
// | unordered_multiset<K> a;     (default construction)| O[1]               |
// | unordered_multiset<K> a(al);                       |                    |
// +----------------------------------------------------+--------------------+
// | unordered_multiset<K> a(b);  (copy construction)   | Average: O[n]      |
// | unordered_multiset<K> a(b, al);                    | Worst:   O[n^2]    |
// +----------------------------------------------------+--------------------+
// | unordered_multiset<K> a(rv); (move construction)   | O[1] if 'a' and    |
// | unordered_multiset<K> a(rv, al);                   | 'rv' use the same  |
// |                                                    | allocator;         |
// |                                                    | otherwise,         |
// |                                                    | Average: O[n]      |
// |                                                    | Worst:   O[n^2]    |
// +----------------------------------------------------+--------------------+
// | unordered_multiset<K> a(li);                       | Average: O[N]      |
// | unordered_multiset<K> a(li, al);                   | Worst:   O[N^2]    |
// | unordered_multiset<K> a(li, w, al);                | where N =          |
// | unordered_multiset<K> a(li, w, hf, al);            |         'li.size()'|
// | unordered_multiset<K> a(li, w, hf, eq, al);        |                    |
// +----------------------------------------------------+--------------------+
// | unordered_multiset<K> a(w);                        | O[n]               |
// | unordered_multiset<K> a(w, hf);                    |                    |
// | unordered_multiset<K> a(w, hf, eq);                |                    |
// | unordered_multiset<K> a(w, hf, eq, al);            |                    |
// +----------------------------------------------------+--------------------+
// | unordered_multiset<K> a(i1, i2);                   | Average: O[        |
// | unordered_multiset<K> a(i1, i2, w)                 |   distance(i1, i2)]|
// | unordered_multiset<K> a(i1, i2, w, hf);            | Worst:   O[n^2]    |
// | unordered_multiset<K> a(i1, i2, w, hf, eq);        |                    |
// | unordered_multiset<K> a(i1, i2, w, hf, eq, al);    |                    |
// +----------------------------------------------------+--------------------+
// | unordered_multiset<K> a(from_range, rg);           | Average: O[N]      |
// | unordered_multiset<K> a(from_range, rg, w);        | Worst:   O[N^2]    |
// | unordered_multiset<K> a(from_range, rg, w, hf);    | where N = range::  |
// | unordered_multiset<K> a(from_range, rg, w, hf, eq);|        distance(rg)|
// | unordered_multiset<K> a(from_range, rg, w, hf, eq, |                    |
// |                         al);                       |                    |
// +----------------------------------------------------+--------------------+
// | a.~unordered_multiset<K>();  (destruction)         | O[n]               |
// +----------------------------------------------------+--------------------+
// | a = b;                       (copy assignment)     | Average: O[n]      |
// |                                                    | Worst:   O[n^2]    |
// +----------------------------------------------------+--------------------+
// | a = rv;                      (move assignment)     | O[1] if 'a' and    |
// |                                                    | 'rv' use the same  |
// |                                                    | allocator;         |
// |                                                    | otherwise,         |
// |                                                    | Average: O[n]      |
// |                                                    | Worst:   O[n^2]    |
// +----------------------------------------------------+--------------------+
// | a = li;                                            | Average: O[N]      |
// |                                                    | Worst:   O[N^2]    |
// |                                                    | where N =          |
// |                                                    |         'li.size()'|
// +----------------------------------------------------+--------------------+
// | a.begin(), a.end(), a.cbegin(), a.cend(),          | O[1]               |
// +----------------------------------------------------+--------------------+
// | a == b, a != b                                     | Best:  O[n]        |
// |                                                    | Worst: O[n^2]      |
// +----------------------------------------------------+--------------------+
// | a.swap(b), swap(a, b)                              | O[1]               |
// +----------------------------------------------------+--------------------+
// | a.key_eq()                                         | O[1]               |
// +----------------------------------------------------+--------------------+
// | a.hash_function()                                  | O[1]               |
// +----------------------------------------------------+--------------------+
// | a.size()                                           | O[1]               |
// +----------------------------------------------------+--------------------+
// | a.max_size()                                       | O[1]               |
// +----------------------------------------------------+--------------------+
// | a.empty()                                          | O[1]               |
// +----------------------------------------------------+--------------------+
// | get_allocator()                                    | O[1]               |
// +----------------------------------------------------+--------------------+
// | a.insert(v)                                        | Average: O[1]      |
// | a.insert(rk)                                       | Worst:   O[n]      |
// | a.emplace(Args&&...)                               |                    |
// +----------------------------------------------------+--------------------+
// | a.insert(p1, v)                                    | Average: O[1]      |
// | a.insert(p1, rk)                                   | Worst:   O[n]      |
// | a.emplace_hint(p1, Args&&...)                      |                    |
// +----------------------------------------------------+--------------------+
// | a.insert(i1, i2)                                   | Average O[         |
// |                                                    |   distance(i1, i2)]|
// |                                                    | Worst:  O[ n *     |
// |                                                    |   distance(i1, i2)]|
// +----------------------------------------------------+--------------------+
// | a.insert(li);                                      | Average: O[N]      |
// |                                                    | Worst:   O[n * N]  |
// |                                                    | where N =          |
// |                                                    |         'li.size()'|
// +----------------------------------------------------+--------------------+
// | a.insert_range(rg)                                 | Average: O[        |
// |                                                    |   ranges::         |
// |                                                    |       distance(rg)]|
// |                                                    | Worst:   O[ n *    |
// |                                                    |   ranges::         |
// |                                                    |       distance(rg)]|
// +----------------------------------------------------+--------------------+
// | a.erase(p1)                                        | Average: O[1]      |
// |                                                    | Worst:   O[n]      |
// +----------------------------------------------------+--------------------+
// | a.erase(k)                                         | Average: O[        |
// |                                                    |         a.count(k)]|
// |                                                    | Worst:   O[n]      |
// +----------------------------------------------------+--------------------+
// | a.erase(p1, p2)                                    | Average: O[        |
// |                                                    |   distance(p1, p2)]|
// |                                                    | Worst:   O[n]      |
// +----------------------------------------------------+--------------------+
// | a.clear()                                          | O[n]               |
// +----------------------------------------------------+--------------------+
// | a.find(k)                                          | Average: O[1]      |
// |                                                    | Worst:   O[n]      |
// +----------------------------------------------------+--------------------+
// | a.contains(k)                                      | Average: O[1]      |
// |                                                    | Worst:   O[n]      |
// +----------------------------------------------------+--------------------+
// | a.count(k)                                         | Average: O[1]      |
// |                                                    | Worst:   O[n]      |
// +----------------------------------------------------+--------------------+
// | a.equal_range(k)                                   | Average: O[        |
// |                                                    |         a.count(k)]|
// |                                                    | Worst:   O[n]      |
// +----------------------------------------------------+--------------------+
// | a.bucket_count()                                   | O[1]               |
// +----------------------------------------------------+--------------------+
// | a.max_bucket_count()                               | O[1]               |
// +----------------------------------------------------+--------------------+
// | a.bucket(k)                                        | O[1]               |
// +----------------------------------------------------+--------------------+
// | a.bucket_size(k)                                   | O[a.bucket_size(k)]|
// +----------------------------------------------------+--------------------+
// | a.load_factor()                                    | O[1]               |
// +----------------------------------------------------+--------------------+
// | a.max_load_factor()                                | O[1]               |
// | a.max_load_factor(z)                               | O[1]               |
// +----------------------------------------------------+--------------------+
// | a.rehash(k)                                        | Average: O[n]      |
// |                                                    | Worst:   O[n^2]    |
// +----------------------------------------------------+--------------------+
// | a.reserve(k)                                       | Average: O[n]      |
// |                                                    | Worst:   O[n^2]    |
// +----------------------------------------------------+--------------------+
// ```
//
///Iterator, Pointer, and Reference Invalidation
///---------------------------------------------
// No method of `unordered_multiset` invalidates a pointer or reference to an
// element in the unordered multiset, unless it also erases that element, such
// as any `erase` overload, `clear`, or the destructor (that erases all
// elements).  Pointers and references are stable through a rehash.
//
// Iterators to elements in the container are invalidated by any rehash, so
// iterators may be invalidated by an `insert` or `emplace` call if it triggers
// a rehash (but not otherwise).  Iterators to specific elements are also
// invalidated when that element is erased.  Note that the `end` iterator is
// not an iterator referring to any element in the container, so may be
// invalidated by any non-`const` method.
//
///Unordered Multiset Configuration
///---------------------------------
// The unordered multiset has interfaces that can provide insight into and
// control of its inner workings.  The syntax and semantics of these interfaces
// for `bslstl_unorderedmultiset` are identical to those of
// `bslstl_unorderedmap`.  See the discussion in
// {`bslstl_unorderedmap`|Unordered Map Configuration} and the illustrative
// material in {`bslstl_unorderedmap`|Example 2}.
//
///Practical Requirements on `HASH`
///--------------------------------
// An important factor in the performance of an unordered multiset (and any of
// the other unordered containers) is the choice of hash function.  Please see
// the discussion in {`bslstl_unorderedmap`|Practical Requirements on `HASH`}.
//
///Usage
///-----
// In this section we show intended use of this component.
//
///Example 1: Categorizing Data
/// - - - - - - - - - - - - - -
// Unordered sets are useful in situations when there is no meaningful way to
// order key values, when the order of the values is irrelevant to the problem
// domain, and (even if there is a meaningful ordering) the value of ordering
// the results is outweighed by the higher performance provided by unordered
// sets (compared to ordered sets).
//
// One uses a multiset (ordered or unordered) when there may be more than one
// instance of an element of a set and when that multiplicity must be
// preserved.
//
// Note that the data type described below is an augmentation of that used in
// {`bslstl_unorderedset`|Example 1}.  The data itself (randomly generated) is
// different.
//
// Suppose one is analyzing data on a set of customers, and each customer is
// categorized by several attributes: customer type, geographic area, and
// (internal) project code; and that each attribute takes on one of a limited
// set of values.  Additionally, there is some financial data associated with
// each customer: past sales and pending sales.
//
// The several customer attributes are modeled by several enumerations:
// ```
// typedef enum {
//     REPEAT
//   , DISCOUNT
//   , IMPULSE
//   , NEED_BASED
//   , BUSINESS
//   , NON_PROFIT
//   , INSTITUTE
//     // ...
// } CustomerCode;
//
// typedef enum {
//     USA_EAST
//   , USA_WEST
//   , CANADA
//   , MEXICO
//   , ENGLAND
//   , SCOTLAND
//   , FRANCE
//   , GERMANY
//   , RUSSIA
//     // ...
// } LocationCode;
//
// typedef enum {
//     TOAST
//   , GREEN
//   , FAST
//   , TIDY
//   , PEARL
//   , SMITH
//     // ...
// } ProjectCode;
// ```
// For printing these values in a human-readable form, we define these helper
// functions:
// ```
// static const char *toAscii(CustomerCode value)
// {
//     switch (value) {
//       case REPEAT:     return "REPEAT";
//       case DISCOUNT:   return "DISCOUNT";
//       case IMPULSE:    return "IMPULSE";
//       case NEED_BASED: return "NEED_BASED";
//       case BUSINESS:   return "BUSINESS";
//       case NON_PROFIT: return "NON_PROFIT";
//       case INSTITUTE:  return "INSTITUTE";
//       // ...
//       default: return "(* UNKNOWN *)";
//     }
// }
//
// static const char *toAscii(LocationCode value)
// {
//     ...
// }
//
// static const char *toAscii(ProjectCode  value)
// {
//     ...
// }
// ```
// The data set (randomly generated for this example) is provided in a
// statically initialized array:
// ```
// static const struct CustomerDatum {
//     CustomerCode d_customer;
//     LocationCode d_location;
//     ProjectCode  d_project;
//     double       d_past;
//     double       d_pending;
// } customerData[] = {
//    { REPEAT    , RUSSIA  , SMITH,   75674.00,     455.00 },
//    { REPEAT    , ENGLAND , TOAST,   35033.00,    8377.00 },
//    { BUSINESS  , USA_EAST, SMITH,   53942.00,    2782.00 },
//    ...
//    { DISCOUNT  , MEXICO  , GREEN,   99737.00,    3872.00 },
// };
//
// const int numCustomerData = sizeof customerData / sizeof *customerData;
// ```
// Suppose, as a step in analysis, we wish to determine the average of the past
// sales and the average of the pending sales for each customer for each unique
// combination of customer attributes (i.e., for each customer profile in the
// data set).  To do so, we must aggregate our data items by customer profile
// but also retain the unique financial data for each item.  The
// `bslstl_unorderedmultiset` provides those semantics.
//
// First, as there are no standard methods for hashing or comparing our user-
// defined types, we define `CustomerDatumHash` and `CustomerDatumEqual`
// classes, each a stateless functor.  Note that there is no meaningful
// ordering of the attribute values, they are merely arbitrary code numbers;
// nothing is lost by using an unordered multiset instead of an ordered
// multiset:
// ```
// class CustomerDatumHash
// {
//   public:
//     // CREATORS
//
//     /// Create a `CustomerDatumHash` object.
//     //! CustomerDatumHash() = default;
//
//     /// Create a `CustomerDatumHash` object.  Note that as
//     /// `CustomerDatumHash` is an empty (stateless) type, this operation
//     /// has no observable effect.
//     //! hash(const CustomerDatumHash& original) = default;
//
//     /// Destroy this object.
//     //! ~CustomerDatumHash() = default;
//
//     // ACCESSORS
//
//     /// Return a hash value computed using the specified `x`.
//     std::size_t operator()(CustomerDatum x) const;
// };
//
// // ACCESSORS
// std::size_t CustomerDatumHash::operator()(CustomerDatum x) const
// {
//     return bsl::hash<int>()(x.d_location * 100 * 100
//                           + x.d_customer * 100
//                           + x.d_project);
// }
//
// class CustomerDatumEqual
// {
//   public:
//     // CREATORS
//
//     /// Create a `CustomerDatumEqual` object.
//     //! CustomerDatumEqual() = default;
//
//     /// Create a `CustomerDatumEqual` object.  Note that as
//     /// `CustomerDatumEqual` is an empty (stateless) type, this
//     /// operation has no observable effect.
//     //! CustomerDatumEqual(const CustomerDatumEqual& original) = default;
//
//     /// Destroy this object.
//     //! ~CustomerDatumEqual() = default;
//
//     // ACCESSORS
//     bool operator()(const CustomerDatum& lhs,
//                     const CustomerDatum& rhs) const;
// };
//
// // ACCESSORS
// bool CustomerDatumEqual::operator()(const CustomerDatum& lhs,
//                                     const CustomerDatum& rhs) const
// {
//     return lhs.d_location == rhs.d_location
//         && lhs.d_customer == rhs.d_customer
//         && lhs.d_project  == rhs.d_project;
// }
// ```
// Notice that many of the required methods of the hash and comparator types
// are compiler generated.  (The declaration of those methods are commented out
// and suffixed by an `= default` comment.)
//
// Also notice that the boolean operation provided by `CustomerDatumEqual` is
// more properly thought of as "equivalence", not "equality".  There may be
// more than one data item with the same customer profile (i.e., the same for
// our purpose here), but they have distinct financial data so the two items
// are not equal (unless the financial data also happens to match).
//
// Next, we define the type of the unordered multiset and a convenience
// aliases:
// ```
// typedef bsl::unordered_multiset<CustomerDatum,
//                                 CustomerDatumHash,
//                                 CustomerDatumEqual> DataByProfile;
// typedef DataByProfile::const_iterator               DataByProfileConstItr;
// ```
// Now, create a helper function to calculate the average financials for a
// category of customer profiles within the unordered multiset.
// ```
// /// Print to the specified `out` in some human-readable format the
// /// averages of the `past` and `pending` attributes of every
// /// `CustomerInfoData` object from the specified `start` up to (but not
// /// including) the specified `end`.  The behavior is undefined unless
// /// `end != start`.
// void processCategory(DataByProfileConstItr  start,
//                      DataByProfileConstItr  end,
//                      FILE                  *out)
// {
//     assert(end != start);
//     assert(out);
//
//     double sumPast    = 0.0;
//     double sumPending = 0.0;
//     int    count      = 0;
//
//     for (DataByProfileConstItr itr = start; end != itr; ++itr) {
//         sumPast    += itr->d_past;
//         sumPending += itr->d_pending;
//         ++count;
//     }
//     printf("%-10s %-8s %-5s %10.2f %10.2f\n",
//            toAscii(start->d_customer),
//            toAscii(start->d_location),
//            toAscii(start->d_project),
//            sumPast/count,
//            sumPending/count);
// }
// ```
// Then, we create an unordered multiset and insert each item of `data`.
// ```
// DataByProfile dataByProfile;
//
// for (int idx = 0; idx < numCustomerData; ++idx) {
//    dataByProfile.insert(customerData[idx]);
// }
// assert(numCustomerData == dataByProfile.size());
// ```
// Finally, to calculate the statistics we need, we must detect the transition
// between categories as we iterate through `customerInfoData`.
// ```
// CustomerDatumEqual    areEquivalent;
// DataByProfileConstItr end             = dataByProfile.end();
// DataByProfileConstItr startOfCategory = end;
//
// for (DataByProfileConstItr itr  = dataByProfile.begin();
//                            end != itr; ++itr) {
//     if (end == startOfCategory) {
//         startOfCategory = itr;
//         continue;
//     }
//
//     if (!areEquivalent(*startOfCategory, *itr)) {
//         processCategory(startOfCategory, itr, stdout);
//         startOfCategory = itr;
//     }
// }
// if (end != startOfCategory) {
//     processCategory(startOfCategory, end, stdout);
// }
// ```
// We find on standard output:
// ```
// BUSINESS   GERMANY  TIDY    84553.00    3379.00
// DISCOUNT   ENGLAND  TIDY    74110.00    2706.00
// NEED_BASED CANADA   FAST    97479.00     681.00
// ...
// NEED_BASED SCOTLAND TOAST   27306.00    5084.50
// INSTITUTE  CANADA   TIDY    83528.00    4722.33
// NEED_BASED FRANCE   FAST    83741.50    5396.50
// REPEAT     MEXICO   TOAST    7469.00    5958.00
// BUSINESS   SCOTLAND FAST    24443.00    4247.00
// INSTITUTE  FRANCE   FAST    19349.00    3982.00
// NEED_BASED RUSSIA   TIDY    50712.00    8647.00
// INSTITUTE  SCOTLAND TIDY    78240.00    6635.00
// BUSINESS   RUSSIA   PEARL   29386.00    3623.00
// INSTITUTE  FRANCE   PEARL   47747.00    3533.00
// ```

