mingw/lib/gcc/mingw32/4.3.3/include/c++/parallel/list_partition.h

   1 // -*- C++ -*-
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  30
  31 /** @file parallel/list_partition.h
  32  *  @brief Functionality to split sequence referenced by only input
  33  *  iterators.
  34  *  This file is a GNU parallel extension to the Standard C++ Library.
  35  */
  36
  37 // Written by Leonor Frias Moya and Johannes Singler.
  38
  39 #ifndef _GLIBCXX_PARALLEL_LIST_PARTITION_H
  40 #define _GLIBCXX_PARALLEL_LIST_PARTITION_H 1
  41
  42 #include <parallel/parallel.h>
  43 #include <vector>
  44
  45 namespace __gnu_parallel
  46 {
  47   /** @brief Shrinks and doubles the ranges.
  48    *  @param os_starts Start positions worked on (oversampled).
  49    *  @param count_to_two Counts up to 2.
  50    *  @param range_length Current length of a chunk.
  51    *  @param make_twice Whether the @c os_starts is allowed to be
  52    *  grown or not
  53    */
  54   template<typename InputIterator>
  55     void
  56     shrink_and_double(std::vector<InputIterator>& os_starts,
  57                       size_t& count_to_two, size_t& range_length,
  58                       const bool make_twice)
  59     {
  60       ++count_to_two;
  61       if (not make_twice or count_to_two < 2)
  62         shrink(os_starts, count_to_two, range_length);
  63       else
  64         {
  65           os_starts.resize((os_starts.size() - 1) * 2 + 1);
  66           count_to_two = 0;
  67         }
  68     }
  69
  70   /** @brief Combines two ranges into one and thus halves the number of ranges.
  71    *  @param os_starts Start positions worked on (oversampled).
  72    *  @param count_to_two Counts up to 2.
  73    *  @param range_length Current length of a chunk. */
  74   template<typename InputIterator>
  75     void
  76     shrink(std::vector<InputIterator>& os_starts, size_t& count_to_two,
  77            size_t& range_length)
  78     {
  79       for (typename std::vector<InputIterator>::size_type i = 0;
  80            i <= (os_starts.size() / 2); ++i)
  81         os_starts[i] = os_starts[i * 2];
  82       range_length *= 2;
  83     }
  84
  85   /** @brief Splits a sequence given by input iterators into parts of
  86    * almost equal size
  87    *
  88    *  The function needs only one pass over the sequence.
  89    *  @param begin Begin iterator of input sequence.
  90    *  @param end End iterator of input sequence.
  91    *  @param starts Start iterators for the resulting parts, dimension
  92    *  @c num_parts+1. For convenience, @c starts @c [num_parts]
  93    *  contains the end iterator of the sequence.
  94    *  @param lengths Length of the resulting parts.
  95    *  @param num_parts Number of parts to split the sequence into.
  96    *  @param f Functor to be applied to each element by traversing it
  97    *  @param oversampling Oversampling factor. If 0, then the
  98    *  partitions will differ in at most @f$ \sqrt{\mathrm{end} -
  99    *  \mathrm{begin}} @f$ elements. Otherwise, the ratio between the
 100    *  longest and the shortest part is bounded by @f$
 101    *  1/(\mathrm{oversampling} \cdot \mathrm{num\_parts}) @f$.
 102    *  @return Length of the whole sequence.
 103    */
 104   template<typename InputIterator, typename FunctorType>
 105     size_t
 106     list_partition(const InputIterator begin, const InputIterator end,
 107                    InputIterator* starts, size_t* lengths, const int num_parts,
 108                    FunctorType& f, int oversampling = 0)
 109     {
 110       bool make_twice = false;
 111
 112       // The resizing algorithm is chosen according to the oversampling factor.
 113       if (oversampling == 0)
 114         {
 115           make_twice = true;
 116           oversampling = 1;
 117         }
 118
 119       std::vector<InputIterator> os_starts(2 * oversampling * num_parts + 1);
 120
 121       os_starts[0]= begin;
 122       InputIterator prev = begin, it = begin;
 123       size_t dist_limit = 0, dist = 0;
 124       size_t cur = 1, next = 1;
 125       size_t range_length = 1;
 126       size_t count_to_two = 0;
 127       while (it != end)
 128         {
 129           cur = next;
 130           for (; cur < os_starts.size() and it != end; ++cur)
 131             {
 132               for (dist_limit += range_length;
 133                    dist < dist_limit and it != end; ++dist)
 134                 {
 135                   f(it);
 136                   ++it;
 137                 }
 138               os_starts[cur] = it;
 139             }
 140
 141           // Must compare for end and not cur < os_starts.size() , because
 142           // cur could be == os_starts.size() as well
 143           if (it == end)
 144             break;
 145
 146           shrink_and_double(os_starts, count_to_two, range_length, make_twice);
 147           next = os_starts.size() / 2 + 1;
 148         }
 149
 150       // Calculation of the parts (one must be extracted from current
 151       // because the partition beginning at end, consists only of
 152       // itself).
 153       size_t size_part = (cur - 1) / num_parts;
 154       int size_greater = static_cast<int>((cur - 1) % num_parts);
 155       starts[0] = os_starts[0];
 156
 157       size_t index = 0;
 158
 159       // Smallest partitions.
 160       for (int i = 1; i < (num_parts + 1 - size_greater); ++i)
 161         {
 162           lengths[i - 1] =  size_part * range_length;
 163           index += size_part;
 164           starts[i] = os_starts[index];
 165         }
 166
 167       // Biggest partitions.
 168       for (int i = num_parts + 1 - size_greater; i <= num_parts; ++i)
 169         {
 170           lengths[i - 1] =  (size_part+1) * range_length;
 171           index += (size_part+1);
 172           starts[i] = os_starts[index];
 173         }
 174
 175       // Correction of the end size (the end iteration has not finished).
 176       lengths[num_parts - 1] -= (dist_limit - dist);
 177
 178       return dist;
 179     }
 180 }
 181
 182 #endif