bdlcc_multipriorityqueue.h

Go to the documentation of this file.

/// @file bdlcc_multipriorityqueue.h
///
/// The content of this file has been pre-processed for Doxygen.
///
 
 
// bdlcc_multipriorityqueue.h                                         -*-C++-*-
#ifndef INCLUDED_BDLCC_MULTIPRIORITYQUEUE
#define INCLUDED_BDLCC_MULTIPRIORITYQUEUE
 
#include <bsls_ident.h>
BSLS_IDENT("$Id: $")
 
/// @defgroup bdlcc_multipriorityqueue bdlcc_multipriorityqueue
/// @brief  Provide a thread-enabled parameterized multi-priority queue.
/// @addtogroup bdl
/// @{
/// @addtogroup bdlcc
/// @{
/// @addtogroup bdlcc_multipriorityqueue
/// @{
///
/// <h1> Outline </h1>
/// * <a href="#bdlcc_multipriorityqueue-purpose"> Purpose</a>
/// * <a href="#bdlcc_multipriorityqueue-classes"> Classes </a>
/// * <a href="#bdlcc_multipriorityqueue-description"> Description </a>
///   * <a href="#bdlcc_multipriorityqueue-thread-enabled-idioms-in-the-bdlcc-multipriorityqueue-interface"> Thread-Enabled Idioms in the bdlcc::MultipriorityQueue Interface </a>
///   * <a href="#bdlcc_multipriorityqueue-possible-future-enhancements"> Possible Future Enhancements </a>
///   * <a href="#bdlcc_multipriorityqueue-warning-synchronization-required-on-destruction"> WARNING: Synchronization Required on Destruction </a>
///   * <a href="#bdlcc_multipriorityqueue-usage"> Usage </a>
///     * <a href="#bdlcc_multipriorityqueue-example-1-simple-thread-pool"> Example 1: Simple Thread Pool </a>
///     * <a href="#bdlcc_multipriorityqueue-example-2-multi-threaded-observer"> Example 2: Multi-Threaded Observer </a>
///
/// # Purpose {#bdlcc_multipriorityqueue-purpose}
/// Provide a thread-enabled parameterized multi-priority queue.
///
/// # Classes {#bdlcc_multipriorityqueue-classes}
///
/// -  bdlcc::MultipriorityQueue: thread-enabled, multi-priority queue
///
/// @see 
///
/// # Description {#bdlcc_multipriorityqueue-description}
/// This component provides a thread-enabled mechanism,
/// `bdlcc::MultipriorityQueue`, implementing a special-purpose priority queue
/// container of items of parameterized `TYPE`.  Each item has a priority which,
/// for efficiency of implementation, is limited to a relatively small number
/// `N` of contiguous integers `[ 0 .. N - 1 ]`, with `N` indicated at
/// construction, and 0 being the most urgent priority.  This queue also takes
/// an optional allocator, supplied at construction.  Once configured, these
/// instance parameters remain unchanged for the life of each multi-priority
/// queue.
///
/// ## Thread-Enabled Idioms in the bdlcc::MultipriorityQueue Interface {#bdlcc_multipriorityqueue-thread-enabled-idioms-in-the-bdlcc-multipriorityqueue-interface}
///
///
/// The thread-enabled `bdlcc::MultipriorityQueue` is, in many regards, similar
/// to a value-semantic type in that there is an obvious abstract notion of
/// "value" that can be described in terms of salient attributes, which for this
/// type is a sequence of priority/element pairs, constrained to be in
/// increasing order of priority.  There are, however, several differences in
/// method behavior and signature that arise due to the thread-enabled nature of
/// the queue and its anticipated usage pattern.
///
/// For example, if a queue object is empty, `popFront` will block indefinitely
/// until an element is added to the queue.  Also, since dynamic instance
/// information, such as the number of elements currently in a queue, can be
/// out-of-date by the time it is returned, some manipulators (e.g.,
/// `tryPopFront`) are deliberately combined with an accessor operation (e.g.,
/// `isEmpty`) in order to guarantee proper behavior.
///
/// Finally, note that although the parameterized `TYPE` is expected to at least
/// support copy construction and assignment, the
/// `bdlcc::MultipriorityQueue<TYPE>` type currently does not support any
/// value-semantic operations, since different queues could have different
/// numbers of priorities, making comparison, assignment and copy construction
/// awkward.
///
/// ## Possible Future Enhancements {#bdlcc_multipriorityqueue-possible-future-enhancements}
///
///
/// In addition to `popFront` and `tryPopFront`, a `bdlcc::MultipriorityQueue`
/// may some day also provide a `timedPopFront` method.  This method would block
/// until it is able to complete successfully or until the specified time limit
/// expires.
///
/// ## WARNING: Synchronization Required on Destruction {#bdlcc_multipriorityqueue-warning-synchronization-required-on-destruction}
///
///
/// The behavior for the destructor is undefined unless all access or
/// modification of the object is completed prior to its destruction.  Some form
/// of synchronization, external to the component, is required to ensure the
/// precondition on the destructor is met.  For example, if two (or more)
/// threads are manipulating a queue, it is *not* safe to anticipate the number
/// of elements added to the queue, and destroy that queue immediately after the
/// last element is popped (without additional synchronization) because one of
/// the corresponding push functions may not have completed (push may, for
/// instance, signal waiting threads after the element is considered added to
/// the queue).
///
/// ## Usage {#bdlcc_multipriorityqueue-usage}
///
///
/// This section illustrates intended use of this component.
///
/// ### Example 1: Simple Thread Pool {#bdlcc_multipriorityqueue-example-1-simple-thread-pool}
///
///
/// This example demonstrates how we might use a `bdlcc::MultipriorityQueue` to
/// communicate between a single "producer" thread and multiple "consumer"
/// threads.  The "producer" pushes work requests of varying priority onto the
/// queue, and each "consumer" iteratively takes the highest priority work
/// request from the queue and services it.
///
/// We begin our example with some utility classes that define a simple "work
/// item":
/// @code
/// enum {
///     k_MAX_CONSUMER_THREADS = 10
/// };
///
/// struct MyWorkData {
///     int d_i;        // input to work to be done
///
///     // Work data...
