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// -*- C++ -*-

// Copyright (C) 2007, 2008, 2009, 2010, 2011 Free Software Foundation, Inc.

//

// This file is part of the GNU ISO C++ Library.  This library is free

// software; you can redistribute it and/or modify it under the terms

// of the GNU General Public License as published by the Free Software

// Foundation; either version 3, or (at your option) any later

// version.

// This library is distributed in the hope that it will be useful, but

// WITHOUT ANY WARRANTY; without even the implied warranty of

// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU

// General Public License for more details.

// Under Section 7 of GPL version 3, you are granted additional

// permissions described in the GCC Runtime Library Exception, version

// 3.1, as published by the Free Software Foundation.

// You should have received a copy of the GNU General Public License and

// a copy of the GCC Runtime Library Exception along with this program;

// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see

// <http://www.gnu.org/licenses/>.

/** @file parallel/partial_sum.h

 *  @brief Parallel implementation of std::partial_sum(), i.e. prefix

*  sums.

 *  This file is a GNU parallel extension to the Standard C++ Library.

 */

// Written by Johannes Singler.

#ifndef _GLIBCXX_PARALLEL_PARTIAL_SUM_H

#define _GLIBCXX_PARALLEL_PARTIAL_SUM_H 1

#include <omp.h>

#include <new>

#include <bits/stl_algobase.h>

#include <parallel/parallel.h>

#include <parallel/numericfwd.h>

namespace __gnu_parallel

{

  // Problem: there is no 0-element given.

  /** @brief Base case prefix sum routine.

   *  @param __begin Begin iterator of input sequence.

   *  @param __end End iterator of input sequence.

   *  @param __result Begin iterator of output sequence.

   *  @param __bin_op Associative binary function.

   *  @param __value Start value. Must be passed since the neutral

   *  element is unknown in general.

   *  @return End iterator of output sequence. */

  template<typename _IIter,

           typename _OutputIterator,

           typename _BinaryOperation>

    _OutputIterator

    __parallel_partial_sum_basecase(_IIter __begin, _IIter __end,

                                    _OutputIterator __result,

                                    _BinaryOperation __bin_op,

      typename std::iterator_traits <_IIter>::value_type __value)

    {

      if (__begin == __end)

        return __result;

      while (__begin != __end)

        {

          __value = __bin_op(__value, *__begin);

          *__result = __value;

          ++__result;

          ++__begin;

        }

      return __result;

    }

  /** @brief Parallel partial sum implementation, two-phase approach,

      no recursion.

      *  @param __begin Begin iterator of input sequence.

      *  @param __end End iterator of input sequence.

      *  @param __result Begin iterator of output sequence.

      *  @param __bin_op Associative binary function.

      *  @param __n Length of sequence.

      *  @return End iterator of output sequence.

      */

  template<typename _IIter,

           typename _OutputIterator,

           typename _BinaryOperation>

    _OutputIterator

    __parallel_partial_sum_linear(_IIter __begin, _IIter __end,

                                  _OutputIterator __result,

                                  _BinaryOperation __bin_op,

      typename std::iterator_traits<_IIter>::difference_type __n)

    {

      typedef std::iterator_traits<_IIter> _TraitsType;

      typedef typename _TraitsType::value_type _ValueType;

      typedef typename _TraitsType::difference_type _DifferenceType;

      if (__begin == __end)

        return __result;

      _ThreadIndex __num_threads =

        std::min<_DifferenceType>(__get_max_threads(), __n - 1);

      if (__num_threads < 2)

        {

          *__result = *__begin;

          return __parallel_partial_sum_basecase(__begin + 1, __end,

                                                 __result + 1, __bin_op,

                                                 *__begin);

        }

      _DifferenceType* __borders;

      _ValueType* __sums;

      const _Settings& __s = _Settings::get();

#     pragma omp parallel num_threads(__num_threads)

      {

#       pragma omp single

        {

          __num_threads = omp_get_num_threads();

          __borders = new _DifferenceType[__num_threads + 2];

          if (__s.partial_sum_dilation == 1.0f)

            __equally_split(__n, __num_threads + 1, __borders);

          else

            {

              _DifferenceType __first_part_length =

                  std::max<_DifferenceType>(1,

                    __n / (1.0f + __s.partial_sum_dilation * __num_threads));

              _DifferenceType __chunk_length =

                  (__n - __first_part_length) / __num_threads;

              _DifferenceType __borderstart =

                  __n - __num_threads * __chunk_length;

              __borders[0] = 0;

              for (_ThreadIndex __i = 1; __i < (__num_threads + 1); ++__i)

                {

                  __borders[__i] = __borderstart;

                  __borderstart += __chunk_length;

                }

              __borders[__num_threads + 1] = __n;

            }

          __sums = static_cast<_ValueType*>(::operator new(sizeof(_ValueType)

                                                           * __num_threads));

          _OutputIterator __target_end;

        } //single

        _ThreadIndex __iam = omp_get_thread_num();

        if (__iam == 0)

          {

            *__result = *__begin;

            __parallel_partial_sum_basecase(__begin + 1,

                                            __begin + __borders[1],

                                            __result + 1,

                                            __bin_op, *__begin);

            ::new(&(__sums[__iam])) _ValueType(*(__result + __borders[1] - 1));

          }

        else

          {

            ::new(&(__sums[__iam]))

              _ValueType(__gnu_parallel::accumulate(

                                         __begin + __borders[__iam] + 1,

                                         __begin + __borders[__iam + 1],

                                         *(__begin + __borders[__iam]),

                                         __bin_op,

                                         __gnu_parallel::sequential_tag()));

          }

#       pragma omp barrier

#       pragma omp single

        __parallel_partial_sum_basecase(__sums + 1, __sums + __num_threads,

                                        __sums + 1, __bin_op, __sums[0]);

#       pragma omp barrier

        // Still same team.

        __parallel_partial_sum_basecase(__begin + __borders[__iam + 1],

                                        __begin + __borders[__iam + 2],

                                        __result + __borders[__iam + 1],

                                        __bin_op, __sums[__iam]);

      } //parallel

      for (_ThreadIndex __i = 0; __i < __num_threads; ++__i)

        __sums[__i].~_ValueType();

      ::operator delete(__sums);

      delete[] __borders;

      return __result + __n;

    }

  /** @brief Parallel partial sum front-__end.

   *  @param __begin Begin iterator of input sequence.

   *  @param __end End iterator of input sequence.

   *  @param __result Begin iterator of output sequence.

   *  @param __bin_op Associative binary function.

   *  @return End iterator of output sequence. */

  template<typename _IIter,

           typename _OutputIterator,

           typename _BinaryOperation>

    _OutputIterator

    __parallel_partial_sum(_IIter __begin, _IIter __end,

                           _OutputIterator __result, _BinaryOperation __bin_op)

    {

      _GLIBCXX_CALL(__begin - __end)

      typedef std::iterator_traits<_IIter> _TraitsType;

      typedef typename _TraitsType::value_type _ValueType;

      typedef typename _TraitsType::difference_type _DifferenceType;

      _DifferenceType __n = __end - __begin;

      switch (_Settings::get().partial_sum_algorithm)

        {

        case LINEAR:

          // Need an initial offset.

          return __parallel_partial_sum_linear(__begin, __end, __result,

                                               __bin_op, __n);

        default:

          // Partial_sum algorithm not implemented.

          _GLIBCXX_PARALLEL_ASSERT(0);

          return __result + __n;

        }

    }

}

#endif /* _GLIBCXX_PARALLEL_PARTIAL_SUM_H */