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 // Copyright (C) 2004-2006 The Trustees of Indiana University.
 
 // Use, modification and distribution is subject to the Boost Software
 // License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
 // http://www.boost.org/LICENSE_1_0.txt)
 
 //  Authors: Douglas Gregor
 //           Andrew Lumsdaine
 #ifndef BOOST_GRAPH_DISTRIBUTED_QUEUE_HPP
 #define BOOST_GRAPH_DISTRIBUTED_QUEUE_HPP
 
 #ifndef BOOST_GRAPH_USE_MPI
 #error "Parallel BGL files should not be included unless <boost/graph/use_mpi.hpp> has been included"
 #endif
 
 #include <boost/graph/parallel/process_group.hpp>
 #include <boost/optional.hpp>
 #include <boost/shared_ptr.hpp>
 #include <vector>
 
 namespace boost { namespace graph { namespace distributed {
 
 /// A unary predicate that always returns "true".
 struct always_push
 {
   template<typename T> bool operator()(const T&) const { return true; }
 };
 
 
 
 /** A distributed queue adaptor.
  *
  * Class template @c distributed_queue implements a distributed queue
  * across a process group. The distributed queue is an adaptor over an
  * existing (local) queue, which must model the @ref Buffer
  * concept. Each process stores a distinct copy of the local queue,
  * from which it draws or removes elements via the @ref pop and @ref
  * top members.
  *
  * The value type of the local queue must be a model of the @ref
  * GlobalDescriptor concept. The @ref push operation of the
  * distributed queue passes (via a message) the value to its owning
  * processor. Thus, the elements within a particular local queue are
  * guaranteed to have the process owning that local queue as an owner.
  *
  * Synchronization of distributed queues occurs in the @ref empty and
  * @ref size functions, which will only return "empty" values (true or
  * 0, respectively) when the entire distributed queue is empty. If the
  * local queue is empty but the distributed queue is not, the
  * operation will block until either condition changes. When the @ref
  * size function of a nonempty queue returns, it returns the size of
  * the local queue. These semantics were selected so that sequential
  * code that processes elements in the queue via the following idiom
  * can be parallelized via introduction of a distributed queue:
  *
  *   distributed_queue<...> Q;
  *   Q.push(x);
  *   while (!Q.empty()) {
  *     // do something, that may push a value onto Q
  *   }
  *
  * In the parallel version, the initial @ref push operation will place
  * the value @c x onto its owner's queue. All processes will
  * synchronize at the call to empty, and only the process owning @c x
  * will be allowed to execute the loop (@ref Q.empty() returns
  * false). This iteration may in turn push values onto other remote
  * queues, so when that process finishes execution of the loop body
  * and all processes synchronize again in @ref empty, more processes
  * may have nonempty local queues to execute. Once all local queues
  * are empty, @ref Q.empty() returns @c false for all processes.
  *
  * The distributed queue can receive messages at two different times:
  * during synchronization and when polling @ref empty. Messages are
  * always received during synchronization, to ensure that accurate
  * local queue sizes can be determines. However, whether @ref empty
  * should poll for messages is specified as an option to the
  * constructor. Polling may be desired when the order in which
  * elements in the queue are processed is not important, because it
  * permits fewer synchronization steps and less communication
  * overhead. However, when more strict ordering guarantees are
  * required, polling may be semantically incorrect. By disabling
  * polling, one ensures that parallel execution using the idiom above
  * will not process an element at a later "level" before an earlier
  * "level".
  *
  * The distributed queue nearly models the @ref Buffer
  * concept. However, the @ref push routine does not necessarily
  * increase the result of @c size() by one (although the size of the
  * global queue does increase by one).
  */
 template<typename ProcessGroup, typename OwnerMap, typename Buffer,
          typename UnaryPredicate = always_push>
 class distributed_queue
 {
   typedef distributed_queue self_type;
 
   enum {
     /** Message indicating a remote push. The message contains a
      * single value x of type value_type that is to be pushed on the
      * receiver's queue.
      */
     msg_push,
     /** Push many elements at once. */
     msg_multipush
   };
 
  public:
   typedef ProcessGroup                     process_group_type;
   typedef Buffer                           buffer_type;
   typedef typename buffer_type::value_type value_type;
   typedef typename buffer_type::size_type  size_type;
 
   /** Construct a new distributed queue.
    *
    * Build a new distributed queue that communicates over the given @p
    * process_group, whose local queue is initialized via @p buffer and
    * which may or may not poll for messages.
    */
   explicit
   distributed_queue(const ProcessGroup& process_group,
                     const OwnerMap& owner,
                     const Buffer& buffer,
                     bool polling = false);
 
   /** Construct a new distributed queue.
    *
    * Build a new distributed queue that communicates over the given @p
    * process_group, whose local queue is initialized via @p buffer and
    * which may or may not poll for messages.
    */
   explicit
   distributed_queue(const ProcessGroup& process_group = ProcessGroup(),
                     const OwnerMap& owner = OwnerMap(),
                     const Buffer& buffer = Buffer(),
                     const UnaryPredicate& pred = UnaryPredicate(),
                     bool polling = false);
 