/// };
///
/// struct MyWorkRequest {
///     enum RequestType {
///         e_WORK = 1,
///         e_STOP = 2
///     };
///
///     RequestType d_type;
///     MyWorkData  d_data;
///
///     // Work data...
/// };
/// @endcode
/// Next, we provide a simple function to service an individual work item, and a
/// function to get a work item.  The details are unimportant for this example:
/// @code
/// void myDoWork(MyWorkData& data)
/// {
///     // Do work...
///     (void)data;
/// }
///
/// int getWorkData(MyWorkData *result)
/// {
///     static int count = 0;
///     result->d_i = rand();   // Only one thread runs this routine, so it
///                             // does not matter that 'rand()' is not
///                             // thread-safe, or that 'count' is 'static'.
///
///     return ++count >= 100;
/// }
/// @endcode
/// The `myConsumer` function (below) will pop elements off the queue in
/// priority order and process them.  As discussed above, note that the call to
/// `queue->popFront(&item)` will block until there is an element available on
/// the queue.  This function will be executed in multiple threads, so that each
/// thread waits in `queue->popFront()`; `bdlcc::MultipriorityQueue` guarantees
/// that each thread gets a unique element from the queue:
/// @code
/// void myConsumer(bdlcc::MultipriorityQueue<MyWorkRequest> *queue)
/// {
///     MyWorkRequest item;
///     while (1) {
///
///         // The 'popFront' function will wait for a 'MyWorkRequest' until
///         // one is available.
///
///         queue->popFront(&item);
///
///         if (MyWorkRequest::e_STOP == item.d_type) {
///             break;
///         }
///
///         myDoWork(item.d_data);
///     }
/// }
/// @endcode
/// The `myConsumerThread` function below is a callback for `bslmt::ThreadUtil`,
/// which requires a "C" signature.  `bslmt::ThreadUtil::create()` expects a
/// pointer to this function, and provides that function pointer to the
/// newly-created thread.  The new thread then executes this function.
///
/// Since `bslmt::ThreadUtil::create()` uses the familiar "C" convention of
/// passing a `void` pointer, our function simply casts that pointer to our
/// required type (`bdlcc::MultipriorityQueue<MyWorkRequest> *`), and then
/// delegates to the queue-specific function `myConsumer` (above):
/// @code
/// extern "C" void *myConsumerThread(void *queuePtr)
/// {
///     myConsumer ((bdlcc::MultipriorityQueue<MyWorkRequest>*) queuePtr);
///     return queuePtr;
/// }
/// @endcode
/// In this simple example, the `myProducer` function (below) serves multiple
/// roles: it creates the `bdlcc::MultipriorityQueue`, starts the consumer
/// threads, and then produces and queues work items.  When work requests are
/// exhausted, this function queues one `e_STOP` item for each consumer thread.
///
/// When each consumer thread reads a `e_STOP`, it terminates its
/// thread-handling function.  Note that, although the producer cannot control
/// which thread pops a particular work item, it can rely on the knowledge that
/// each consumer thread will read a single `e_STOP` item and then terminate.
///
/// Finally, the `myProducer` function "joins" each consumer thread, which
/// ensures that the thread itself will terminate correctly (see the
/// @ref bslmt_threadutil  component-level documentation for details):
/// @code
/// void myProducer()
/// {
///     enum {
///         k_NUM_PRIORITIES = 8,
///         k_NUM_THREADS    = 8
///     };
///
///     MyWorkRequest item;
///     MyWorkData    workData;
///
///     // Create multi-priority queue with specified number of priorities.
///
///     bdlcc::MultipriorityQueue<MyWorkRequest> queue(k_NUM_PRIORITIES);
///
///     // Start the specified number of threads.
///
///     assert(0 < k_NUM_THREADS
///         && k_NUM_THREADS <= static_cast<int>(k_MAX_CONSUMER_THREADS));
///     bslmt::ThreadUtil::Handle consumerHandles[k_MAX_CONSUMER_THREADS];
///
///     for (int i = 0; i < k_NUM_THREADS; ++i) {
///         bslmt::ThreadUtil::create(&consumerHandles[i],
///                                  myConsumerThread,
///                                  &queue);
///     }
///
///     // Load work data into work requests and push them onto the queue with
///     // varying priority until all work data has been exhausted.
///
///     int count = 0;                          // used to generate priorities
///
///     while (!getWorkData(&workData)) {       // see declaration (above)
///         item.d_type = MyWorkRequest::e_WORK;
///         item.d_data = workData;
///         queue.pushBack(item, count % k_NUM_PRIORITIES);  // mixed
///                                                          // priorities
///         ++count;
///     }
///
///     // Load as many stop requests as there are active consumer threads.
///
///     for (int i = 0; i < k_NUM_THREADS; ++i) {
///         item.d_type = MyWorkRequest::e_STOP;
///         queue.pushBack(item, k_NUM_PRIORITIES - 1);  // lowest priority
///     }
///
///     // Join all of the consumer threads back with the main thread.
///
///     for (int i = 0; i < k_NUM_THREADS; ++i) {
///         bslmt::ThreadUtil::join(consumerHandles[i]);
///     }
/// }
/// @endcode
///
/// ### Example 2: Multi-Threaded Observer {#bdlcc_multipriorityqueue-example-2-multi-threaded-observer}
///
///
/// The previous example shows a simple mechanism for distributing work requests
/// over multiple threads.  This approach works well for large tasks that can be
/// decomposed into discrete, independent tasks that can benefit from parallel
/// execution.  Note also that the various threads are synchronized only at the
/// end of execution, when the producer "joins" the various consumer threads.
///
/// The simple strategy used in the first example works well for tasks that
/// share no state, and are completely independent of one another.  For
/// instance, a web server might use a similar strategy to distribute `http`
/// requests across multiple worker threads.
///
