libstdc++
quicksort.h
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24 
25 /** @file parallel/quicksort.h
26  * @brief Implementation of a unbalanced parallel quicksort (in-place).
27  * This file is a GNU parallel extension to the Standard C++ Library.
28  */
29 
30 // Written by Johannes Singler.
31 
32 #ifndef _GLIBCXX_PARALLEL_QUICKSORT_H
33 #define _GLIBCXX_PARALLEL_QUICKSORT_H 1
34 
35 #include <parallel/parallel.h>
36 #include <parallel/partition.h>
37 
38 namespace __gnu_parallel
39 {
40  /** @brief Unbalanced quicksort divide step.
41  * @param __begin Begin iterator of subsequence.
42  * @param __end End iterator of subsequence.
43  * @param __comp Comparator.
44  * @param __pivot_rank Desired __rank of the pivot.
45  * @param __num_samples Choose pivot from that many samples.
46  * @param __num_threads Number of threads that are allowed to work on
47  * this part.
48  */
49  template<typename _RAIter, typename _Compare>
50  typename std::iterator_traits<_RAIter>::difference_type
51  __parallel_sort_qs_divide(_RAIter __begin, _RAIter __end,
52  _Compare __comp, typename std::iterator_traits
53  <_RAIter>::difference_type __pivot_rank,
54  typename std::iterator_traits
55  <_RAIter>::difference_type
56  __num_samples, _ThreadIndex __num_threads)
57  {
58  typedef std::iterator_traits<_RAIter> _TraitsType;
59  typedef typename _TraitsType::value_type _ValueType;
60  typedef typename _TraitsType::difference_type _DifferenceType;
61 
62  _DifferenceType __n = __end - __begin;
63  __num_samples = std::min(__num_samples, __n);
64 
65  // Allocate uninitialized, to avoid default constructor.
66  _ValueType* __samples = static_cast<_ValueType*>
67  (::operator new(__num_samples * sizeof(_ValueType)));
68 
69  for (_DifferenceType __s = 0; __s < __num_samples; ++__s)
70  {
71  const unsigned long long __index = static_cast<unsigned long long>
72  (__s) * __n / __num_samples;
73  ::new(&(__samples[__s])) _ValueType(__begin[__index]);
74  }
75 
76  __gnu_sequential::sort(__samples, __samples + __num_samples, __comp);
77 
78  _ValueType& __pivot = __samples[__pivot_rank * __num_samples / __n];
79 
81  __pred(__comp, __pivot);
82  _DifferenceType __split = __parallel_partition(__begin, __end,
83  __pred, __num_threads);
84 
85  for (_DifferenceType __s = 0; __s < __num_samples; ++__s)
86  __samples[__s].~_ValueType();
87  ::operator delete(__samples);
88 
89  return __split;
90  }
91 
92  /** @brief Unbalanced quicksort conquer step.
93  * @param __begin Begin iterator of subsequence.
94  * @param __end End iterator of subsequence.
95  * @param __comp Comparator.
96  * @param __num_threads Number of threads that are allowed to work on
97  * this part.
98  */
99  template<typename _RAIter, typename _Compare>
100  void
101  __parallel_sort_qs_conquer(_RAIter __begin, _RAIter __end,
102  _Compare __comp,
103  _ThreadIndex __num_threads)
104  {
105  typedef std::iterator_traits<_RAIter> _TraitsType;
106  typedef typename _TraitsType::value_type _ValueType;
107  typedef typename _TraitsType::difference_type _DifferenceType;
108 
109  if (__num_threads <= 1)
110  {
111  __gnu_sequential::sort(__begin, __end, __comp);
112  return;
113  }
114 
115  _DifferenceType __n = __end - __begin, __pivot_rank;
116 
117  if (__n <= 1)
118  return;
119 
120  _ThreadIndex __num_threads_left;
121 
122  if ((__num_threads % 2) == 1)
123  __num_threads_left = __num_threads / 2 + 1;
124  else
125  __num_threads_left = __num_threads / 2;
126 
127  __pivot_rank = __n * __num_threads_left / __num_threads;
128 
129  _DifferenceType __split = __parallel_sort_qs_divide
130  (__begin, __end, __comp, __pivot_rank,
131  _Settings::get().sort_qs_num_samples_preset, __num_threads);
132 
133 #pragma omp parallel sections num_threads(2)
134  {
135 #pragma omp section
136  __parallel_sort_qs_conquer(__begin, __begin + __split,
137  __comp, __num_threads_left);
138 #pragma omp section
139  __parallel_sort_qs_conquer(__begin + __split, __end,
140  __comp, __num_threads - __num_threads_left);
141  }
142  }
143 
144 
145  /** @brief Unbalanced quicksort main call.
146  * @param __begin Begin iterator of input sequence.
147  * @param __end End iterator input sequence, ignored.
148  * @param __comp Comparator.
149  * @param __num_threads Number of threads that are allowed to work on
150  * this part.
151  */
152  template<typename _RAIter, typename _Compare>
153  void
154  __parallel_sort_qs(_RAIter __begin, _RAIter __end,
155  _Compare __comp,
156  _ThreadIndex __num_threads)
157  {
158  _GLIBCXX_CALL(__n)
159 
160  typedef std::iterator_traits<_RAIter> _TraitsType;
161  typedef typename _TraitsType::value_type _ValueType;
162  typedef typename _TraitsType::difference_type _DifferenceType;
163 
164  _DifferenceType __n = __end - __begin;
165 
166  // At least one element per processor.
167  if (__num_threads > __n)
168  __num_threads = static_cast<_ThreadIndex>(__n);
169 
171  __begin, __begin + __n, __comp, __num_threads);
172  }
173 
174 } //namespace __gnu_parallel
175 
176 #endif /* _GLIBCXX_PARALLEL_QUICKSORT_H */