#include <bslscm_version.h>

#include <bslstl_algorithm.h>
#include <bslstl_equalto.h>
#include <bslstl_hash.h>
#include <bslstl_hashtable.h>
#include <bslstl_hashtablebucketiterator.h>
#include <bslstl_hashtableiterator.h>
#include <bslstl_iteratorutil.h>
#include <bslstl_pair.h>  // result type of 'equal_range' method
#include <bslstl_ranges.h>
#include <bslstl_unorderedsetkeyconfiguration.h>

#include <bslalg_bidirectionallink.h>
#include <bslalg_bidirectionalnode.h>
#include <bslalg_typetraithasstliterators.h>

#include <bslma_allocatortraits.h>
#include <bslma_isstdallocator.h>
#include <bslma_bslallocator.h>        // Can probably escape with a fwd-decl,
                                       // but not very user friendly
#include <bslma_usesbslmaallocator.h>

#include <bslmf_containercompatiblerange.h>
#include <bslmf_enableif.h>
#include <bslmf_isbitwisemoveable.h>
#include <bslmf_isnothrowswappable.h>
#include <bslmf_nestedtraitdeclaration.h>
#include <bslmf_typeidentity.h>
#include <bslmf_util.h>    // 'forward(V)'

#include <bsls_assert.h>
#include <bsls_compilerfeatures.h>
#include <bsls_keyword.h>
#include <bsls_performancehint.h>
#include <bsls_util.h>     // 'forward<T>(V)'

#include <cstddef>  // for 'std::size_t'

#if defined(BSLS_COMPILERFEATURES_SUPPORT_GENERALIZED_INITIALIZERS)
# include <initializer_list>
#endif

#ifdef BSLS_COMPILERFEATURES_SUPPORT_TRAITS_HEADER
#include <type_traits>  // 'std::is_nothrow_move_assignable'
#endif

#if defined(BSLS_LIBRARYFEATURES_HAS_CPP20_CONCEPTS) \
 && defined(BSLS_LIBRARYFEATURES_HAS_CPP20_RANGES)
# define BSLSTL_UNORDEREDMULTISET_REQUIRES_CONTAINER_COMPATIBLE_RANGE(R, T) \
                  requires ::BloombergLP::bslmf::ContainerCompatibleRange<R, T>
#else
# define BSLSTL_UNORDEREDMULTISET_REQUIRES_CONTAINER_COMPATIBLE_RANGE(R, T)
#endif

#if BSLS_COMPILERFEATURES_SIMULATE_CPP11_FEATURES
// clang-format off
// Include version that can be compiled with C++03
// Generated on Mon Jan 13 08:31:40 2025
// Command line: sim_cpp11_features.pl bslstl_unorderedmultiset.h

# define COMPILING_BSLSTL_UNORDEREDMULTISET_H
# include <bslstl_unorderedmultiset_cpp03.h>
# undef COMPILING_BSLSTL_UNORDEREDMULTISET_H

// clang-format on
#else

namespace bsl {

                        // ========================
                        // class unordered_multiset
                        // ========================

/// This class template implements a value-semantic container type holding
/// an unordered multiset of values (of template parameter type `KEY`).
///
/// This class:
/// * supports a complete set of *value-semantic* operations
///   - except for BDEX serialization
/// * is *exception-neutral* (agnostic except for the `at` method)
/// * is *alias-safe*
/// * is `const` *thread-safe*
/// For terminology see {`bsldoc_glossary`}.
template <class KEY,
          class HASH      = bsl::hash<KEY>,
          class EQUAL     = bsl::equal_to<KEY>,
          class ALLOCATOR = bsl::allocator<KEY> >
class unordered_multiset
{

  private:

    // PRIVATE TYPE

    /// This typedef is an alias for the allocator traits type associated
    /// with this container.
    typedef bsl::allocator_traits<ALLOCATOR>                 AllocatorTraits;

    /// This typedef is an alias for the type of values maintained by this
    /// unordered multiset.
    typedef KEY                                              ValueType;

    /// This typedef is an alias for the policy used internally by this
    /// container to extract the `KEY` value from the values maintained by
    /// this unordered multiset.
    typedef ::BloombergLP::bslstl::UnorderedSetKeyConfiguration<ValueType>
                                                             ListConfiguration;

    /// This typedef is an alias for the template instantiation of the
    /// underlying `bslstl::HashTable` used to implement this unordered
    /// multiset.
    typedef ::BloombergLP::bslstl::HashTable<ListConfiguration,
                                             HASH,
                                             EQUAL,
                                             ALLOCATOR>      HashTable;

    /// This typedef is an alias for the type of links maintained by the
    /// linked list of elements held by the underlying `bslstl::HashTable`.
    typedef ::BloombergLP::bslalg::BidirectionalLink         HashTableLink;

    /// This typedef is a convenient alias for the utility associated with
    /// movable references.
    typedef BloombergLP::bslmf::MovableRefUtil               MoveUtil;

    // FRIENDS
    template <class KEY2,
              class HASH2,
              class EQUAL2,
              class ALLOCATOR2>
    friend bool operator==(
                   const unordered_multiset<KEY2, HASH2, EQUAL2, ALLOCATOR2>&,
                   const unordered_multiset<KEY2, HASH2, EQUAL2, ALLOCATOR2>&);

  public:
    // PUBLIC TYPES
    typedef KEY                                        key_type;
    typedef KEY                                        value_type;
    typedef HASH                                       hasher;
    typedef EQUAL                                      key_equal;
    typedef ALLOCATOR                                  allocator_type;
    typedef value_type&                                reference;
    typedef const value_type&                          const_reference;

    typedef typename AllocatorTraits::size_type        size_type;
    typedef typename AllocatorTraits::difference_type  difference_type;
    typedef typename AllocatorTraits::pointer          pointer;
    typedef typename AllocatorTraits::const_pointer    const_pointer;

    typedef ::BloombergLP::bslstl::HashTableIterator<
                    const value_type, difference_type> iterator;

    typedef ::BloombergLP::bslstl::HashTableBucketIterator<
                    const value_type, difference_type> local_iterator;

    typedef iterator                                   const_iterator;
    typedef local_iterator                             const_local_iterator;

  public:
    // TRAITS
    BSLMF_NESTED_TRAIT_DECLARATION_IF(
                    unordered_multiset,
                    ::BloombergLP::bslmf::IsBitwiseMoveable,
                    ::BloombergLP::bslmf::IsBitwiseMoveable<HashTable>::value);

  private:
    // DATA
    HashTable d_impl;

  public:
    // CREATORS

    /// Create an empty unordered multiset.  Optionally specify an
    /// `initialNumBuckets` indicating the initial size of the array of
    /// buckets of this container.  If `initialNumBuckets` is not supplied,
    /// a single bucket is used.  Optionally specify a `hashFunction` used
    /// to generate the hash values for the keys contained in this unordered
    /// multiset.  If `hashFunction` is not supplied, a default-constructed
    /// object of the (template parameter) type `HASH` is used.  Optionally
    /// specify a key-equality functor `keyEqual` used to verify that two
    /// keys are equivalent.  If `keyEqual` is not supplied, a
    /// default-constructed object of the (template parameter) type `EQUAL`
    /// is used.  Optionally specify a `basicAllocator` used to supply
    /// memory.  If `basicAllocator` is not supplied, a default-constructed
    /// object of the (template parameter) type `ALLOCATOR` is used.  If the
    /// type `ALLOCATOR` is `bsl::allocator` (the default), then
    /// `basicAllocator`, if supplied, shall be convertible to
    /// `bslma::Allocator *`.  If the type `ALLOCATOR` is `bsl::allocator`
    /// and `basicAllocator` is not supplied, the currently installed
    /// default allocator is used.
    unordered_multiset();
    explicit unordered_multiset(size_type        initialNumBuckets,
                                const HASH&      hashFunction = HASH(),
                                const EQUAL&     keyEqual = EQUAL(),
                                const ALLOCATOR& basicAllocator = ALLOCATOR());
    unordered_multiset(size_type        initialNumBuckets,
                       const HASH&      hashFunction,
                       const ALLOCATOR& basicAllocator);
    unordered_multiset(size_type        initialNumBuckets,
                       const ALLOCATOR& basicAllocator);
    explicit unordered_multiset(const ALLOCATOR& basicAllocator);

    /// Create an unordered multiset having the same value as the specified
    /// `original` object.  Use a copy of `original.hash_function()` to
    /// generate hash values for the keys contained in this unordered
    /// multiset.  Use a copy of `original.key_eq()` to verify that two keys
    /// are equivalent.  Use the allocator returned by
    /// `bsl::allocator_traits<ALLOCATOR>::
    /// select_on_container_copy_construction(original.get_allocator())` to
    /// allocate memory.  This method requires that the (template parameter)
    /// type `KEY` be `copy-insertable` into this unordered multiset (see
    /// {Requirements on `KEY`}).
    unordered_multiset(const unordered_multiset& original);