/// In more complicated examples, it is often necessary or desirable to
/// synchronize the separate tasks during execution.  The second example below
/// shows a single "Observer" mechanism that receives event notification from
/// the various worker threads.
///
/// We first create a simple `MyEvent` data type.  Worker threads will use this
/// type to report information about their work.  In our example, we will report
/// the "worker Id", the event number, and some arbitrary text.
///
/// As with the previous example, class `MyEvent` also contains an `EventType`,
/// an enumeration that indicates whether the worker has completed all work.
/// The "Observer" will use this enumerated value to note when a worker thread
/// has completed its work:
/// @code
/// enum {
///     k_MAX_CONSUMER_THREADS = 10,
///     k_MAX_EVENT_TEXT       = 80
/// };
///
/// struct MyEvent {
///     enum EventType {
///         e_IN_PROGRESS   = 1,
///         e_TASK_COMPLETE = 2
///     };
///
///     EventType d_type;
///     int       d_workerId;
///     int       d_eventNumber;
///     char      d_eventText[k_MAX_EVENT_TEXT];
/// };
/// @endcode
/// As noted in the previous example, `bslmt::ThreadUtil::create()` spawns a new
/// thread, which invokes a simple "C" function taking a `void` pointer.  In the
/// previous example, we simply converted that `void` pointer into a pointer to
/// `bdlcc::MultipriorityQueue<MyWorkRequest>`.
///
/// In this example, however, we want to pass an additional data item.  Each
/// worker thread is initialized with a unique integer value ("worker Id"),
/// which identifies that thread.  We therefore create a simple `struct` that
/// contains both of these values:
/// @code
/// struct MyWorkerData {
///     int                               d_workerId;
///     bdlcc::MultipriorityQueue<MyEvent> *d_queue;
/// };
/// @endcode
/// Function `myWorker` (below) simulates a working thread by enqueuing multiple
/// `MyEvent` events during execution.  In a realistic application, each
/// `MyEvent` structure would likely contain different textual information.  For
/// the sake of simplicity, however, our loop uses a constant value for the text
/// field.  Note that various priorities are generated to illustrate the
/// multi-priority aspect of this particular queue:
/// @code
/// void myWorker(int workerId, bdlcc::MultipriorityQueue<MyEvent> *queue)
/// {
///     const int N = queue->numPriorities();
///     const int NUM_EVENTS = 5;
///     int eventNumber;    // used also to generate mixed priorities
///
///     // First push 'NUM_EVENTS' events onto 'queue' with mixed priorities.
///
///     for (eventNumber = 0; eventNumber < NUM_EVENTS; ++eventNumber) {
///         MyEvent ev = {
///             MyEvent::e_IN_PROGRESS,
///             workerId,
///             eventNumber,
///             "In-Progress Event"         // constant (for simplicity)
///         };
///         queue->pushBack(ev, eventNumber % N);       // mixed priorities
///     }
///
///     // Now push an event to end this task.
///
///     MyEvent ev = {
///         MyEvent::e_TASK_COMPLETE,
///         workerId,
///         eventNumber,
///         "Task Complete"
///     };
///     queue->pushBack(ev, N - 1);                     // lowest priority
/// }
/// @endcode
/// The callback function `myWorkerThread` (below) invoked by
/// `bslmt::ThreadUtil::create` takes the traditional `void` pointer.  The
/// expected data is the composite structure `MyWorkerData`.  The callback
/// function casts the `void` pointer to the application-specific data type and
/// then uses the referenced object to construct a call to the `myWorker`
/// function:
/// @code
/// extern "C" void *myWorkerThread(void *vWorkerPtr)
/// {
///     MyWorkerData *workerPtr = (MyWorkerData *)vWorkerPtr;
///     myWorker(workerPtr->d_workerId, workerPtr->d_queue);
///     return vWorkerPtr;
/// }
/// @endcode
/// For the sake of simplicity, we will implement the Observer behavior (below)
/// in the main thread.  The `void` function `myObserver` starts multiple
/// threads running the `myWorker` function, reads `MyEvent` values from the
/// queue, and logs all messages in the order of arrival.
///
/// As each `myWorker` thread terminates, it sends a `e_TASK_COMPLETE` event.
/// Upon receiving this event, the `myObserver` function uses the `d_workerId`
/// to find the relevant thread, and then "joins" that thread.
///
/// The `myObserver` function determines when all tasks have completed simply by
/// counting the number of `e_TASK_COMPLETE` messages received:
/// @code
/// void myObserver()
/// {
///     const int k_NUM_THREADS    = 10;
///     const int k_NUM_PRIORITIES = 4;
///
///     bdlcc::MultipriorityQueue<MyEvent> queue(k_NUM_PRIORITIES);
///
///     assert(0 < k_NUM_THREADS
///         && k_NUM_THREADS <= static_cast<int>(k_MAX_CONSUMER_THREADS));
///     bslmt::ThreadUtil::Handle workerHandles[k_MAX_CONSUMER_THREADS];
///
///     // Create `k_NUM_THREADS` threads, each having a unique "worker id".
///
///     MyWorkerData workerData[k_NUM_THREADS];
///     for (int i = 0; i < k_NUM_THREADS; ++i) {
///         workerData[i].d_queue = &queue;
///         workerData[i].d_workerId = i;
///         bslmt::ThreadUtil::create(&workerHandles[i],
///                                  myWorkerThread,
///                                  &workerData[i]);
///     }
///
///     // Now print out each of the `MyEvent` values as the threads complete.
///     // This function ends after a total of `k_NUM_THREADS`
///     // `MyEvent::e_TASK_COMPLETE` events have been printed.
///
///     int nStop = 0;
///     while (nStop < k_NUM_THREADS) {
///         MyEvent ev;
///         queue.popFront(&ev);
///         bsl::cout << "[" << ev.d_workerId << "] "
///                   << ev.d_eventNumber << ". "
///                   << ev.d_eventText << bsl::endl;
///         if (MyEvent::e_TASK_COMPLETE == ev.d_type) {
///             ++nStop;
///             bslmt::ThreadUtil::join(workerHandles[ev.d_workerId]);
///         }
///     }
/// }
/// @endcode
/// @}
/** @} */
/** @} */
 
