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  #ifndef QTCONCURRENT_REDUCEKERNEL_H
  #define QTCONCURRENT_REDUCEKERNEL_H
  
  #include <QtConcurrent/qtconcurrent_global.h>
  
  #if !defined(QT_NO_CONCURRENT) || defined(Q_CLANG_QDOC)
  
  #include <QtCore/qatomic.h>
  #include <QtCore/qlist.h>
  #include <QtCore/qmap.h>
  #include <QtCore/qmutex.h>
  #include <QtCore/qthread.h>
  #include <QtCore/qthreadpool.h>
  #include <QtCore/qvector.h>
  
  QT_BEGIN_NAMESPACE
  
  
  namespace QtConcurrent {
  
  /*
      The ReduceQueueStartLimit and ReduceQueueThrottleLimit constants
      limit the reduce queue size for MapReduce. When the number of
      reduce blocks in the queue exceeds ReduceQueueStartLimit,
      MapReduce won't start any new threads, and when it exceeds
      ReduceQueueThrottleLimit running threads will be stopped.
  */
  #ifdef Q_CLANG_QDOC
  enum ReduceQueueLimits {
      ReduceQueueStartLimit = 20,
      ReduceQueueThrottleLimit = 30
  };
  #else
  enum {
      ReduceQueueStartLimit = 20,
      ReduceQueueThrottleLimit = 30
  };
  #endif
  
  // IntermediateResults holds a block of intermediate results from a
  // map or filter functor. The begin/end offsets indicates the origin
  // and range of the block.
  template <typename T>
  class IntermediateResults
  {
  public:
      int begin, end;
      QVector<T> vector;
  };
  
  enum ReduceOption {
      UnorderedReduce = 0x1,
      OrderedReduce = 0x2,
      SequentialReduce = 0x4
      // ParallelReduce = 0x8
  };
  Q_DECLARE_FLAGS(ReduceOptions, ReduceOption)
  #ifndef Q_CLANG_QDOC
  Q_DECLARE_OPERATORS_FOR_FLAGS(ReduceOptions)
  #endif
  // supports both ordered and out-of-order reduction
  template <typename ReduceFunctor, typename ReduceResultType, typename T>
  class ReduceKernel
  {
      typedef QMap<int, IntermediateResults<T> > ResultsMap;
  
      const ReduceOptions reduceOptions;
  
      QMutex mutex;
      int progress, resultsMapSize, threadCount;
      ResultsMap resultsMap;
  
      bool canReduce(int begin) const
      {
          return (((reduceOptions & UnorderedReduce)
                   && progress == 0)
                  || ((reduceOptions & OrderedReduce)
                      && progress == begin));
      }
  
      void reduceResult(ReduceFunctor &reduce,
                        ReduceResultType &r,
                        const IntermediateResults<T> &result)
      {
          for (int i = 0; i < result.vector.size(); ++i) {
              reduce(r, result.vector.at(i));
          }
      }
  
      void reduceResults(ReduceFunctor &reduce,
                         ReduceResultType &r,
                         ResultsMap &map)
      {
          typename ResultsMap::iterator it = map.begin();
          while (it != map.end()) {
              reduceResult(reduce, r, it.value());
              ++it;
          }
      }
  
  public:
      ReduceKernel(ReduceOptions _reduceOptions)
          : reduceOptions(_reduceOptions), progress(0), resultsMapSize(0),
            threadCount(QThreadPool::globalInstance()->maxThreadCount())
      { }
  
      void runReduce(ReduceFunctor &reduce,
                     ReduceResultType &r,
                     const IntermediateResults<T> &result)
      {
          QMutexLocker locker(&mutex);
          if (!canReduce(result.begin)) {
              ++resultsMapSize;
              resultsMap.insert(result.begin, result);
              return;
          }
  
          if (reduceOptions & UnorderedReduce) {
              // UnorderedReduce
              progress = -1;
  
              // reduce this result
              locker.unlock();
              reduceResult(reduce, r, result);
              locker.relock();
  
              // reduce all stored results as well
              while (!resultsMap.isEmpty()) {
                  ResultsMap resultsMapCopy = resultsMap;
                  resultsMap.clear();
  
                  locker.unlock();
                  reduceResults(reduce, r, resultsMapCopy);
                  locker.relock();
  
                  resultsMapSize -= resultsMapCopy.size();
              }
  
              progress = 0;
          } else {
              // reduce this result
              locker.unlock();
              reduceResult(reduce, r, result);
              locker.relock();
  
              // OrderedReduce
              progress += result.end - result.begin;
  
              // reduce as many other results as possible
              typename ResultsMap::iterator it = resultsMap.begin();
              while (it != resultsMap.end()) {
                  if (it.value().begin != progress)
                      break;
  
                  locker.unlock();
                  reduceResult(reduce, r, it.value());
                  locker.relock();
  
                  --resultsMapSize;
                  progress += it.value().end - it.value().begin;
                  it = resultsMap.erase(it);
              }
          }
      }
  
      // final reduction
      void finish(ReduceFunctor &reduce, ReduceResultType &r)
      {
          reduceResults(reduce, r, resultsMap);
      }
  
      inline bool shouldThrottle()
      {
          return (resultsMapSize > (ReduceQueueThrottleLimit * threadCount));
      }
  
      inline bool shouldStartThread()
      {
          return (resultsMapSize <= (ReduceQueueStartLimit * threadCount));
      }
  };
  
  template <typename Sequence, typename Base, typename Functor1, typename Functor2>
  struct SequenceHolder2 : public Base
  {
      SequenceHolder2(const Sequence &_sequence,
                      Functor1 functor1,
                      Functor2 functor2,
                      ReduceOptions reduceOptions)
          : Base(_sequence.begin(), _sequence.end(), functor1, functor2, reduceOptions),
            sequence(_sequence)
      { }
  
      Sequence sequence;
  
      void finish() override
      {
          Base::finish();
          // Clear the sequence to make sure all temporaries are destroyed
          // before finished is signaled.
          sequence = Sequence();
      }
  };
  
  } // namespace QtConcurrent
  
  QT_END_NAMESPACE
  
  #endif // QT_NO_CONCURRENT
  
  #endif
  