    /// Create an unordered multiset having the same value as the specified
    /// `original` object by moving (in constant time) the contents of
    /// `original` to the new unordered multiset.  Use a copy of
    /// `original.hash_function()` to generate hash values for the keys
    /// contained in this unordered multiset.  Use a copy of
    /// `original.key_eq()` to verify that two keys are equivalent.  The
    /// allocator associated with `original` is propagated for use in the
    /// newly-created unordered multiset.  `original` is left in a valid but
    /// unspecified state.
    unordered_multiset(
                  BloombergLP::bslmf::MovableRef<unordered_multiset> original);

    /// Create an unordered multiset having the same value as the specified
    /// `original` object that uses the specified `basicAllocator` to supply
    /// memory.  Use a copy of `original.hash_function()` to generate hash
    /// values for the keys contained in this unordered multiset.  Use a
    /// copy of `original.key_eq()` to verify that two keys are equivalent.
    /// This method requires that the (template parameter) type `KEY` be
    /// `copy-insertable` into this unordered multiset (see {Requirements on
    /// `KEY`}).  Note that a `bslma::Allocator *` can be supplied for
    /// `basicAllocator` if the (template parameter) type `ALLOCATOR` is
    /// `bsl::allocator` (the default).
    unordered_multiset(
                const unordered_multiset&                      original,
                const typename type_identity<ALLOCATOR>::type& basicAllocator);

    /// Create an unordered multiset having the same value as the specified
    /// `original` object that uses the specified `basicAllocator` to supply
    /// memory.  The contents of `original` are moved (in constant time) to
    /// the new unordered multiset if `basicAllocator ==
    /// original.get_allocator()`, and are move-inserted (in linear time)
    /// using `basicAllocator` otherwise.  `original` is left in a valid but
    /// unspecified state.  Use a copy of `original.hash_function()` to
    /// generate hash values for the keys contained in this unordered
    /// multiset.  Use a copy of `original.key_eq()` to verify that two keys
    /// are equivalent.  This method requires that the (template parameter)
    /// type `KEY` be `move-insertable` into this unordered multiset (see
    /// {Requirements on `KEY`}).  Note that a `bslma::Allocator *` can be
    /// supplied for `basicAllocator` if the (template parameter) type
    /// `ALLOCATOR` is `bsl::allocator` (the default).
    unordered_multiset(
            BloombergLP::bslmf::MovableRef<unordered_multiset> original,
            const typename type_identity<ALLOCATOR>::type&     basicAllocator);

    /// Create an unordered multiset, and insert each `value_type` object in
    /// the sequence starting at the specified `first` element, and ending
    /// immediately before the specified `last` element.  Optionally specify
    /// an `initialNumBuckets` indicating the initial size of the array of
    /// buckets of this container.  If `initialNumBuckets` is not supplied,
    /// a single bucket is used.  Optionally specify a `hashFunction` used
    /// to generate hash values for the keys contained in this unordered
    /// multiset.  If `hashFunction` is not supplied, a default-constructed
    /// object of (template parameter) type `HASH` is used.  Optionally
    /// specify a key-equality functor `keyEqual` used to verify that two
    /// keys are equivalent.  If `keyEqual` is not supplied, a
    /// default-constructed object of (template parameter) type `EQUAL` is
    /// used.  Optionally specify a `basicAllocator` used to supply memory.
    /// If `basicAllocator` is not supplied, a default-constructed object of
    /// the (template parameter) type `ALLOCATOR` is used.  If the type
    /// `ALLOCATOR` is `bsl::allocator` and `basicAllocator` is not
    /// supplied, the currently installed default allocator is used to
    /// supply memory.  The (template parameter) type `INPUT_ITERATOR` shall
    /// meet the requirements of an input iterator defined in the C++11
    /// standard [24.2.3] providing access to values of a type convertible
    /// to `value_type`, and `value_type` must be `emplace-constructible`
    /// from `*i` into this unordered multiset, where `i` is a
    /// dereferenceable iterator in the range `[first .. last)` (see
    /// {Requirements on `KEY`}).  The behavior is undefined unless `first`
    /// and `last` refer to a sequence of valid values where `first` is at a
    /// position at or before `last`.  Note that a `bslma::Allocator *` can
    /// be supplied for `basicAllocator` if the type `ALLOCATOR` is
    /// `bsl::allocator` (the default).
    template <class INPUT_ITERATOR>
    unordered_multiset(INPUT_ITERATOR   first,
                       INPUT_ITERATOR   last,
                       size_type        initialNumBuckets = 0,
                       const HASH&      hashFunction = HASH(),
                       const EQUAL&     keyEqual = EQUAL(),
                       const ALLOCATOR& basicAllocator = ALLOCATOR());
    template <class INPUT_ITERATOR>
    unordered_multiset(INPUT_ITERATOR   first,
                       INPUT_ITERATOR   last,
                       size_type        initialNumBuckets,
                       const HASH&      hashFunction,
                       const ALLOCATOR& basicAllocator);
    template <class INPUT_ITERATOR>
    unordered_multiset(INPUT_ITERATOR   first,
                       INPUT_ITERATOR   last,
                       size_type        initialNumBuckets,
                       const ALLOCATOR& basicAllocator);
    template <class INPUT_ITERATOR>
    unordered_multiset(INPUT_ITERATOR   first,
                       INPUT_ITERATOR   last,
                       const ALLOCATOR& basicAllocator);

#if defined(BSLS_COMPILERFEATURES_SUPPORT_GENERALIZED_INITIALIZERS)
# ifdef BSLS_COMPILERFEATURES_SUPPORT_CTAD
    template <
    class = bsl::enable_if_t<std::is_invocable_v<HASH, const KEY &>>,
    class = bsl::enable_if_t<
                         std::is_invocable_v<EQUAL, const KEY &, const KEY &>>,
    class = bsl::enable_if_t< bsl::IsStdAllocator_v<ALLOCATOR>>
    >
# endif
    unordered_multiset(
                      std::initializer_list<KEY> values,
                      size_type                  initialNumBuckets = 0,
                      const HASH&                hashFunction = HASH(),
                      const EQUAL&               keyEqual = EQUAL(),
                      const ALLOCATOR&           basicAllocator = ALLOCATOR());
# ifdef BSLS_COMPILERFEATURES_SUPPORT_CTAD
    template <
    class = bsl::enable_if_t<std::is_invocable_v<HASH, const KEY &>>,
    class = bsl::enable_if_t<bsl::IsStdAllocator<ALLOCATOR>::value>
    >
# endif
    unordered_multiset(std::initializer_list<KEY> values,
                       size_type                  initialNumBuckets,
                       const HASH&                hashFunction,
                       const ALLOCATOR&           basicAllocator);
# ifdef BSLS_COMPILERFEATURES_SUPPORT_CTAD
    template <class = bsl::enable_if_t<bsl::IsStdAllocator<ALLOCATOR>::value>>
# endif
    unordered_multiset(std::initializer_list<KEY> values,
                       size_type                  initialNumBuckets,
                       const ALLOCATOR&           basicAllocator);
# ifdef BSLS_COMPILERFEATURES_SUPPORT_CTAD
    /// Create an unordered multiset and insert each `value_type` object in
    /// the specified `values` initializer list.  Optionally specify an
    /// `initialNumBuckets` indicating the initial size of the array of
    /// buckets of this container.  If `initialNumBuckets` is not supplied,
    /// a single bucket is used.  Optionally specify a `hashFunction` used
    /// to generate the hash values for the keys contained in this unordered
    /// multiset.  If `hashFunction` is not supplied, a default-constructed
    /// object of the (template parameter) type `HASH` is used.  Optionally
    /// specify a key-equality functor `keyEqual` used to verify that two
    /// keys are equivalent.  If `keyEqual` is not supplied, a
    /// default-constructed object of the (template parameter) type `EQUAL`
    /// is used.  Optionally specify a `basicAllocator` used to supply
    /// memory.  If `basicAllocator` is not supplied, a default-constructed
    /// object of the (template parameter) type `ALLOCATOR` is used.  If the
    /// type `ALLOCATOR` is `bsl::allocator` and `basicAllocator` is not
    /// supplied, the currently installed default allocator is used to
    /// supply memory.  This method requires that the (template parameter)
    /// type `KEY` be `copy-insertable` into this unordered multiset (see
    /// {Requirements on `KEY`}).  Note that a `bslma::Allocator *` can be
    /// supplied for `basicAllocator` if the type `ALLOCATOR` is
    /// `bsl::allocator` (the default).
    template <class = bsl::enable_if_t<bsl::IsStdAllocator<ALLOCATOR>::value>>
# endif
    unordered_multiset(std::initializer_list<KEY> values,
                       const ALLOCATOR&           basicAllocator);
#endif

    /// Create an unordered multiset, and insert each `value_type` object in
    /// the specified `range`.  Optionally specify an `initialNumBuckets`
    /// indicating the initial size of the array of buckets of this
    /// container.  If `initialNumBuckets` is not supplied, an
    /// implementation-defined value is used.  Optionally specify a
    /// `hashFunction` used to generate the hash values for each key value
    /// contained in this unordered multiset.  If `hashFunction` is not
    /// supplied, a default-constructed object of the (template parameter)
    /// type `HASH` is used.  Optionally specify a key-equality functor
    /// `keyEqual` used to determine whether two keys have the same value.
    /// If `keyEqual` is not supplied, a default-constructed object of the
    /// (template parameter) type `EQUAL` is used.  Optionally specify a
    /// `basicAllocator` used to supply memory.  If `basicAllocator` is not
    /// supplied, a default-constructed object of the (template parameter)
    /// type `ALLOCATOR` is used.  If the type `ALLOCATOR` is
    /// `bsl::allocator` (the default), then `basicAllocator`, if supplied,
    /// shall be convertible to `bslma::Allocator *`.  If the type
    /// `ALLOCATOR` is `bsl::allocator` and `basicAllocator` is not
    /// supplied, the currently installed default allocator is used.  This
    /// operation has `O[N]` complexity, where `N` is the number of elements
    /// in `range`.  Note that `RANGE` must meet the requirements of an
    /// input range and the values from `range` must have a type matching or
    /// convertible to `value_type`.
    template <class RANGE>
    BSLSTL_UNORDEREDMULTISET_REQUIRES_CONTAINER_COMPATIBLE_RANGE(RANGE,
                                                                 value_type)
    unordered_multiset(
         bsl::from_range_t                        ,
         BSLS_COMPILERFEATURES_FORWARD_REF(RANGE) range,
         size_type                                initialNumBuckets = 0,
         const HASH&                              hashFunction = HASH(),
         const EQUAL&                             keyEqual = EQUAL(),
         const ALLOCATOR&                         basicAllocator = ALLOCATOR())
    : d_impl(hashFunction, keyEqual, initialNumBuckets, 1.0f, basicAllocator)
    {
        // Defined inline for Windows.

#if defined(BSLS_LIBRARYFEATURES_HAS_CPP20_CONCEPTS) \
&& defined(BSLS_LIBRARYFEATURES_HAS_CPP20_RANGES)
        if constexpr (ranges::sized_range<RANGE>) {
            insertFromRange(bsl::ranges::begin(range),
                            bsl::ranges::end  (range),
                            bsl::ranges::size (range));
        } else // ...
#endif
        {
            insertFromRange(bsl::ranges::begin(range),
                            bsl::ranges::end  (range));
        }
    }

    template <class RANGE>
    BSLSTL_UNORDEREDMULTISET_REQUIRES_CONTAINER_COMPATIBLE_RANGE(RANGE,
                                                                 value_type)
    unordered_multiset(
                    bsl::from_range_t                        ,
                    BSLS_COMPILERFEATURES_FORWARD_REF(RANGE) range,
                    size_type                                initialNumBuckets,
                    const HASH&                              hashFunction,
                    const ALLOCATOR&                         basicAllocator)
    : d_impl(hashFunction, EQUAL(), initialNumBuckets, 1.0f, basicAllocator)
    {
        // Defined inline for Windows.

        unordered_multiset other(bsl::from_range,
                                 range,
                                 initialNumBuckets,
                                 hashFunction,
                                 EQUAL(),
                                 basicAllocator);
        this->swap(other);
    }

    template <class RANGE>
    BSLSTL_UNORDEREDMULTISET_REQUIRES_CONTAINER_COMPATIBLE_RANGE(RANGE,
                                                                 value_type)
    unordered_multiset(
                    bsl::from_range_t                        ,
                    BSLS_COMPILERFEATURES_FORWARD_REF(RANGE) range,
                    size_type                                initialNumBuckets,
                    const ALLOCATOR&                         basicAllocator)
    : d_impl(HASH(), EQUAL(), initialNumBuckets, 1.0f, basicAllocator)
    {
        // Defined inline for Windows.

        unordered_multiset other(bsl::from_range,
                                 range,
                                 initialNumBuckets,
                                 HASH(),
                                 EQUAL(),
                                 basicAllocator);
        this->swap(other);
    }

    template <class RANGE>
    BSLSTL_UNORDEREDMULTISET_REQUIRES_CONTAINER_COMPATIBLE_RANGE(RANGE,
                                                                 value_type)
    unordered_multiset(bsl::from_range_t                        ,
                       BSLS_COMPILERFEATURES_FORWARD_REF(RANGE) range,
                       const ALLOCATOR&                         basicAllocator)
    : d_impl(HASH(), EQUAL(), 0, 1.0f, basicAllocator)
    {
        // Defined inline for Windows.

        unordered_multiset other(bsl::from_range,
                                 range,
                                 0,
                                 HASH(),
                                 EQUAL(),
                                 basicAllocator);
        this->swap(other);
    }

    /// Destroy this object.
    ~unordered_multiset();