/** @addtogroup bdl
 * @{
 */
/** @addtogroup bdlcc
 * @{
 */
/** @addtogroup bdlcc_multipriorityqueue
 * @{
 */
 
#include <bdlscm_version.h>
 
#include <bdlb_bitutil.h>
 
#include <bdlma_concurrentpool.h>
 
#include <bslalg_constructorproxy.h>
 
#include <bslma_allocator.h>
#include <bslma_deallocatorproctor.h>
#include <bslma_default.h>
#include <bslma_managedptr.h>
#include <bslma_usesbslmaallocator.h>
 
#include <bslmf_movableref.h>
#include <bslmf_nestedtraitdeclaration.h>
 
#include <bslmt_condition.h>
#include <bslmt_lockguard.h>
#include <bslmt_mutex.h>
#include <bslmt_threadutil.h>
 
#include <bsls_assert.h>
#include <bsls_atomic.h>
 
#include <bsl_climits.h>
#include <bsl_cstdint.h>
#include <bsl_new.h>
#include <bsl_vector.h>
 
#ifndef BDE_DONT_ALLOW_TRANSITIVE_INCLUDES
#include <bslalg_typetraits.h>
#endif // BDE_DONT_ALLOW_TRANSITIVE_INCLUDES
 
 
namespace bdlcc {
 
                 // =========================================
                 // local class MultipriorityQueue_Node<TYPE>
                 // =========================================
 
/// This class handles storage of one item of parameterized `TYPE` as a node
/// in a linked list of items stored in a multipriority queue for a given
/// priority.  This class is not to be used from outside this component.
///
/// See @ref bdlcc_multipriorityqueue 
template <class TYPE>
class MultipriorityQueue_Node {
 
    // DATA
    bslalg::ConstructorProxy<TYPE>  d_item;    // object stored in node
    MultipriorityQueue_Node<TYPE>  *d_next_p;  // next node on linked list
 
  private:
    // NOT IMPLEMENTED
    MultipriorityQueue_Node(const MultipriorityQueue_Node&);
    MultipriorityQueue_Node& operator=(const MultipriorityQueue_Node&);
 
  public:
    // TRAITS
    BSLMF_NESTED_TRAIT_DECLARATION(MultipriorityQueue_Node,
                                   bslma::UsesBslmaAllocator);
 
    // CREATORS
 
    /// Create a node containing a copy of the specified `item` and having
    /// the specified `next` pointer.  Use the specified `basicAllocator` to supply memory.  
    /// 
    /// \pre The behavior is undefined   unless `basicAllocator` is non-null.  
    /// 
    /// \note Note that   `item` must be copyable and assignable.
    MultipriorityQueue_Node(const TYPE&              item,
                            bslma::Allocator        *basicAllocator);
 
    /// Create a node containing the value of the specified `item` and
    /// having the specified `next` pointer.  `item` is left in a valid but
    /// unspecified state.  Use the specified `basicAllocator` to supply memory.  
    /// 
    /// \pre The behavior is undefined   unless `basicAllocator` is
    /// non-null.
    MultipriorityQueue_Node(bslmf::MovableRef<TYPE>  item,
                            bslma::Allocator        *basicAllocator);
 
    /// Destroy this node and free all memory that was allocated on its
    /// behalf, if any.
    ~MultipriorityQueue_Node();
 
    // MANIPULATORS
 
    /// Return a reference to the non-modifiable item stored in this node.
    TYPE& item();
 
    /// Return a reference to the modifiable pointer to the node following
    /// this node on the linked list.
    MultipriorityQueue_Node*& nextPtr();
 
    // ACCESSORS
 
    /// Return a pointer to the non-modifiable node following this node on
    /// the linked list, or 0 if this node has no successor.
    const MultipriorityQueue_Node *nextPtr() const;
};
 
                       // ==============================
                       // class MultipriorityQueue<TYPE>
                       // ==============================
 
/// This class implements a thread-enabled multipriority queue whose
/// priorities are restricted to a (small) set of contiguous `N` integer
/// values, `[ 0 .. N - 1 ]`, with 0 being the most urgent.
///
/// This class does have a notion of value, namely the sequence of
/// priority/element pairs, constrained to be in decreasing order of urgency
/// (i.e., monotonically increasing priority values).  However, no value-semantic operations are implemented.  
/// 
/// \note Note that   elements having
/// the same priority are maintained in First-In-First-Out (FIFO) order. 
/// 
/// \note Note that   the current implementation supports up to a maximum of
/// `sizeof(int) * CHAR_BIT` priorities.
///
/// This class is implemented as a set of linked lists, one for each
/// priority.  Two vectors are used to maintain head and tail pointers for
/// the lists.
///
/// See @ref bdlcc_multipriorityqueue 
template <class TYPE>
class MultipriorityQueue {
 