   /** Construct a new distributed queue.
    *
    * Build a new distributed queue that communicates over the given @p
    * process_group, whose local queue is default-initalized and which
    * may or may not poll for messages.
    */
   distributed_queue(const ProcessGroup& process_group, const OwnerMap& owner,
                     const UnaryPredicate& pred, bool polling = false);
 
   /** Virtual destructor required with virtual functions.
    *
    */
   virtual ~distributed_queue() {}
 
   /** Push an element onto the distributed queue.
    *
    * The element will be sent to its owner process to be added to that
    * process's local queue. If polling is enabled for this queue and
    * the owner process is the current process, the value will be
    * immediately pushed onto the local queue.
    *
    * Complexity: O(1) messages of size O(sizeof(value_type)) will be
    * transmitted.
    */
   void push(const value_type& x);
 
   /** Pop an element off the local queue.
    *
    * @p @c !empty()
    */
   void pop() { buffer.pop(); }
 
   /**
    * Return the element at the top of the local queue.
    *
    * @p @c !empty()
    */
   value_type& top() { return buffer.top(); }
 
   /**
    * \overload
    */
   const value_type& top() const { return buffer.top(); }
 
   /** Determine if the queue is empty.
    *
    * When the local queue is nonempty, returns @c true. If the local
    * queue is empty, synchronizes with all other processes in the
    * process group until either (1) the local queue is nonempty
    * (returns @c true) (2) the entire distributed queue is empty
    * (returns @c false).
    */
   bool empty() const;
 
   /** Determine the size of the local queue.
    *
    * The behavior of this routine is equivalent to the behavior of
    * @ref empty, except that when @ref empty returns true this
    * function returns the size of the local queue and when @ref empty
    * returns false this function returns zero.
    */
   size_type size() const;
 
   // private:
   /** Synchronize the distributed queue and determine if all queues
    * are empty.
    *
    * \returns \c true when all local queues are empty, or false if at least
    * one of the local queues is nonempty.
    * Defined as virtual for derived classes like depth_limited_distributed_queue.
    */
   virtual bool do_synchronize() const;
 
  private:
   // Setup triggers
   void setup_triggers();
 
   // Message handlers
   void 
   handle_push(int source, int tag, const value_type& value, 
               trigger_receive_context);
 
   void 
   handle_multipush(int source, int tag, const std::vector<value_type>& values, 
                    trigger_receive_context);
 
   mutable ProcessGroup process_group;
   OwnerMap owner;
   mutable Buffer buffer;
   UnaryPredicate pred;
   bool polling;
 
   typedef std::vector<value_type> outgoing_buffer_t;
   typedef std::vector<outgoing_buffer_t> outgoing_buffers_t;
   shared_ptr<outgoing_buffers_t> outgoing_buffers;
 };
 
 /// Helper macro containing the normal names for the template
 /// parameters to distributed_queue.
 #define BOOST_DISTRIBUTED_QUEUE_PARMS                           \
   typename ProcessGroup, typename OwnerMap, typename Buffer,    \
   typename UnaryPredicate
 
 /// Helper macro containing the normal template-id for
 /// distributed_queue.
 #define BOOST_DISTRIBUTED_QUEUE_TYPE                                    \
   distributed_queue<ProcessGroup, OwnerMap, Buffer, UnaryPredicate>
 
 /** Synchronize all processes involved with the given distributed queue.
  *
  * This function will synchronize all of the local queues for a given
  * distributed queue, by ensuring that no additional messages are in
  * transit. It is rarely required by the user, because most
  * synchronization of distributed queues occurs via the @c empty or @c
  * size methods.
  */
 template<BOOST_DISTRIBUTED_QUEUE_PARMS>
 inline void
 synchronize(const BOOST_DISTRIBUTED_QUEUE_TYPE& Q)
 { Q.do_synchronize(); }
 
 /// Construct a new distributed queue.
 template<typename ProcessGroup, typename OwnerMap, typename Buffer>
 inline distributed_queue<ProcessGroup, OwnerMap, Buffer>
 make_distributed_queue(const ProcessGroup& process_group,
                        const OwnerMap& owner,
                        const Buffer& buffer,
                        bool polling = false)
 {
   typedef distributed_queue<ProcessGroup, OwnerMap, Buffer> result_type;
   return result_type(process_group, owner, buffer, polling);
 }
 
 } } } // end namespace boost::graph::distributed
 
 #include <boost/graph/distributed/detail/queue.ipp>
 
 #undef BOOST_DISTRIBUTED_QUEUE_TYPE
 #undef BOOST_DISTRIBUTED_QUEUE_PARMS
 
 #endif // BOOST_GRAPH_DISTRIBUTED_QUEUE_HPP

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