    // MANIPULATORS

    /// Assign to this object the value, hash function, and equality
    /// comparator of the specified `rhs` object, propagate to this object
    /// the allocator of `rhs` if the `ALLOCATOR` type has trait
    /// `propagate_on_container_copy_assignment`, and return a reference
    /// providing modifiable access to this object.  If an exception is
    /// thrown, `*this` is left in a valid but unspecified state.  This
    /// method requires that the (template parameter) type `KEY` be both
    /// `copy-assignable` and `copy-insertable` into this unordered multiset
    /// (see {Requirements on `KEY`}).
    unordered_multiset& operator=(const unordered_multiset& rhs);

    /// Assign to this object the value, hash function, and equality
    /// comparator of the specified `rhs` object, propagate to this object
    /// the allocator of `rhs` if the `ALLOCATOR` type has trait
    /// `propagate_on_container_move_assignment`, and return a reference
    /// providing modifiable access to this object.  The contents of `rhs`
    /// are moved (in constant time) to this unordered multiset if
    /// `get_allocator() == rhs.get_allocator()` (after accounting for the
    /// aforementioned trait); otherwise, all elements in this unordered
    /// multiset are either destroyed or move-assigned to and each
    /// additional element in `rhs` is move-inserted into this unordered
    /// multiset.  `rhs` is left in a valid but unspecified state, and if an
    /// exception is thrown, `*this` is left in a valid but unspecified
    /// state.  This method requires that the (template parameter) type
    /// `KEY` be both `move-assignable` and `move-insertable` into this
    /// unordered multiset (see {Requirements on `KEY`}).
    unordered_multiset&
    operator=(BloombergLP::bslmf::MovableRef<unordered_multiset> rhs)
                            BSLS_KEYWORD_NOEXCEPT_SPECIFICATION(
                                AllocatorTraits::is_always_equal::value
                             && std::is_nothrow_move_assignable<HASH>::value
                             && std::is_nothrow_move_assignable<EQUAL>::value);

#if defined(BSLS_COMPILERFEATURES_SUPPORT_GENERALIZED_INITIALIZERS)
    /// Assign to this object the value resulting from first clearing this
    /// unordered multiset and then inserting each `value_type` object in
    /// the specified `values` initializer list, and return a reference
    /// providing modifiable access to this object.  This method requires
    /// that the (template parameter) type `KEY` be `copy-insertable` into
    /// this unordered multiset (see {Requirements on `KEY`}).
    unordered_multiset& operator=(std::initializer_list<KEY> values);
#endif

    /// Return an iterator providing modifiable access to the first
    /// `value_type` object (in the sequence of `value_type` objects)
    /// maintained by this unordered multiset, or the `end` iterator if this
    /// unordered multiset is empty.
    iterator begin() BSLS_KEYWORD_NOEXCEPT;

    /// Return an iterator providing modifiable access to the past-the-end
    /// element in the sequence of `value_type` objects maintained by this
    /// unordered multiset.
    iterator end() BSLS_KEYWORD_NOEXCEPT;

    /// Return a local iterator providing modifiable access to the first
    /// `value_type` object in the sequence of `value_type` objects of the
    /// bucket having the specified `index`, in the array of buckets
    /// maintained by this unordered multiset, or the `end(index)`
    /// otherwise.
    local_iterator begin(size_type index);

    /// Return a local iterator providing modifiable access to the
    /// past-the-end element in the sequence of `value_type` objects of the
    /// bucket having the specified `index`, in the array of buckets
    /// maintained by this unordered multiset.
    local_iterator end(size_type index);

    /// Remove all entries from this unordered multiset.  Note that the
    /// container is empty after this call, but allocated memory may be
    /// retained for future use.
    void clear() BSLS_KEYWORD_NOEXCEPT;

    /// Return a pair of iterators providing modifiable access to the
    /// sequence of `value_type` objects in this unordered multiset
    /// equivalent to the specified `key`, where the first iterator is
    /// positioned at the start of the sequence, and the second is
    /// positioned one past the end of the sequence.  If this unordered
    /// multiset contains no `value_type` objects equivalent to the `key`,
    /// then the two returned iterators will have the same value.  The
    /// behavior is undefined unless `key` is equivalent to the elements of
    /// at most one equivalent-key group in this unordered multiset.
    template <class LOOKUP_KEY>
    typename enable_if<
           BloombergLP::bslmf::IsTransparentPredicate<HASH, LOOKUP_KEY>::value
        && BloombergLP::bslmf::IsTransparentPredicate<EQUAL,LOOKUP_KEY>::value,
                      pair<iterator, iterator> >::type
    equal_range(const LOOKUP_KEY& key)
        {
            // Note: implemented inline due to Sun CC compilation error.
            typedef bsl::pair<iterator, iterator> ResultType;
            HashTableLink *first;
            HashTableLink *last;
            d_impl.findRange(&first, &last, key);
            return ResultType(iterator(first), iterator(last));
        }

    /// Return a pair of iterators providing modifiable access to the
    /// sequence of `value_type` objects in this unordered multiset
    /// equivalent to the specified `key`, where the first iterator is
    /// positioned at the start of the sequence, and the second is
    /// positioned one past the end of the sequence.  If this unordered
    /// multiset contains no `value_type` objects equivalent to the `key`,
    /// then the two returned iterators will have the same value.
    pair<iterator, iterator> equal_range(const key_type& key);

    /// Remove from this unordered multiset all `value_type` objects that
    /// are equivalent to the specified `key`, if they exist, and return the
    /// number of object erased; otherwise, if there are no `value_type`
    /// objects equivalent to `key`, return 0 with no other effect.  This
    /// method invalidates only iterators and references to the removed
    /// element and previously saved values of the `end()` iterator, and
    /// preserves the relative order of the elements not removed.
    size_type erase(const key_type& key);
    template <class t_KEY>
    typename enable_if<
        BloombergLP::bslmf::IsTransparentPredicate<HASH, t_KEY>::value &&
        BloombergLP::bslmf::IsTransparentPredicate<EQUAL,t_KEY>::value &&
        !is_convertible<BSLS_COMPILERFEATURES_FORWARD_REF(t_KEY),
                        iterator>::value &&
        !is_convertible<BSLS_COMPILERFEATURES_FORWARD_REF(t_KEY),
                        const_iterator>::value,
    size_type>::type erase(BSLS_COMPILERFEATURES_FORWARD_REF(t_KEY) key)
    {
        // Implemented inline due to Sun CC compilation error.
        size_type count = 0;
        // Our implementation always finds the first element
        iterator it = this->find(key);
        if (it != end()) {
            do {
                it = erase(it);
                count++;
            } while (it != end() && key_eq()(*it, key));
        }
        return count;
    }

    /// Remove from this unordered multiset the `value_type` object at the
    /// specified `position`, and return an iterator referring to the
    /// element immediately following the removed element, or to the
    /// past-the-end position if the removed element was the last element in
    /// the sequence of elements maintained by this unordered multiset.
    /// This method invalidates only iterators and references to the removed
    /// element and previously saved values of the `end()` iterator, and
    /// preserves the relative order of the elements not removed.  The
    /// behavior is undefined unless `position` refers to a `value_type`
    /// object in this unordered multiset.
    iterator erase(const_iterator position);

    /// Remove from unordered multiset the `value_type` objects starting at
    /// the specified `first` position up to, but not including the
    /// specified `last` position, and return `last`.  This method
    /// invalidates only iterators and references to the removed element and
    /// previously saved values of the `end()` iterator, and preserves the
    /// relative order of the elements not removed.  The behavior is
    /// undefined unless `first` and `last` either refer to elements in this
    /// unordered multiset or are the `end` iterator, and the `first`
    /// position is at or before the `last` position in the sequence
    /// provided by this container.
    iterator erase(const_iterator first, const_iterator last);

    /// Return an iterator providing modifiable access to the first
    /// `value_type` object in the sequence of all the value elements of
    /// this unordered multiset equivalent to the specified `key`, if such
    /// entries exist, and the past-the-end (`end`) iterator otherwise.  The
    /// behavior is undefined unless `key` is equivalent to the elements of
    /// at most one equivalent-key group in this unordered multiset.
    template <class LOOKUP_KEY>
    typename enable_if<
           BloombergLP::bslmf::IsTransparentPredicate<HASH, LOOKUP_KEY>::value
        && BloombergLP::bslmf::IsTransparentPredicate<EQUAL,LOOKUP_KEY>::value,
                      iterator>::type
    find(const LOOKUP_KEY& key)
        {
            // Note: implemented inline due to Sun CC compilation error.
            return iterator(d_impl.find(key));
        }

    /// Return an iterator providing modifiable access to the first
    /// `value_type` object in the sequence of all the value elements of
    /// this unordered multiset equivalent to the specified `key`, if such
    /// entries exist, and the past-the-end (`end`) iterator otherwise.
    iterator find(const key_type& key);

    /// Insert the specified `value` into this unordered multiset.  If one
    /// or more keys equivalent to `value` already exist in this unordered
    /// multiset, this method is guaranteed to insert `value` in a position
    /// contiguous to one of those equivalent keys.  Return an iterator
    /// referring to the newly inserted `value_type` object that is
    /// equivalent to `value`.  Note that this method requires that the
    /// (template parameter) type `KEY` be `copy-insertable` into this
    /// unordered multiset (see {Requirements on `KEY`}).
    iterator insert(const value_type& value);

    /// Insert the specified `value` into this unordered multiset.  If one
    /// or more keys equivalent to `value` already exist in this unordered
    /// multiset, this method is guaranteed to insert `value` in a position
    /// contiguous to one of those equivalent keys.  Return an iterator
    /// referring to the newly inserted `value_type` object that is
    /// equivalent to `value`.  This method requires that the (template
    /// parameter) type `KEY` be `move-insertable` into this unordered
    /// multiset (see {Requirements on `KEY`}).
    iterator insert(BloombergLP::bslmf::MovableRef<value_type> value);

    /// Insert the specified `value` into this unordered multiset (in
    /// constant time if the specified `hint` refers to an element in this
    /// container equivalent to `value`).  If one or more keys equivalent to
    /// `value` already exist in this unordered multiset, this method is
    /// guaranteed to insert `value` in a position contiguous to one of
    /// those equivalent keys.  Return an iterator referring to the newly
    /// inserted `value_type` object that is equivalent to `value`.  If
    /// `hint` does not refer to an element in this container equivalent to
    /// `value`, this operation has worst case `O[N]` and average case
    /// constant-time complexity, where `N` is the size of this unordered
    /// multiset.  This method requires that the (template parameter) type
    /// `KEY` be `copy-insertable` into this unordered multiset (see
    /// {Requirements on `KEY`}).  The behavior is undefined unless `hint`
    /// is an iterator in the range `[begin() .. end()]` (both endpoints
    /// included).
    iterator insert(const_iterator hint, const value_type& value);

    /// Insert the specified `value` into this unordered multiset (in
    /// constant time if the specified `hint` refers to an element in this
    /// container equivalent to `value`).  If one or more keys equivalent to
    /// `value` already exist in this unordered multiset, this method is
    /// guaranteed to insert `value` in a position contiguous to one of
    /// those equivalent keys.  Return an iterator referring to the newly
    /// inserted `value_type` object that is equivalent to `value`.  If
    /// `hint` does not refer to an element in this container equivalent to
    /// `value`, this operation has worst case `O[N]` and average case
    /// constant-time complexity, where `N` is the size of this unordered
    /// multiset.  This method requires that the (template parameter) type
    /// `KEY` be `move-insertable` into this unordered multiset (see
    /// {Requirements on `KEY`}).  The behavior is undefined unless `hint`
    /// is an iterator in the range `[begin() .. end()]` (both endpoints
    /// included).
    iterator insert(const_iterator                             hint,
                    BloombergLP::bslmf::MovableRef<value_type> value);

    /// Insert into this unordered multiset the value of each `value_type`
    /// object in the range starting at the specified `first` iterator and
    /// ending immediately before the specified `last` iterator.  The
    /// (template parameter) type `INPUT_ITERATOR` shall meet the
    /// requirements of an input iterator defined in the C++11 standard
    /// [24.2.3] providing access to values of a type convertible to
    /// `value_type`, and `value_type` must be `emplace-constructible` from
    /// `*i` into this unordered multiset, where `i` is a dereferenceable
    /// iterator in the range `[first .. last)` (see {Requirements on
    /// `KEY`}).  The behavior is undefined unless `first` and `last` refer
    /// to a sequence of valid values where `first` is at a position at or
    /// before `last`.
    template <class INPUT_ITERATOR>
    void insert(INPUT_ITERATOR first, INPUT_ITERATOR last);

#if defined(BSLS_COMPILERFEATURES_SUPPORT_GENERALIZED_INITIALIZERS)
    /// Insert into this unordered multiset the value of each `value_type`
    /// object in the specified `values` initializer list.  This method
    /// requires that the (template parameter) type `KEY` be
    /// `copy-insertable` into this unordered multiset (see {Requirements on
    /// `KEY`}).
    void insert(std::initializer_list<KEY> values);
#endif

    /// Insert into this unordered multiset the value of each `value_type`
    /// object in the specified `range`.  The (template parameter) type
    /// `RANGE` must meet the requirements the C++20 standard [ranges]
    /// providing access to values of a type convertible to `value_type`,
    /// and `value_type` must be `emplace-constructible` from `*i` into this
    /// unordered multiset, where `i` is a dereferenceable iterator obtained
    /// from `range` (see {Requirements on `KEY`}).  The behavior is
    /// undefined if `range` overlaps this unordered multiset.
    template <class RANGE>
    BSLSTL_UNORDEREDMULTISET_REQUIRES_CONTAINER_COMPATIBLE_RANGE(RANGE,
                                                                 value_type)
    void insert_range(BSLS_COMPILERFEATURES_FORWARD_REF(RANGE) range)
    {
        // Defined inline for Windows.