    // PRIVATE CONSTANTS
    enum {
        k_BITS_PER_INT           = sizeof(int) * CHAR_BIT,
        k_DEFAULT_NUM_PRIORITIES = k_BITS_PER_INT,
        k_MAX_NUM_PRIORITIES     = k_BITS_PER_INT
    };
 
    // PRIVATE TYPES
 
    /// The type of the elements on the linked lists of items that are
    /// maintained for the `N` priorities handled by this multipriority
    /// queue.
    typedef MultipriorityQueue_Node<TYPE> Node;
 
    /// The type of the vectors of list head and tail pointers.
    typedef bsl::vector<Node *>                NodePtrVector;
 
    // DATA
    mutable bslmt::Mutex  d_mutex;         // used to synchronize access
                                           // (including 'const' access)
 
    bslmt::Condition      d_notEmptyCondition;
                                           // signaled on each push
 
    NodePtrVector         d_heads;         // pointers to heads of linked lists
                                           // -- one for each priority
 
    NodePtrVector         d_tails;         // pointers to tails of linked lists
                                           // -- one for each priority
 
    int                   d_notEmptyFlags; // bit mask indicating priorities
                                           // for which there is data, where
                                           // bit 0 is the lowest order bit,
                                           // representing most urgent priority
 
    bdlma::ConcurrentPool d_pool;          // memory pool used for node storage
 
    bsls::AtomicInt       d_length;        // total number of items in this
                                           // multipriority queue
 
    bool                  d_enabledFlag;   // enabled/disabled state of pushes
                                           // to the multipriority queue (does
                                           // not affect pops)
 
    bslma::Allocator     *d_allocator_p;   // memory allocator (held)
 
  private:
    // NOT IMPLEMENTED
    MultipriorityQueue(const MultipriorityQueue&);
    MultipriorityQueue& operator=(const MultipriorityQueue&);
 
  private:
    // PRIVATE MANIPULATORS
 
    /// Attempt to remove (immediately) the least-recently added item having
    /// the most urgent priority (lowest value) from this multipriority
    /// queue.  If the specified `blockFlag` is `true`, this method blocks
    /// the calling thread until an item becomes available.  On success,
    /// load the value of the popped item into the specified `item`; if the
    /// specified `itemPriority` is non-null, load the priority of the
    /// popped item into `itemPriority`; and return 0.  Otherwise, leave
    /// `item` and `itemPriority` unmodified, and return a non-zero value
    /// indicating that this multipriority queue was empty.  
    /// 
    /// \pre The behavior is undefined   unless `item` is non-null.  
    /// \note Note that  a non-zero value can
    /// be returned only if `blockFlag` is `false`.
    int tryPopFrontImpl(TYPE *item, int *itemPriority, bool blockFlag);
 
  public:
    // TRAITS
    BSLMF_NESTED_TRAIT_DECLARATION(MultipriorityQueue,
                                   bslma::UsesBslmaAllocator);
 
    // CREATORS
 
    /// Create a multipriority queue.  Optionally specify `numPriorities`,
    /// the number of distinct priorities supported by the multipriority
    /// queue.  If `numPriorities` is not specified, the
    /// (implementation-imposed maximum) number 32 is used.  Optionally
    /// specify a `basicAllocator` used to supply memory.  If
    /// `basicAllocator` is 0, the currently installed default allocator is used.  
    /// 
    /// \pre The behavior is undefined   unless `1 <= numPriorities <= 32`
    /// (if specified).
    explicit MultipriorityQueue(bslma::Allocator *basicAllocator = 0);
    explicit MultipriorityQueue(int               numPriorities,
                                bslma::Allocator *basicAllocator = 0);
 
    /// Destroy this container.  
    /// \pre The behavior is undefined  unless all access
    /// or modification of the container has completed prior to this call.
    ~MultipriorityQueue();
 
    // MANIPULATORS
 
    /// Remove the least-recently added item having the most urgent priority
    /// (lowest value) from this multi-priority queue and load its value
    /// into the specified `item`.  If this queue is empty, this method
    /// blocks the calling thread until an item becomes available.  If the
    /// optionally specified `itemPriority` is non-null, load the priority
    /// of the popped item into `itemPriority`.  
    /// 
    /// \pre The behavior is undefined   unless `item` is non-null.  Note this is unaffected by the enabled /
    /// disabled state of the queue.
    void popFront(TYPE *item, int *itemPriority = 0);
 
    /// Insert the value of the specified `item` with the specified
    /// `itemPriority` into this multipriority queue before any queued items
    /// having a less urgent priority (higher value) than `itemPriority`,
    /// and after any items having the same or more urgent priority (lower
    /// value) than `itemPriority`.  If the multipriority queue is enabled,
    /// the push succeeds and `0` is returned, otherwise the push fails, the
    /// queue remains unchanged, and a nonzero value is returned.  
    /// 
    /// \pre The behavior is undefined   unless `0 <= itemPriority < numPriorities()`.
    int pushBack(const TYPE& item, int itemPriority);
 
    /// Insert the value of the specified `item` with the specified
    /// `itemPriority` into this multipriority queue before any queued items
    /// having a less urgent priority (higher value) than `itemPriority`,
    /// and after any items having the same or more urgent priority (lower
    /// value) than `itemPriority`.  `item` is left in a valid but
    /// unspecified state.  If the multipriority queue is enabled, the push
    /// succeeds and `0` is returned, otherwise the push fails, the queue
    /// remains unchanged, and a nonzero value is returned.  
    /// 
    /// \pre The behavior is undefined   unless `0 <= itemPriority < numPriorities()`.
    int pushBack(bslmf::MovableRef<TYPE> item, int itemPriority);
 