#if defined(BSLS_LIBRARYFEATURES_HAS_CPP20_CONCEPTS) \
&& defined(BSLS_LIBRARYFEATURES_HAS_CPP20_RANGES)
        if constexpr (ranges::sized_range<RANGE>) {
            insertFromRange(bsl::ranges::begin(range),
                            bsl::ranges::end  (range),
                            bsl::ranges::size (range));
        } else // ...
#endif
        {
            insertFromRange(bsl::ranges::begin(range),
                            bsl::ranges::end  (range));
        }
    }

#if !BSLS_COMPILERFEATURES_SIMULATE_CPP11_FEATURES
    /// Insert into this unordered multiset a newly created `value_type`
    /// object, constructed by forwarding `get_allocator()` (if required)
    /// and the specified (variable number of) `args` to the corresponding
    /// constructor of `value_type`.  Return an iterator referring to the
    /// newly created and inserted object in this unordered multiset whose
    /// value is equivalent to that of an object constructed from `args`.
    /// This method requires that the (template parameter) type `KEY` be
    /// `emplace-constructible` into this unordered multiset from `args`
    /// (see {Requirements on `KEY`}).
    template <class... Args>
    iterator emplace(Args&&... args);

    /// Insert into this unordered multiset a newly created `value_type`
    /// object, constructed by forwarding `get_allocator()` (if required)
    /// and the specified (variable number of) `args` to the corresponding
    /// constructor of `value_type` (in constant time if the specified
    /// `hint` refers to an element in this container equivalent to the
    /// newly created `value_type` object).  Return an iterator referring to
    /// the newly created and inserted object in this unordered multiset
    /// whose value is equivalent to that of an object constructed from
    /// `args`.  If `hint` does not refer to an element in this container
    /// equivalent to the newly created `value_type` object, this operation
    /// has worst case `O[N]` and average case constant-time complexity,
    /// where `N` is the size of this unordered multiset.  This method
    /// requires that the (template parameter) type `KEY` be
    /// `emplace-constructible` into this unordered multiset from `args`
    /// (see {Requirements on `KEY`}).  The behavior is undefined unless
    /// `hint` is an iterator in the range `[begin() .. end()]` (both
    /// endpoints included).
    template <class... Args>
    iterator emplace_hint(const_iterator hint, Args&&... args);

#endif

    /// Set the maximum load factor of this container to the specified
    /// `newLoadFactor`.
    void max_load_factor(float newLoadFactor);

    /// Change the size of the array of buckets maintained by this container
    /// to at least the specified `numBuckets`, and redistribute all the
    /// contained elements into the new sequence of buckets, according to
    /// their hash values.  Note that this operation has no effect if
    /// rehashing the elements into `numBuckets` would cause this unordered
    /// multiset to exceed its `max_load_factor`.
    void rehash(size_type numBuckets);

    /// Increase the number of buckets of this unordered multiset to a
    /// quantity such that the ratio between the specified `numElements` and
    /// this quantity does not exceed `max_load_factor`.  Note that this
    /// guarantees that, after the reserve, elements can be inserted to grow
    /// the container to `size() == numElements` without rehashing.  Also
    /// note that memory allocations may still occur when growing the
    /// container to `size() == numElements`.  Also note that this operation
    /// has no effect if `numElements <= size()`.
    void reserve(size_type numElements);

    /// Exchange the value, hasher, key-equality functor, and
    /// `max_load_factor` of this object with those of the specified `other`
    /// object; also exchange the allocator of this object with that of
    /// `other` if the (template parameter) type `ALLOCATOR` has the
    /// `propagate_on_container_swap` trait, and do not modify either
    /// allocator otherwise.  This method provides the no-throw
    /// exception-safety guarantee if and only if both the (template
    /// parameter) types `HASH` and `EQUAL` provide no-throw swap
    /// operations; if an exception is thrown, both objects are left in
    /// valid but unspecified states.  This operation guarantees `O[1]`
    /// complexity.  The behavior is undefined unless either this object was
    /// created with the same allocator as `other` or `ALLOCATOR` has the
    /// `propagate_on_container_swap` trait.
    void swap(unordered_multiset& other)
                                 BSLS_KEYWORD_NOEXCEPT_SPECIFICATION(
                                      AllocatorTraits::is_always_equal::value
                                  &&  bsl::is_nothrow_swappable<HASH>::value
                                  &&  bsl::is_nothrow_swappable<EQUAL>::value);

  private:
    // PRIVATE MANIPULATORS

    /// Insert the values between the specified `first` and `last` into an
    /// initially empty unordered multiset.
    template <class INPUT_ITERATOR, class SENTINEL>
    void constructFromRange(INPUT_ITERATOR first, SENTINEL last);

#if defined(BSLS_LIBRARYFEATURES_HAS_CPP20_CONCEPTS) \
 && defined(BSLS_LIBRARYFEATURES_HAS_CPP20_RANGES)

    /// Insert the values between the specified `first` and `last` into an
    /// initially empty unordered multiset.  The specified `numElements` is
    /// used to improve performance.  The behavior is undefined if the iterators
    /// support the calculation of distance and `numElements` is not the
    /// distance from `first` to `last`.
    template <class INPUT_ITERATOR, class SENTINEL>
    void constructFromRange(INPUT_ITERATOR first,
                            SENTINEL       last,
                            size_t         numElements);
#endif

    /// Insert the values between the specified `first` and `last` into this
    /// unordered multiset.
    template <class INPUT_ITERATOR, class SENTINEL>
    void insertFromRange(INPUT_ITERATOR first, SENTINEL last);

#if defined(BSLS_LIBRARYFEATURES_HAS_CPP20_CONCEPTS) \
 && defined(BSLS_LIBRARYFEATURES_HAS_CPP20_RANGES)

    /// Insert the values between the specified `first` and `last` into this
    /// unordered multiset.  The specified `numElements` is used to improve
    /// performance.  The behavior is undefined if the iterators support the
    /// calculation of distance and `numElements` is not the distance from
    /// `first` to `last`.
    template <class INPUT_ITERATOR, class SENTINEL>
    void insertFromRange(INPUT_ITERATOR first,
                         SENTINEL       last,
                         size_t         numElements);
#endif

  public:
    // ACCESSORS

    /// Return (a copy of) the allocator used for memory allocation by this
    /// unordered multiset.
    ALLOCATOR get_allocator() const BSLS_KEYWORD_NOEXCEPT;

    const_iterator  begin() const BSLS_KEYWORD_NOEXCEPT;

    /// Return an iterator providing non-modifiable access to the first
    /// `value_type` object in the sequence of `value_type` objects
    /// maintained by this unordered multiset, or the `end` iterator if this
    /// unordered multiset is empty.
    const_iterator cbegin() const BSLS_KEYWORD_NOEXCEPT;

    const_iterator  end() const BSLS_KEYWORD_NOEXCEPT;

    /// Return an iterator providing non-modifiable access to the
    /// past-the-end element in the sequence of `value_type` objects
    /// maintained by this unordered multiset.
    const_iterator cend() const BSLS_KEYWORD_NOEXCEPT;

    /// Return `true` if this unordered multiset contains an element whose
    /// key is equivalent to the specified `key`.
    bool contains(const key_type &key) const;

    /// Return `true` if this unordered multiset contains an element whose
    /// key is equivalent to the specified `key`.
    template <class LOOKUP_KEY>
    typename enable_if<
        BloombergLP::bslmf::IsTransparentPredicate<HASH, LOOKUP_KEY>::value &&
            BloombergLP::bslmf::IsTransparentPredicate<EQUAL,
                                                       LOOKUP_KEY>::value,
        bool>::type
    contains(const LOOKUP_KEY& key) const
    {
        // Note: implemented inline due to Sun CC compilation error
        return find(key) != end();
    }

    /// Return `true` if this unordered multiset contains no elements, and
    /// `false` otherwise.
    bool empty() const BSLS_KEYWORD_NOEXCEPT;

    /// Return the number of elements in this unordered multiset.
    size_type size() const BSLS_KEYWORD_NOEXCEPT;

    /// Return a theoretical upper bound on the largest number of elements
    /// that this unordered multiset could possibly hold.  Note that there
    /// is no guarantee that the unordered multiset can successfully grow to
    /// the returned size, or even close to that size without running out of
    /// resources.
    size_type max_size() const BSLS_KEYWORD_NOEXCEPT;

    /// Return (a copy of) the key-equality binary functor that returns
    /// `true` if the value of two `key_type` objects are equivalent, and
    /// `false` otherwise.
    EQUAL key_eq() const;

    /// Return (a copy of) the hash unary functor used by this unordered
    /// multiset to generate a hash value (of type `size_t`) for a
    /// `key_type` object.
    HASH hash_function() const;

    /// Return an iterator providing non-modifiable access to the first
    /// `value_type` object in the sequence of all the value elements of
    /// this unordered multiset equivalent to the specified `key`, if such
    /// entries exist, and the past-the-end (`end`) iterator otherwise.  The
    /// behavior is undefined unless `key` is equivalent to the elements of
    /// at most one equivalent-key group in this unordered multiset.
    template <class LOOKUP_KEY>
    typename enable_if<
           BloombergLP::bslmf::IsTransparentPredicate<HASH, LOOKUP_KEY>::value
        && BloombergLP::bslmf::IsTransparentPredicate<EQUAL,LOOKUP_KEY>::value,
                      const_iterator>::type
    find(const LOOKUP_KEY& key) const
        {
            // Note: implemented inline due to Sun CC compilation error.
            return const_iterator(d_impl.find(key));
        }

    /// Return an iterator providing non-modifiable access to the first
    /// `value_type` object in the sequence of all the value elements of
    /// this unordered multiset equivalent to the specified `key`, if such
    /// entries exist, and the past-the-end (`end`) iterator otherwise.
    const_iterator find(const key_type& key) const;

    /// Return the number of `value_type` objects within this unordered
    /// multiset that are equivalent to the specified `key`.  The behavior
    /// is undefined unless `key` is equivalent to the elements of at most
    /// one equivalent-key group in this unordered multiset.
    template <class LOOKUP_KEY>
    typename enable_if<
           BloombergLP::bslmf::IsTransparentPredicate<HASH, LOOKUP_KEY>::value
        && BloombergLP::bslmf::IsTransparentPredicate<EQUAL,LOOKUP_KEY>::value,
                      size_type>::type
    count(const LOOKUP_KEY& key) const
        {
            // Note: implemented inline due to Sun CC compilation error.
            typedef ::BloombergLP::bslalg::BidirectionalNode<value_type> BNode;

            size_type result = 0;
            for (HashTableLink *cursor = d_impl.find(key);
                 cursor;
                 ++result, cursor = cursor->nextLink()) {

                BNode *cursorNode = static_cast<BNode *>(cursor);
                if (!this->key_eq()(
                         key,
                         ListConfiguration::extractKey(cursorNode->value()))) {
                    break;
                }
            }
            return result;
        }

    /// Return the number of `value_type` objects within this unordered
    /// multiset that are equivalent to the specified `key`.
    size_type count(const key_type& key) const;

    /// Return a pair of iterators providing non-modifiable access to the
    /// sequence of `value_type` objects in this unordered multiset
    /// equivalent to the specified `key`, where the first iterator is
    /// positioned at the start of the sequence, and the second is
    /// positioned one past the end of the sequence.  If this unordered
    /// multiset contains no `value_type` objects equivalent to the `key`,
    /// then the two returned iterators will have the same value.  The
    /// behavior is undefined unless `key` is equivalent to the elements of
    /// at most one equivalent-key group in this unordered multiset.
    template <class LOOKUP_KEY>
    typename enable_if<
           BloombergLP::bslmf::IsTransparentPredicate<HASH, LOOKUP_KEY>::value
        && BloombergLP::bslmf::IsTransparentPredicate<EQUAL,LOOKUP_KEY>::value,
                      pair<const_iterator, const_iterator> >::type
    equal_range(const LOOKUP_KEY& key) const
        {
            // Note: implemented inline due to Sun CC compilation error.
            typedef bsl::pair<const_iterator, const_iterator> ResultType;
            HashTableLink *first;
            HashTableLink *last;
            d_impl.findRange(&first, &last, key);
            return ResultType(const_iterator(first), const_iterator(last));
        }

    /// Return a pair of iterators providing non-modifiable access to the
    /// sequence of `value_type` objects in this unordered multiset
    /// equivalent to the specified `key`, where the first iterator is
    /// positioned at the start of the sequence, and the second is
    /// positioned one past the end of the sequence.  If this unordered
    /// multiset contains no `value_type` objects equivalent to the `key`,
    /// then the two returned iterators will have the same value.
    pair<const_iterator, const_iterator> equal_range(
                                                    const key_type& key) const;

    const_local_iterator begin(size_type index) const;

    /// Return a local iterator providing non-modifiable access to the first
    /// `value_type` object (in the sequence of `value_type` objects) of the
    /// bucket having the specified `index` in the array of buckets
    /// maintained by this unordered multiset, or the `end(index)`
    /// otherwise.  The behavior is undefined unless 'index <
    /// bucket_count()'.
    const_local_iterator cbegin(size_type index) const;

    const_local_iterator end(size_type index) const;

    /// Return a local iterator providing non-modifiable access to the
    /// past-the-end element (in the sequence of `value_type` objects) of
    /// the bucket having the specified `index` in the array of buckets
    /// maintained by this unordered multiset.  The behavior is undefined
    /// unless `index < bucket_count()`.
    const_local_iterator cend(size_type index) const;

    /// Return the index of the bucket, in the array of buckets of this
    /// container, where a value equivalent to the specified `key` would be
    /// inserted.
    size_type bucket(const key_type& key) const;