    /// Insert the value of the specified `item` with the specified
    /// `itemPriority` onto the back of this multipriority queue before any
    /// queued items having a less urgent priority (higher value) than
    /// `itemPriority`, and after any items having the same or more urgent
    /// priority (lower value) than `itemPriority`.  All of the specified
    /// `numItems` items are pushed as a single atomic action, unless the
    /// copy constructor for one of them throws an exception, in which case
    /// a possibly empty subset of the pushes will have completed and no
    /// memory will be leaked.  `Raw` means that the push will succeed even if the multipriority queue is disabled.  
    /// 
    /// \note Note that   this method is
    /// targeted for specific use by the class `bdlmt::MultipriorityThreadPool`.  
    /// 
    /// \pre The behavior is undefined   unless
    /// `0 <= itemPriority < numPriorities()`.
    void pushBackMultipleRaw(const TYPE& item, int itemPriority, int numItems);
 
    /// Insert the value of the specified `item` with the specified
    /// `itemPriority` into the front of this multipriority queue the
    /// specified `numItems` times, before any queued items having the same
    /// or less urgent priority (higher value) than `itemPriority`, and
    /// after any items having more urgent priority (lower value) than
    /// `itemPriority`.  All `numItems` items are pushed as a single atomic
    /// action, unless the copy constructor throws while creating one of
    /// them, in which case a possibly empty subset of the pushes will have
    /// completed and no memory will be leaked.  `Raw` means that the push
    /// will succeed even if the multipriority queue is disabled.  
    /// 
    /// \pre The behavior is undefined   unless `0 <= itemPriority < numPriorities()`. 
    /// 
    /// \note Note that   this method is targeted at specific uses by the class
    /// `bdlmt::MultipriorityThreadPool`.
    void pushFrontMultipleRaw(const TYPE& item,
                              int         itemPriority,
                              int         numItems);
 
    /// Attempt to remove (immediately) the least-recently added item having
    /// the most urgent priority (lowest value) from this multi-priority
    /// queue.  On success, load the value of the popped item into the
    /// specified `item`; if the optionally specified `itemPriority` is
    /// non-null, load the priority of the popped item into `itemPriority`;
    /// and return 0.  Otherwise, leave `item` and `itemPriority`
    /// unmodified, and return a non-zero value indicating that this queue was empty.  
    /// 
    /// \pre The behavior is undefined   unless `item` is non-null.
    /// Note this is unaffected by the enabled / disabled state of the
    /// queue.
    int tryPopFront(TYPE *item, int *itemPriority = 0);
 
    /// Remove and destroy all items from this multi-priority queue.
    void removeAll();
 
    /// Enable pushes to this multipriority queue.  This method has no
    /// effect unless the queue was disabled.
    void enable();
 
    /// Disable pushes to this multipriority queue.  This method has no
    /// effect unless the queue was enabled.
    void disable();
 
    // ACCESSORS
 
    /// Return the number of distinct priorities (indicated at construction)
    /// that are supported by this multi-priority queue.
    int numPriorities() const;
 
    /// Return the total number of items in this multi-priority queue.
    int length() const;
 
    /// Return `true` if there are no items in this multi-priority queue,
    /// and `false` otherwise.
    bool isEmpty() const;
 
    /// Return `true` if this multipriority queue is enable and `false`
    /// otherwise.
    bool isEnabled() const;
};
 
// ============================================================================
//                            INLINE DEFINITIONS
// ============================================================================
 
                 // -----------------------------------------
                 // local class MultipriorityQueue_Node<TYPE>
                 // -----------------------------------------
 
// CREATORS
template <class TYPE>
inline
MultipriorityQueue_Node<TYPE>::MultipriorityQueue_Node(
                                              const TYPE&       item,
                                              bslma::Allocator *basicAllocator)
: d_item(item, basicAllocator)
, d_next_p(0)
{}
 
template <class TYPE>
inline
MultipriorityQueue_Node<TYPE>::MultipriorityQueue_Node(
                                       bslmf::MovableRef<TYPE>  item,
                                       bslma::Allocator        *basicAllocator)
: d_item(bslmf::MovableRefUtil::move(item), basicAllocator)
, d_next_p(0)
{}
 
template <class TYPE>
inline
MultipriorityQueue_Node<TYPE>::~MultipriorityQueue_Node()
{}
 
// MANIPULATORS
template <class TYPE>
inline
TYPE& MultipriorityQueue_Node<TYPE>::item()
{
    return d_item.object();
}
 
template <class TYPE>
inline
MultipriorityQueue_Node<TYPE>*& MultipriorityQueue_Node<TYPE>::nextPtr()
{
    return d_next_p;
}
 
// ACCESSORS
template <class TYPE>
inline
const MultipriorityQueue_Node<TYPE> *
                            MultipriorityQueue_Node<TYPE>::nextPtr() const
{
    return d_next_p;
}
 
                       // ------------------------------
                       // class MultipriorityQueue<TYPE>
                       // ------------------------------
 
// PRIVATE MANIPULATORS
template <class TYPE>
int MultipriorityQueue<TYPE>::tryPopFrontImpl(TYPE *item,
                                              int  *itemPriority,
                                              bool  blockFlag)
{
    enum { e_SUCCESS = 0, e_FAILURE = -1 };
 
    Node *condemned;
    int priority;
 
    BSLS_ASSERT(item);
 
    {
        bslmt::LockGuard<bslmt::Mutex> lock(&d_mutex);
 
        while (0 == d_length) {
            // Note that if we get a spurious signal, we will check the
            // 'blockFlag' unnecessarily, but that will typically be a rare
            // occurrence.  This arrangement minimizes the time taken in the
            // case where '0 != d_length', which will typically be a frequent
            // occurrence.
 
            if (blockFlag) {
                d_notEmptyCondition.wait(&d_mutex);
            }
            else {
                return e_FAILURE;                                     // RETURN
            }
        }
 
        priority = bdlb::BitUtil::numTrailingUnsetBits(
                                               (bsl::uint32_t)d_notEmptyFlags);
        BSLS_ASSERT(priority < k_MAX_NUM_PRIORITIES);
            // verifies there is at least one priority bit set.  Note that
            // 'numTrailingUnsetBits' cannot return a negative value.
 
        Node *& head = d_heads[priority];
        condemned = head;
 
        *item = bslmf::MovableRefUtil::move(condemned->item());  // might throw
 
        head = head->nextPtr();
        if (0 == head) {
            // The last item with this priority was just popped.
 
            BSLS_ASSERT(d_tails[priority] == condemned);
            d_notEmptyFlags &= ~(1 << priority);
        }
 
        --d_length;
    }
 
    if (itemPriority) {
        *itemPriority = priority;
    }
 
    condemned->~Node();
    d_pool.deallocate(condemned);
 
    return e_SUCCESS;
}
 
// CREATORS
template <class TYPE>
MultipriorityQueue<TYPE>::MultipriorityQueue(bslma::Allocator *basicAllocator)
: d_heads((typename NodePtrVector::size_type)k_DEFAULT_NUM_PRIORITIES, 0,
          basicAllocator)
, d_tails((typename NodePtrVector::size_type)k_DEFAULT_NUM_PRIORITIES, 0,
          basicAllocator)
, d_notEmptyFlags(0)
, d_pool(sizeof(Node), bslma::Default::allocator(basicAllocator))
, d_length(0)
, d_enabledFlag(true)
, d_allocator_p(bslma::Default::allocator(basicAllocator))
{
}
 
template <class TYPE>
MultipriorityQueue<TYPE>::MultipriorityQueue(int               numPriorities,
                                             bslma::Allocator *basicAllocator)
: d_heads((typename NodePtrVector::size_type)numPriorities, 0, basicAllocator)
, d_tails((typename NodePtrVector::size_type)numPriorities, 0, basicAllocator)
, d_notEmptyFlags(0)
, d_pool(sizeof(Node), bslma::Default::allocator(basicAllocator))
, d_length(0)
, d_enabledFlag(true)
, d_allocator_p(bslma::Default::allocator(basicAllocator))
{
    BSLS_ASSERT(1                       <= numPriorities);
    BSLS_ASSERT(k_MAX_NUM_PRIORITIES >= numPriorities);
}
 
template <class TYPE>
MultipriorityQueue<TYPE>::~MultipriorityQueue()
{
    removeAll();
 
    typename NodePtrVector::iterator it;
    typename NodePtrVector::iterator endIt;
 
    for (it = d_heads.begin(), endIt = d_heads.end(); endIt != it; ++it) {
        BSLS_ASSERT(!*it);
    }
 
    // Tails do not get set to null by 'removeAll', so are indeterminate.
 
    BSLS_ASSERT(isEmpty());
    BSLS_ASSERT(0 == d_notEmptyFlags);
}
 
// MANIPULATORS
template <class TYPE>
inline
void MultipriorityQueue<TYPE>::popFront(TYPE *item, int *itemPriority)
{
    tryPopFrontImpl(item, itemPriority, true);
}
 
template <class TYPE>
int MultipriorityQueue<TYPE>::pushBack(const TYPE& item, int itemPriority)
{
    enum { e_SUCCESS = 0, e_FAILURE = -1 };
 
    BSLS_ASSERT((unsigned)itemPriority < d_heads.size());
 
    // Allocate and copy construct.  Note we are doing this work outside the
    // mutex, which is advantageous in that no one is waiting on us, but it has
    // the disadvantage that we haven't checked whether this multipriority
    // queue is disabled, in which case we'll throw the new node away.
 
    // Note the queue being disabled is not the usual case.  Note a race
    // condition occurs if we check d_enabledFlag outside the mutex.
 
    Node *newNode = (Node *)d_pool.allocate();
    bslma::DeallocatorProctor<bdlma::ConcurrentPool> deallocator(newNode,
                                                                 &d_pool);
 
    ::new (newNode) Node(item, d_allocator_p);                   // might throw
    deallocator.release();
    bslma::ManagedPtr<Node> deleter(newNode, &d_pool);
 
    {
        bslmt::LockGuard<bslmt::Mutex> lock(&d_mutex);
 
        if (!d_enabledFlag) {
            return e_FAILURE;                                         // RETURN
        }
 
        deleter.release();
 
        const int mask = 1 << itemPriority;
        if (d_notEmptyFlags & mask) {
            d_tails[itemPriority]->nextPtr() = newNode;
        }
        else {
            d_heads[itemPriority] = newNode;
            d_notEmptyFlags |= mask;
        }
        d_tails[itemPriority] = newNode;
 
        ++d_length;
    }
 
    d_notEmptyCondition.signal();
 
    return e_SUCCESS;
}
 
template <class TYPE>
int MultipriorityQueue<TYPE>::pushBack(bslmf::MovableRef<TYPE> item,
                                       int                     itemPriority)
{
    enum { e_SUCCESS = 0, e_FAILURE = -1 };
 
    BSLS_ASSERT((unsigned)itemPriority < d_heads.size());
 
    // Allocate and copy construct.  Note we are doing this work outside the
    // mutex, which is advantageous in that no one is waiting on us, but it has
    // the disadvantage that we haven't checked whether this multipriority
    // queue is disabled, in which case we'll throw the new node away.
    //
    // Note the queue being disabled is not the usual case.  Note a race
    // condition occurs if we check d_enabledFlag outside the mutex.
 