    /// Return the index of the bucket, in the array of buckets of this
    /// container, where a value equivalent to the specified `key` would be
    /// inserted.
    ///
    /// Note: implemented inline due to Sun CC compilation error.
    template <class LOOKUP_KEY>
    typename enable_if<
           BloombergLP::bslmf::IsTransparentPredicate<HASH, LOOKUP_KEY>::value
        && BloombergLP::bslmf::IsTransparentPredicate<EQUAL,LOOKUP_KEY>::value,
                      size_type>::type
    bucket(const LOOKUP_KEY& key) const
    {
        return d_impl.bucketIndexForKey(key);
    }

    /// Return the number of buckets in the array of buckets maintained by
    /// this unordered multiset.
    size_type bucket_count() const BSLS_KEYWORD_NOEXCEPT;

    /// Return a theoretical upper bound on the largest number of buckets
    /// that this container could possibly manage.  Note that there is no
    /// guarantee that the unordered multiset can successfully grow to the
    /// returned size, or even close to that size without running out of
    /// resources.
    size_type max_bucket_count() const BSLS_KEYWORD_NOEXCEPT;

    /// Return the number of elements contained in the bucket at the
    /// specified `index` in the array of buckets maintained by this
    /// container.  The behavior is undefined unless 'index <
    /// bucket_count()'.
    size_type bucket_size(size_type index) const;

    /// Return the current ratio between the `size` of this container and
    /// the number of buckets.  The `load_factor` is a measure of how full
    /// the container is, and a higher load factor leads to an increased
    /// number of collisions, thus resulting in a loss performance.
    float load_factor() const BSLS_KEYWORD_NOEXCEPT;

    /// Return the maximum load factor allowed for this container.  If an
    /// insert operation would cause `load_factor` to exceed the
    /// `max_load_factor`, that same insert operation will increase the
    /// number of buckets and rehash the elements of the container into
    /// those buckets the (see rehash).
    float max_load_factor() const BSLS_KEYWORD_NOEXCEPT;

};

#ifdef BSLS_COMPILERFEATURES_SUPPORT_CTAD
// CLASS TEMPLATE DEDUCTION GUIDES

/// Deduce the template parameter `KEY` from the `value_type` of the
/// iterators supplied to the constructor of `unordered_multiset`.  Deduce
/// the template parameters `HASH`, `EQUAL` and `ALLOCATOR` from the other
/// parameters passed to the constructor.  This deduction guide does not
/// participate unless: (1) the supplied `HASH` is invocable with a `KEY`,
/// (2) the supplied `EQUAL` is invocable with two `KEY`s, and (3) the
/// supplied allocator meets the requirements of a standard allocator.
template <
    class INPUT_ITERATOR,
    class KEY = BloombergLP::bslstl::IteratorUtil::IterVal_t<INPUT_ITERATOR>,
    class HASH = bsl::hash<KEY>,
    class EQUAL = bsl::equal_to<KEY>,
    class ALLOCATOR = bsl::allocator<KEY>,
    class = bsl::enable_if_t<std::is_invocable_v<HASH, const KEY&>>,
    class = bsl::enable_if_t<
                           std::is_invocable_v<EQUAL, const KEY&, const KEY&>>,
    class = bsl::enable_if_t< bsl::IsStdAllocator_v<ALLOCATOR>>
    >
unordered_multiset(INPUT_ITERATOR,
                   INPUT_ITERATOR,
                   typename bsl::allocator_traits<ALLOCATOR>::size_type = 0,
                   HASH      = HASH(),
                   EQUAL     = EQUAL(),
                   ALLOCATOR = ALLOCATOR())
-> unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>;

/// Deduce the template parameter `KEY` from the `value_type` of the
/// iterators supplied to the constructor of `unordered_multiset`.  Deduce
/// the template parameters `HASH` and `EQUAL` from the other parameters
/// passed to the constructor.  This deduction guide does not participate
/// unless the supplied allocator is convertible to `bsl::allocator<KEY>`.
template <
    class INPUT_ITERATOR,
    class KEY = BloombergLP::bslstl::IteratorUtil::IterVal_t<INPUT_ITERATOR>,
    class HASH,
    class EQUAL,
    class ALLOC,
    class DEFAULT_ALLOCATOR = bsl::allocator<KEY>,
    class = bsl::enable_if_t<bsl::is_convertible_v<ALLOC *, DEFAULT_ALLOCATOR>>
    >
unordered_multiset(
    INPUT_ITERATOR,
    INPUT_ITERATOR,
    typename bsl::allocator_traits<DEFAULT_ALLOCATOR>::size_type,
    HASH,
    EQUAL,
    ALLOC *)
-> unordered_multiset<KEY, HASH, EQUAL>;

/// Deduce the template parameter `KEY` from the `value_type` of the
/// iterators supplied to the constructor of `unordered_multiset`.  Deduce
/// the template parameters `HASH` and `ALLOCATOR` from the other parameters
/// passed to the constructor.  This deduction guide does not participate
/// unless the supplied `HASH` is invocable with a `KEY`, and the supplied
/// allocator meets the requirements of a standard allocator.
template <
    class INPUT_ITERATOR,
    class KEY = BloombergLP::bslstl::IteratorUtil::IterVal_t<INPUT_ITERATOR>,
    class HASH,
    class ALLOCATOR,
    class = bsl::enable_if_t<std::is_invocable_v<HASH, const KEY &>>,
    class = bsl::enable_if_t< bsl::IsStdAllocator_v<ALLOCATOR>>
    >
unordered_multiset(INPUT_ITERATOR,
                   INPUT_ITERATOR,
                   typename bsl::allocator_traits<ALLOCATOR>::size_type,
                   HASH,
                   ALLOCATOR)
-> unordered_multiset<KEY, HASH, bsl::equal_to<KEY>, ALLOCATOR>;

/// Deduce the template parameter `KEY` from the `value_type` of the
/// iterators supplied to the constructor of `unordered_multiset`.  Deduce
/// the template parameter `HASH` from the other parameters passed to the
/// constructor.  This deduction guide does not participate unless the
/// supplied allocator is convertible to `bsl::allocator<KEY>`.
template <
    class INPUT_ITERATOR,
    class KEY = BloombergLP::bslstl::IteratorUtil::IterVal_t<INPUT_ITERATOR>,
    class HASH,
    class ALLOC,
    class DEFAULT_ALLOCATOR = bsl::allocator<KEY>,
    class = bsl::enable_if_t<bsl::is_convertible_v<ALLOC *, DEFAULT_ALLOCATOR>>
    >
unordered_multiset(
    INPUT_ITERATOR,
    INPUT_ITERATOR,
    typename bsl::allocator_traits<DEFAULT_ALLOCATOR>::size_type,
    HASH,
    ALLOC *)
-> unordered_multiset<KEY, HASH>;

/// Deduce the template parameter `KEY` from the `value_type` of the
/// iterators supplied to the constructor of `unordered_multiset`.   This
/// deduction guide does not participate unless the supplied allocator meets
/// the requirements of a standard allocator.
template <
    class INPUT_ITERATOR,
    class ALLOCATOR,
    class KEY = BloombergLP::bslstl::IteratorUtil::IterVal_t<INPUT_ITERATOR>,
    class = bsl::enable_if_t<bsl::IsStdAllocator_v<ALLOCATOR>>
    >
unordered_multiset(INPUT_ITERATOR,
                   INPUT_ITERATOR,
                   typename bsl::allocator_traits<ALLOCATOR>::size_type,
                   ALLOCATOR)
-> unordered_multiset<KEY, bsl::hash<KEY>, bsl::equal_to<KEY>, ALLOCATOR>;

/// Deduce the template parameter `KEY` from the `value_type` of the
/// iterators supplied to the constructor of `unordered_multiset`.  This
/// deduction guide does not participate unless the supplied allocator is
/// convertible to `bsl::allocator<KEY>`.
template <
    class INPUT_ITERATOR,
    class KEY = BloombergLP::bslstl::IteratorUtil::IterVal_t<INPUT_ITERATOR>,
    class ALLOC,
    class DEFAULT_ALLOCATOR = bsl::allocator<KEY>,
    class = bsl::enable_if_t<bsl::is_convertible_v<ALLOC *, DEFAULT_ALLOCATOR>>
    >
unordered_multiset(
    INPUT_ITERATOR,
    INPUT_ITERATOR,
    typename bsl::allocator_traits<DEFAULT_ALLOCATOR>::size_type,
    ALLOC *)
-> unordered_multiset<KEY>;

/// Deduce the template parameter `KEY` from the `value_type` of the
/// iterators supplied to the constructor of `unordered_multiset`.   This
/// deduction guide does not participate unless the supplied allocator meets
/// the requirements of a standard allocator.
template <
    class INPUT_ITERATOR,
    class ALLOCATOR,
    class KEY = BloombergLP::bslstl::IteratorUtil::IterVal_t<INPUT_ITERATOR>,
    class = bsl::enable_if_t<bsl::IsStdAllocator_v<ALLOCATOR>>
    >
unordered_multiset(INPUT_ITERATOR, INPUT_ITERATOR, ALLOCATOR)
-> unordered_multiset<KEY, bsl::hash<KEY>, bsl::equal_to<KEY>, ALLOCATOR>;

/// Deduce the template parameter `KEY` from the `value_type` of the
/// iterators supplied to the constructor of `unordered_multiset`.  This
/// deduction guide does not participate unless the supplied allocator is
/// convertible to `bsl::allocator<KEY>`.
template <
    class INPUT_ITERATOR,
    class KEY = BloombergLP::bslstl::IteratorUtil::IterVal_t<INPUT_ITERATOR>,
    class ALLOC,
    class DEFAULT_ALLOCATOR = bsl::allocator<KEY>,
    class = bsl::enable_if_t<bsl::is_convertible_v<ALLOC *, DEFAULT_ALLOCATOR>>
    >
unordered_multiset(INPUT_ITERATOR, INPUT_ITERATOR, ALLOC *)
-> unordered_multiset<KEY>;

/// Deduce the template parameter `KEY` from the `value_type` of the
/// initializer_list supplied to the constructor of `unordered_multiset`.
/// Deduce the template parameters `HASH`, EQUAL and `ALLOCATOR` from the
/// other parameters passed to the constructor.  This deduction guide does
/// not participate unless: (1) the supplied `HASH` is invocable with a
/// `KEY`, (2) the supplied `EQUAL` is invocable with two `KEY`s, and (3)
/// the supplied allocator meets the requirements of a standard allocator.
template <
    class KEY,
    class HASH = bsl::hash<KEY>,
    class EQUAL = bsl::equal_to<KEY>,
    class ALLOCATOR = bsl::allocator<KEY>,
    class = bsl::enable_if_t<std::is_invocable_v<HASH, const KEY&>>,
    class = bsl::enable_if_t<
                           std::is_invocable_v<EQUAL, const KEY&, const KEY&>>,
    class = bsl::enable_if_t< bsl::IsStdAllocator_v<ALLOCATOR>>
    >
unordered_multiset(std::initializer_list<KEY>,
                   typename bsl::allocator_traits<ALLOCATOR>::size_type = 0,
                   HASH      = HASH(),
                   EQUAL     = EQUAL(),
                   ALLOCATOR = ALLOCATOR())
-> unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>;

/// Deduce the template parameter `KEY` from the `value_type` of the
/// initializer_list supplied to the constructor of `unordered_multiset`.
/// Deduce the template parameters `HASH` and `EQUAL` from the other
/// parameters passed to the constructor.  This deduction guide does not
/// participate unless the supplied allocator is convertible to
/// `bsl::allocator<KEY>`.
template <
    class KEY,
    class HASH,
    class EQUAL,
    class ALLOC,
    class DEFAULT_ALLOCATOR = bsl::allocator<KEY>,
    class = bsl::enable_if_t<bsl::is_convertible_v<ALLOC *, DEFAULT_ALLOCATOR>>
    >
unordered_multiset(
                  std::initializer_list<KEY>,
                  typename bsl::allocator_traits<DEFAULT_ALLOCATOR>::size_type,
                  HASH,
                  EQUAL,
                  ALLOC *)
-> unordered_multiset<KEY, HASH, EQUAL>;

/// Deduce the template parameter `KEY` from the `value_type` of the
/// initializer_list supplied to the constructor of `unordered_multiset`.
/// Deduce the template parameters `HASH` and `ALLOCATOR` from the other
/// parameters passed to the constructor.  This deduction guide does not
/// participate unless the supplied `HASH` is invocable with a `KEY`, and
/// the supplied allocator meets the requirements of a standard allocator.
template <
    class KEY,
    class HASH,
    class ALLOCATOR,
    class = bsl::enable_if_t<std::is_invocable_v<HASH, const KEY &>>,
    class = bsl::enable_if_t< bsl::IsStdAllocator_v<ALLOCATOR>>
    >
unordered_multiset(std::initializer_list<KEY>,
                   typename bsl::allocator_traits<ALLOCATOR>::size_type,
                   HASH,
                   ALLOCATOR)
-> unordered_multiset<KEY, HASH, bsl::equal_to<KEY>, ALLOCATOR>;

/// Deduce the template parameter `KEY` from the `value_type` of the
/// initializer_list supplied to the constructor of `unordered_multiset`.
/// Deduce the template parameter `HASH` from the other parameters passed to
/// the constructor.  This deduction guide does not participate unless the
/// supplied allocator is convertible to `bsl::allocator<KEY>`.
template <
    class KEY,
    class HASH,
    class ALLOC,
    class DEFAULT_ALLOCATOR = bsl::allocator<KEY>,
    class = bsl::enable_if_t<bsl::is_convertible_v<ALLOC *, DEFAULT_ALLOCATOR>>
    >
unordered_multiset(
                  std::initializer_list<KEY>,
                  typename bsl::allocator_traits<DEFAULT_ALLOCATOR>::size_type,
                  HASH,
                  ALLOC *)
-> unordered_multiset<KEY, HASH>;

/// Deduce the template parameter `KEY` from the `value_type` of the
/// initializer_list supplied to the constructor of `unordered_multiset`.
/// This deduction guide does not participate unless the supplied allocator
/// meets the requirements of a standard allocator.
template <
    class KEY,
    class ALLOCATOR,
    class = bsl::enable_if_t<bsl::IsStdAllocator_v<ALLOCATOR>>
    >
unordered_multiset(std::initializer_list<KEY>,
                   typename bsl::allocator_traits<ALLOCATOR>::size_type,
                   ALLOCATOR)
-> unordered_multiset<KEY, bsl::hash<KEY>, bsl::equal_to<KEY>, ALLOCATOR>;

/// Deduce the template parameter `KEY` from the `value_type` of the
/// initializer_list supplied to the constructor of `unordered_multiset`.
/// This deduction guide does not participate unless the supplied allocator
/// is convertible to `bsl::allocator<KEY>`.
template <
    class KEY,
    class ALLOC,
    class DEFAULT_ALLOCATOR = bsl::allocator<KEY>,
    class = bsl::enable_if_t<bsl::is_convertible_v<ALLOC *, DEFAULT_ALLOCATOR>>
    >
unordered_multiset(
                  std::initializer_list<KEY>,
                  typename bsl::allocator_traits<DEFAULT_ALLOCATOR>::size_type,
                  ALLOC *)
-> unordered_multiset<KEY>;

/// Deduce the template parameter `KEY` from the `value_type` of the
/// initializer_list supplied to the constructor of `unordered_multiset`.
/// This deduction guide does not participate unless the supplied allocator
/// meets the requirements of a standard allocator.
template <
    class KEY,
    class ALLOCATOR,
    class = bsl::enable_if_t<bsl::IsStdAllocator_v<ALLOCATOR>>
    >
unordered_multiset(std::initializer_list<KEY>, ALLOCATOR)
-> unordered_multiset<KEY, bsl::hash<KEY>, bsl::equal_to<KEY>, ALLOCATOR>;

/// Deduce the template parameter `KEY` from the `value_type` of the
/// initializer_list supplied to the constructor of `unordered_multiset`.
/// This deduction guide does not participate unless the supplied allocator
/// is convertible to `bsl::allocator<KEY>`.
template <
    class KEY,
    class ALLOC,
    class DEFAULT_ALLOCATOR = bsl::allocator<KEY>,
    class = bsl::enable_if_t<bsl::is_convertible_v<ALLOC *, DEFAULT_ALLOCATOR>>
    >
unordered_multiset(std::initializer_list<KEY>, ALLOC *)
-> unordered_multiset<KEY>;
#endif

// FREE OPERATORS

/// Return `true` if the specified `lhs` and `rhs` objects have the same
/// value, and `false` otherwise.  Two `unordered_multiset` objects have the
/// same value if they have the same number of value elements, and for each
/// value-element that is contained in `lhs` there is a value-element
/// contained in `rhs` having the same value, and vice-versa.  Note that
/// this method requires that the (template parameter) type `KEY` be
/// `equality-comparable` (see {Requirements on `KEY`}).
template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
bool operator==(const unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>& lhs,
                const unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>& rhs);

#ifndef BSLS_COMPILERFEATURES_SUPPORT_THREE_WAY_COMPARISON
template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
bool operator!=(const unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>& lhs,
                const unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>& rhs);
    // Return 'true' if the specified 'lhs' and 'rhs' objects do not have the
    // same value, and 'false' otherwise.  Two 'unordered_multiset' objects do
    // not have the same value if they do not have the same number of
    // value elements, or that for some value-element contained in 'lhs' there
    // is not a value-element in 'rhs' having the same value, and vice-versa.
    // Note that this method requires that the (template parameter) type 'KEY'
    // and be 'equality-comparable' (see {Requirements on 'KEY'}).
#endif

// FREE FUNCTIONS

/// Erase all the elements in the specified unordered_multiset `ms` that
/// satisfy the specified predicate `predicate`.  Return the number of
/// elements erased.
template <class KEY, class HASH, class EQUAL, class ALLOCATOR, class PREDICATE>
typename unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::size_type
erase_if(unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>& ms,
         PREDICATE                                        predicate);

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
void swap(unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>& a,
          unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>& b)
                                BSLS_KEYWORD_NOEXCEPT_SPECIFICATION(
                                    BSLS_KEYWORD_NOEXCEPT_OPERATOR(a.swap(b)));
    // Exchange the value, hasher, key-equality functor, and 'max_load_factor'
    // of the specified 'a' object with those of the specified 'b' object; also
    // exchange the allocator of 'a' with that of 'b' if the (template
    // parameter) type 'ALLOCATOR' has the 'propagate_on_container_swap' trait,
    // and do not modify either allocator otherwise.  This function provides
    // the no-throw exception-safety guarantee if and only if both the
    // (template parameter) types 'HASH' and 'EQUAL' provide no-throw swap
    // operations; if an exception is thrown, both objects are left in valid
    // but unspecified states.  This operation guarantees 'O[1]' complexity.
    // The behavior is undefined unless either 'a' was created with the same
    // allocator as 'b' or 'ALLOCATOR' has the 'propagate_on_container_swap'
    // trait.

// ============================================================================
//                  TEMPLATE AND INLINE FUNCTION DEFINITIONS
// ============================================================================

                        //-------------------------
                        // class unordered_multiset
                        //-------------------------

// PRIVATE MANIPULATORS
template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
template <class INPUT_ITERATOR, class SENTINEL>
inline
void unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::constructFromRange(
                                                          INPUT_ITERATOR first,
                                                          SENTINEL       last)
{
    ///Implementation Notes
    ///--------------------
    // If we can calculate the number of elements, reserve space for them
    // upfront to reduce rehashing.  The calculation is done once since
    // `IteratorUtil::insertDistance` may be expensive for non-random-access
    // iterators.

    if (first == last) {
        return;                                                       // RETURN
    }

    if BSLS_KEYWORD_CONSTEXPR_CPP17 (
        BloombergLP::bslstl::IteratorUtil::
                       canCalculateInsertDistance<INPUT_ITERATOR,SENTINEL>()) {
        this->reserve(
               BloombergLP::bslstl::IteratorUtil::insertDistance(first, last));
    }

    while (first != last) {
        d_impl.insert(*first);
        ++first;
    }
}

#if defined(BSLS_LIBRARYFEATURES_HAS_CPP20_CONCEPTS) \
 && defined(BSLS_LIBRARYFEATURES_HAS_CPP20_RANGES)

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
template <class INPUT_ITERATOR, class SENTINEL>
inline
void unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::constructFromRange(
                                                    INPUT_ITERATOR first,
                                                    SENTINEL       last,
                                                    size_t         numElements)
{
    BSLS_ASSERT_SAFE((
        !BloombergLP::bslstl::IteratorUtil
                       ::canCalculateInsertDistance<INPUT_ITERATOR, SENTINEL>()
        || numElements == static_cast<size_t>(
             BloombergLP::bslstl::IteratorUtil::insertDistance(first, last))));

    if (0 < numElements) {
        this->reserve(numElements);
    }

    while (first != last) {
        d_impl.insert(*first);
        ++first;
    }
}

#endif

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
template <class INPUT_ITERATOR, class SENTINEL>
inline
void unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::insertFromRange(
                                                          INPUT_ITERATOR first,
                                                          SENTINEL       last)
{
    ///Implementation Notes
    ///--------------------
    // If we can calculate the number of elements, reserve space for them
    // upfront to reduce rehashing.  The calculation is done once since
    // `IteratorUtil::insertDistance` may be expensive for non-random-access
    /// iterators.

    if BSLS_KEYWORD_CONSTEXPR_CPP17 (BloombergLP::bslstl::IteratorUtil
                    ::canCalculateInsertDistance<INPUT_ITERATOR, SENTINEL>()) {
        this->reserve(this->size()
             + BloombergLP::bslstl::IteratorUtil::insertDistance(first, last));
    }

    while (first != last) {
        d_impl.insert(*first);
        ++first;
    }
}

#if defined(BSLS_LIBRARYFEATURES_HAS_CPP20_CONCEPTS) \
 && defined(BSLS_LIBRARYFEATURES_HAS_CPP20_RANGES)

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
template <class INPUT_ITERATOR, class SENTINEL>
inline
void unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::insertFromRange(
                                                    INPUT_ITERATOR first,
                                                    SENTINEL       last,
                                                    size_t         numElements)
{
    BSLS_ASSERT_SAFE((
        !BloombergLP::bslstl::IteratorUtil
                       ::canCalculateInsertDistance<INPUT_ITERATOR, SENTINEL>()
        || numElements == static_cast<size_t>(
             BloombergLP::bslstl::IteratorUtil::insertDistance(first, last))));

    this->reserve(this->size() + numElements);

    while (first != last) {
        d_impl.insert(*first);
        ++first;
    }
}

#endif

// CREATORS
template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::unordered_multiset()
: d_impl(HASH(), EQUAL(), 0, 1.0f, ALLOCATOR())
{
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::unordered_multiset(
                                            size_type        initialNumBuckets,
                                            const HASH&      hashFunction,
                                            const EQUAL&     keyEqual,
                                            const ALLOCATOR& basicAllocator)
: d_impl(hashFunction, keyEqual, initialNumBuckets, 1.0f, basicAllocator)
{
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::unordered_multiset(
                                            size_type        initialNumBuckets,
                                            const HASH&      hashFunction,
                                            const ALLOCATOR& basicAllocator)
: d_impl(hashFunction, EQUAL(), initialNumBuckets, 1.0f, basicAllocator)
{
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::unordered_multiset(
                                            size_type        initialNumBuckets,
                                            const ALLOCATOR& basicAllocator)
: d_impl(HASH(), EQUAL(), initialNumBuckets, 1.0f, basicAllocator)
{
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::unordered_multiset(
                                               const ALLOCATOR& basicAllocator)
: d_impl(basicAllocator)
{
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::unordered_multiset(
                                            const unordered_multiset& original)
: d_impl(original.d_impl,
         AllocatorTraits::select_on_container_copy_construction(
                                                     original.get_allocator()))
{
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::unordered_multiset(
                   BloombergLP::bslmf::MovableRef<unordered_multiset> original)
: d_impl(MoveUtil::move(MoveUtil::access(original).d_impl))
{
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::unordered_multiset(
                 const unordered_multiset&                      original,
                 const typename type_identity<ALLOCATOR>::type& basicAllocator)
: d_impl(original.d_impl, basicAllocator)
{
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::unordered_multiset(
             BloombergLP::bslmf::MovableRef<unordered_multiset> original,
             const typename type_identity<ALLOCATOR>::type&     basicAllocator)
: d_impl(MoveUtil::move(MoveUtil::access(original).d_impl), basicAllocator)
{
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
template <class INPUT_ITERATOR>
inline
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::unordered_multiset(
                                            INPUT_ITERATOR   first,
                                            INPUT_ITERATOR   last,
                                            size_type        initialNumBuckets,
                                            const HASH&      hashFunction,
                                            const EQUAL&     keyEqual,
                                            const ALLOCATOR& basicAllocator)
: d_impl(hashFunction, keyEqual, initialNumBuckets, 1.0f, basicAllocator)
{
    constructFromRange(first, last);
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
template <class INPUT_ITERATOR>
inline
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::unordered_multiset(
                                            INPUT_ITERATOR   first,
                                            INPUT_ITERATOR   last,
                                            size_type        initialNumBuckets,
                                            const HASH&      hashFunction,
                                            const ALLOCATOR& basicAllocator)
: d_impl(hashFunction, EQUAL(), initialNumBuckets, 1.0f, basicAllocator)
{
    this->insert(first, last);
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
template <class INPUT_ITERATOR>
inline
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::unordered_multiset(
                                            INPUT_ITERATOR   first,
                                            INPUT_ITERATOR   last,
                                            size_type        initialNumBuckets,
                                            const ALLOCATOR& basicAllocator)
: d_impl(HASH(), EQUAL(), initialNumBuckets, 1.0f, basicAllocator)
{
    this->insert(first, last);
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
template <class INPUT_ITERATOR>
inline
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::unordered_multiset(
                                               INPUT_ITERATOR   first,
                                               INPUT_ITERATOR   last,
                                               const ALLOCATOR& basicAllocator)
: d_impl(HASH(), EQUAL(), 0, 1.0f, basicAllocator)
{
    this->insert(first, last);
}

#if defined(BSLS_COMPILERFEATURES_SUPPORT_GENERALIZED_INITIALIZERS)
template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
#ifdef BSLS_COMPILERFEATURES_SUPPORT_CTAD
template <class, class, class>
#endif
inline
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::unordered_multiset(
                                  std::initializer_list<KEY> values,
                                  size_type                  initialNumBuckets,
                                  const hasher&              hashFunction,
                                  const key_equal&           keyEqual,
                                  const ALLOCATOR&           basicAllocator)
: unordered_multiset(values.begin(),
                     values.end(),
                     initialNumBuckets,
                     hashFunction,
                     keyEqual,
                     basicAllocator)
{
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
#ifdef BSLS_COMPILERFEATURES_SUPPORT_CTAD
template <class, class>
#endif
inline
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::unordered_multiset(
                                  std::initializer_list<KEY> values,
                                  size_type                  initialNumBuckets,
                                  const HASH&                hashFunction,
                                  const ALLOCATOR&           basicAllocator)
: unordered_multiset(values.begin(),
                     values.end(),
                     initialNumBuckets,
                     hashFunction,
                     EQUAL(),
                     basicAllocator)
{
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
#ifdef BSLS_COMPILERFEATURES_SUPPORT_CTAD
template <class>
#endif
inline
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::unordered_multiset(
                                  std::initializer_list<KEY> values,
                                  size_type                  initialNumBuckets,
                                  const ALLOCATOR&           basicAllocator)
: unordered_multiset(values.begin(),
                     values.end(),
                     initialNumBuckets,
                     HASH(),
                     EQUAL(),
                     basicAllocator)
{
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
#ifdef BSLS_COMPILERFEATURES_SUPPORT_CTAD
template <class>
#endif
inline
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::unordered_multiset(
                                     std::initializer_list<KEY> values,
                                     const ALLOCATOR&           basicAllocator)
: unordered_multiset(values.begin(),
                     values.end(),
                     0,
                     HASH(),
                     EQUAL(),
                     basicAllocator)
{
}
#endif  // defined(BSLS_COMPILERFEATURES_SUPPORT_GENERALIZED_INITIALIZERS)