    Node *newNode = static_cast<Node *>(d_pool.allocate());
    bslma::DeallocatorProctor<bdlma::ConcurrentPool> deallocator(newNode,
                                                                 &d_pool);
 
    {
        bslmt::LockGuard<bslmt::Mutex> lock(&d_mutex);
 
        // Do the enable check before the move, since if it is a move and not a
        // copy, there's no backing out after that.
 
        if (!d_enabledFlag) {
            return e_FAILURE;                                         // RETURN
        }
 
        ::new (newNode) Node(bslmf::MovableRefUtil::move(item),  // might throw
                             d_allocator_p);
        deallocator.release();
 
        const int mask = 1 << itemPriority;
        if (d_notEmptyFlags & mask) {
            d_tails[itemPriority]->nextPtr() = newNode;
        }
        else {
            d_heads[itemPriority] = newNode;
            d_notEmptyFlags |= mask;
        }
        d_tails[itemPriority] = newNode;
 
        ++d_length;
    }
 
    d_notEmptyCondition.signal();
 
    return e_SUCCESS;
}
 
template <class TYPE>
void MultipriorityQueue<TYPE>::pushBackMultipleRaw(const TYPE& item,
                                                   int         itemPriority,
                                                   int         numItems)
{
    BSLS_ASSERT((unsigned)itemPriority < d_heads.size());
 
    const int mask = 1 << itemPriority;
 
    {
        bslmt::LockGuard<bslmt::Mutex> lock(&d_mutex);
 
        for (int ii = 0; ii < numItems; ++ii) {
            Node *newNode = (Node *)d_pool.allocate();
            bslma::DeallocatorProctor<bdlma::ConcurrentPool> deallocator(
                                                             newNode, &d_pool);
 
            ::new (newNode) Node(item, d_allocator_p);           // might throw
            deallocator.release();
 
            if (d_notEmptyFlags & mask) {
                d_tails[itemPriority]->nextPtr() = newNode;
            }
            else {
                d_heads[itemPriority] = newNode;
                d_notEmptyFlags |= mask;
            }
            d_tails[itemPriority] = newNode;
 
            ++d_length;
        }
    }
 
    for (int ii = 0; ii < numItems; ++ii) {
        d_notEmptyCondition.signal();
    }
}
 
template <class TYPE>
void MultipriorityQueue<TYPE>::pushFrontMultipleRaw(const TYPE& item,
                                                    int         itemPriority,
                                                    int         numItems)
{
    BSLS_ASSERT((unsigned)itemPriority < d_heads.size());
 
    const int mask = 1 << itemPriority;
 
    {
        bslmt::LockGuard<bslmt::Mutex> lock(&d_mutex);
 
        for (int ii = 0; ii < numItems; ++ii) {
            Node *newNode = (Node *)d_pool.allocate();
            bslma::DeallocatorProctor<bdlma::ConcurrentPool> deallocator(
                                                             newNode, &d_pool);
 
            ::new (newNode) Node(item, d_allocator_p);           // might throw
            deallocator.release();
 
            Node *& head = d_heads[itemPriority];
            if (!head) {
                d_tails[itemPriority] = newNode;
                d_notEmptyFlags |= mask;
            }
            newNode->nextPtr() = head;
            head = newNode;
 
            ++d_length;
        }
    }
 
    for (int ii = 0; ii < numItems; ++ii) {
        d_notEmptyCondition.signal();
    }
}
 
template <class TYPE>
inline
int MultipriorityQueue<TYPE>::tryPopFront(TYPE *item, int *itemPriority)
{
    return tryPopFrontImpl(item, itemPriority, false);
}
 
template <class TYPE>
void MultipriorityQueue<TYPE>::removeAll()
{
    Node *condemnedList = 0;
 
    {
        bslmt::LockGuard<bslmt::Mutex> lock(&d_mutex);
 
        while (d_notEmptyFlags) {
            const int priority = bdlb::BitUtil::numTrailingUnsetBits(
                                  static_cast<bsl::uint32_t>(d_notEmptyFlags));
 
            Node *& head = d_heads[priority];
            BSLS_ASSERT(head);
 
            d_tails[priority]->nextPtr() = condemnedList;
            condemnedList = head;
 
            head = 0;
 
            d_notEmptyFlags &= ~(1 << priority);
        }
 
        BSLS_ASSERT(0 == d_notEmptyFlags);
 
        d_length = 0;
    }
 
    Node *node = condemnedList;
    while (node) {
        Node *condemnedNode = node;
        node = node->nextPtr();
 
        condemnedNode->~Node();
        d_pool.deallocate(condemnedNode);
    }
}
 
template <class TYPE>
inline
void MultipriorityQueue<TYPE>::enable()
{
    bslmt::LockGuard<bslmt::Mutex> lock(&d_mutex);
 
    d_enabledFlag = true;
}
 
template <class TYPE>
inline
void MultipriorityQueue<TYPE>::disable()
{
    bslmt::LockGuard<bslmt::Mutex> lock(&d_mutex);
 
    d_enabledFlag = false;
}
 
// ACCESSORS
template <class TYPE>
inline
int MultipriorityQueue<TYPE>::numPriorities() const
{
    return static_cast<int>(d_heads.size());
}
 
template <class TYPE>
inline
int MultipriorityQueue<TYPE>::length() const
{
    return d_length;
}
 
template <class TYPE>
inline
bool MultipriorityQueue<TYPE>::isEmpty() const
{
    return 0 == d_length;
}
 
template <class TYPE>
inline
bool MultipriorityQueue<TYPE>::isEnabled() const
{
    return d_enabledFlag;
}
 
}  // close package namespace
 
 
#endif
 
// ----------------------------------------------------------------------------
// Copyright 2015 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 ----------------------------------
 
/** @} */
/** @} */
/** @} */