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::~unordered_multiset()
{
    // All memory management is handled by the base 'd_impl' member.
}

// MANIPULATORS
template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>&
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::operator=(
                                                 const unordered_multiset& rhs)
{
    // Note that we have delegated responsibility for correct handling of
    // allocator propagation to the 'HashTable' implementation.

    d_impl = rhs.d_impl;

    return *this;
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>&
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::operator=(
                        BloombergLP::bslmf::MovableRef<unordered_multiset> rhs)
                             BSLS_KEYWORD_NOEXCEPT_SPECIFICATION(
                                 AllocatorTraits::is_always_equal::value
                              && std::is_nothrow_move_assignable<HASH>::value
                              && std::is_nothrow_move_assignable<EQUAL>::value)
{
    // Note that we have delegated responsibility for correct handling of
    // allocator propagation to the 'HashTable' implementation.

    unordered_multiset& lvalue = rhs;

    d_impl = MoveUtil::move(lvalue.d_impl);

    return *this;
}

#if defined(BSLS_COMPILERFEATURES_SUPPORT_GENERALIZED_INITIALIZERS)
template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>&
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::operator=(
                                             std::initializer_list<KEY> values)
{
    unordered_multiset tmp(values, d_impl.allocator());

    d_impl.swap(tmp.d_impl);

    return *this;
}
#endif

#if !BSLS_COMPILERFEATURES_SIMULATE_CPP11_FEATURES
template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
template <class... Args>
inline
typename unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::iterator
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::emplace(Args&&... arguments)
{
    return iterator(d_impl.emplace(
                           BSLS_COMPILERFEATURES_FORWARD(Args, arguments)...));

}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
template <class... Args>
inline
typename unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::iterator
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::emplace_hint(
                                                      const_iterator hint,
                                                      Args&&...      arguments)
{
    return iterator(d_impl.emplaceWithHint(hint.node(),
                           BSLS_COMPILERFEATURES_FORWARD(Args, arguments)...));
}
#endif

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
typename unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::iterator
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::begin() BSLS_KEYWORD_NOEXCEPT
{
    return iterator(d_impl.elementListRoot());
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
typename unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::iterator
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::end() BSLS_KEYWORD_NOEXCEPT
{
    return iterator();
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
typename unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::local_iterator
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::begin(size_type index)
{
    BSLS_ASSERT_SAFE(index < this->bucket_count());

    return local_iterator(&d_impl.bucketAtIndex(index));
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
typename unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::local_iterator
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::end(size_type index)
{
    BSLS_ASSERT_SAFE(index < this->bucket_count());

    return local_iterator(0, &d_impl.bucketAtIndex(index));
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
void
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::clear() BSLS_KEYWORD_NOEXCEPT
{
    d_impl.removeAll();
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
typename unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::iterator
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::find(const key_type& key)
{
    return iterator(d_impl.find(key));
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
bsl::pair<typename unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::iterator,
          typename unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::iterator>
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::equal_range(
                                                           const key_type& key)
{
    HashTableLink *first;
    HashTableLink *last;
    d_impl.findRange(&first, &last, key);
    return bsl::pair<iterator, iterator>(iterator(first), iterator(last));
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
typename unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::iterator
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::erase(const_iterator position)
{
    BSLS_ASSERT(position != this->end());

    return iterator(d_impl.remove(position.node()));
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
typename unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::size_type
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::erase(const key_type& key)
{
    typedef ::BloombergLP::bslalg::BidirectionalNode<value_type> BNode;

    HashTableLink *target = d_impl.find(key);
    if (target) {
        target = d_impl.remove(target);
        size_type result = 1;
        while (target &&
               this->key_eq()(key, ListConfiguration::extractKey(
                                     static_cast<BNode *>(target)->value()))) {
            target = d_impl.remove(target);
            ++result;
        }
        return result;                                                // RETURN
    }

    return 0;
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
typename unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::iterator
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::erase(const_iterator first,
                                                       const_iterator last)
{

#if defined BDE_BUILD_TARGET_SAFE_2
    if (first != last) {
        iterator it        = this->begin();
        const iterator end = this->end();
        for (; it != first; ++it) {
            BSLS_ASSERT(last != it);
            BSLS_ASSERT(end  != it);
        }
        for (; it != last; ++it) {
            BSLS_ASSERT(end  != it);
        }
    }
#endif

    while (first != last) {
        first = this->erase(first);
    }

    return iterator(first.node()); // convert from const_iterator
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
typename unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::iterator
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::insert(
                                                       const value_type& value)
{
    return iterator(d_impl.insert(value));
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
typename unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::iterator
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::insert(
                              BloombergLP::bslmf::MovableRef<value_type> value)
{
    return iterator(d_impl.insert(MoveUtil::move(value)));
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
typename unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::iterator
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::insert(
                                                       const_iterator    hint,
                                                       const value_type& value)
{
    return iterator(d_impl.insert(value, hint.node()));
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
typename unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::iterator
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::insert(
                              const_iterator                             hint,
                              BloombergLP::bslmf::MovableRef<value_type> value)
{
    return iterator(d_impl.insert(MoveUtil::move(value), hint.node()));
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
template <class INPUT_ITERATOR>
void
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::insert(INPUT_ITERATOR first,
                                                        INPUT_ITERATOR last)
{
    insertFromRange(first, last);
}

#if defined(BSLS_COMPILERFEATURES_SUPPORT_GENERALIZED_INITIALIZERS)
template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
void unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::insert(
                                             std::initializer_list<KEY> values)
{
    insert(values.begin(), values.end());
}
#endif

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
void unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::max_load_factor(
                                                           float newLoadFactor)
{
    d_impl.setMaxLoadFactor(newLoadFactor);
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
void
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::rehash(size_type numBuckets)
{
    d_impl.rehashForNumBuckets(numBuckets);
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
void
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::reserve(size_type numElements)
{
    d_impl.reserveForNumElements(numElements);
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
void
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::swap(
                                                     unordered_multiset& other)
                                   BSLS_KEYWORD_NOEXCEPT_SPECIFICATION(
                                       AllocatorTraits::is_always_equal::value
                                   &&  bsl::is_nothrow_swappable<HASH>::value
                                   &&  bsl::is_nothrow_swappable<EQUAL>::value)
{
    d_impl.swap(other.d_impl);
}

// ACCESSORS
template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
ALLOCATOR
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::get_allocator() const
                                                          BSLS_KEYWORD_NOEXCEPT
{
    return d_impl.allocator();
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
typename unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::const_iterator
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::begin() const
                                                          BSLS_KEYWORD_NOEXCEPT
{
    return const_iterator(d_impl.elementListRoot());
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
typename unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::const_iterator
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::end() const
                                                          BSLS_KEYWORD_NOEXCEPT
{
    return const_iterator();
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
typename unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::const_iterator
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::cbegin() const
                                                          BSLS_KEYWORD_NOEXCEPT
{
    return const_iterator(d_impl.elementListRoot());
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
typename unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::const_iterator
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::cend() const
                                                          BSLS_KEYWORD_NOEXCEPT
{
    return const_iterator();
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
typename unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::const_local_iterator
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::begin(size_type index) const
{
    BSLS_ASSERT_SAFE(index < this->bucket_count());

    return const_local_iterator(&d_impl.bucketAtIndex(index));
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
typename
 unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::const_local_iterator
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::end(size_type index) const
{
    BSLS_ASSERT_SAFE(index < this->bucket_count());

    return const_local_iterator(0, &d_impl.bucketAtIndex(index));
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
typename
 unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::const_local_iterator
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::cbegin(size_type index) const
{
    BSLS_ASSERT_SAFE(index < this->bucket_count());

    return const_local_iterator(&d_impl.bucketAtIndex(index));
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
typename unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::const_local_iterator
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::cend(size_type index) const
{
    BSLS_ASSERT_SAFE(index < this->bucket_count());

    return const_local_iterator(0, &d_impl.bucketAtIndex(index));
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
typename unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::size_type
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::bucket(
                                                     const key_type& key) const
{
    return d_impl.bucketIndexForKey(key);
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
typename unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::size_type
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::bucket_count() const
                                                          BSLS_KEYWORD_NOEXCEPT
{
    return d_impl.numBuckets();
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
typename unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::size_type
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::bucket_size(
                                                         size_type index) const
{
    BSLS_ASSERT_SAFE(index < this->bucket_count());

    return d_impl.countElementsInBucket(index);
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
typename unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::size_type
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::count(
                                                     const key_type& key) const
{
    typedef ::BloombergLP::bslalg::BidirectionalNode<value_type> BNode;

    size_type result = 0;
    for (HashTableLink *cursor = d_impl.find(key);
         cursor;
         ++result, cursor = cursor->nextLink()) {

        BNode *cursorNode = static_cast<BNode *>(cursor);
        if (!this->key_eq()(
                         key,
                         ListConfiguration::extractKey(cursorNode->value()))) {
            break;
        }
    }
    return result;
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
typename unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::const_iterator
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::find(
                                                     const key_type& key) const
{
    return const_iterator(d_impl.find(key));
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
bool unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::contains(
                                                     const key_type& key) const
{
    return find(key) != end();
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
bool
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::empty() const
                                                          BSLS_KEYWORD_NOEXCEPT
{
    return 0 == d_impl.size();
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
typename unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::size_type
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::size() const
                                                          BSLS_KEYWORD_NOEXCEPT
{
    return d_impl.size();
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
typename unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::size_type
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::max_size() const
                                                          BSLS_KEYWORD_NOEXCEPT
{
    return AllocatorTraits::max_size(get_allocator());
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
typename unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::hasher
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::hash_function() const
{
    return d_impl.hasher();
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
typename unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::key_equal
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::key_eq() const
{
    return d_impl.comparator();
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
bsl::pair<
      typename unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::const_iterator,
      typename unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::const_iterator>
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::equal_range(
                                                     const key_type& key) const
{
    HashTableLink *first;
    HashTableLink *last;
    d_impl.findRange(&first, &last, key);
    return bsl::pair<const_iterator, const_iterator>(const_iterator(first),
                                                     const_iterator(last));
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
typename unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::size_type
unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::max_bucket_count() const
                                                          BSLS_KEYWORD_NOEXCEPT
{
    return d_impl.maxNumBuckets();
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
float unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::load_factor() const
                                                          BSLS_KEYWORD_NOEXCEPT
{
    return d_impl.loadFactor();
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
float unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::max_load_factor() const
                                                          BSLS_KEYWORD_NOEXCEPT
{
    return d_impl.maxLoadFactor();
}

}  // close namespace bsl

// FREE OPERATORS
template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
bool bsl::operator==(
               const bsl::unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>& lhs,
               const bsl::unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>& rhs)
{
    return lhs.d_impl == rhs.d_impl;
}

#ifndef BSLS_COMPILERFEATURES_SUPPORT_THREE_WAY_COMPARISON
template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
bool bsl::operator!=(
               const bsl::unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>& lhs,
               const bsl::unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>& rhs)
{
    return !(lhs == rhs);
}
#endif

// FREE FUNCTIONS
template <class KEY, class HASH, class EQUAL, class ALLOCATOR, class PREDICATE>
inline
typename bsl::unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>::size_type
bsl::erase_if(unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>& ms,
              PREDICATE                                        predicate)
{
    return BloombergLP::bslstl::AlgorithmUtil::containerEraseIf(ms, predicate);
}

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
inline
void
bsl::swap(bsl::unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>& a,
          bsl::unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR>& b)
                                 BSLS_KEYWORD_NOEXCEPT_SPECIFICATION(
                                     BSLS_KEYWORD_NOEXCEPT_OPERATOR(a.swap(b)))
{
    a.swap(b);
}

// ============================================================================
//                                TYPE TRAITS
// ============================================================================

// Type traits for STL *unordered* *associative* containers:
//: o An unordered associative container defines STL iterators.
//: o An unordered associative container is bitwise movable if both functors
//:   and the allocator are bitwise movable.
//: o An unordered associative container uses 'bslma' allocators if the
//:   (template parameter) type 'ALLOCATOR' is convertible from
//:   'bslma::Allocator *'.

namespace BloombergLP {

namespace bslalg {

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
struct HasStlIterators<bsl::unordered_multiset<KEY, HASH, EQUAL, ALLOCATOR> >
     : bsl::true_type
{};

}  // close namespace bslalg

namespace bslma {

template <class KEY, class HASH, class EQUAL, class ALLOCATOR>
struct UsesBslmaAllocator<bsl::unordered_multiset<KEY,
                                                  HASH,
                                                  EQUAL,
                                                  ALLOCATOR> >
    : bsl::is_convertible<Allocator*, ALLOCATOR>::type
{};

}  // close namespace bslma

}  // close enterprise namespace

#endif // End C++11 code

#endif

// ----------------------------------------------------------------------------
// Copyright 2013 Bloomberg Finance L.P.
//
// 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
//
//     http://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.
// ----------------------------- END-OF-FILE ----------------------------------
