1 //===----------------------------------------------------------------------===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 
9 #ifndef _LIBCPP___ALGORITHM_SAMPLE_H
10 #define _LIBCPP___ALGORITHM_SAMPLE_H
11 
12 #include <__algorithm/min.h>
13 #include <__assert>
14 #include <__config>
15 #include <__iterator/distance.h>
16 #include <__iterator/iterator_traits.h>
17 #include <__random/uniform_int_distribution.h>
18 #include <type_traits>
19 
20 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
21 #  pragma GCC system_header
22 #endif
23 
24 _LIBCPP_PUSH_MACROS
25 #include <__undef_macros>
26 
27 _LIBCPP_BEGIN_NAMESPACE_STD
28 
29 template <class _PopulationIterator, class _SampleIterator, class _Distance,
30           class _UniformRandomNumberGenerator>
31 _LIBCPP_INLINE_VISIBILITY
32 _SampleIterator __sample(_PopulationIterator __first,
33                          _PopulationIterator __last, _SampleIterator __output_iter,
34                          _Distance __n,
35                          _UniformRandomNumberGenerator & __g,
36                          input_iterator_tag) {
37 
38   _Distance __k = 0;
39   for (; __first != __last && __k < __n; ++__first, (void) ++__k)
40     __output_iter[__k] = *__first;
41   _Distance __sz = __k;
42   for (; __first != __last; ++__first, (void) ++__k) {
43     _Distance __r = uniform_int_distribution<_Distance>(0, __k)(__g);
44     if (__r < __sz)
45       __output_iter[__r] = *__first;
46   }
47   return __output_iter + _VSTD::min(__n, __k);
48 }
49 
50 template <class _PopulationIterator, class _SampleIterator, class _Distance,
51           class _UniformRandomNumberGenerator>
52 _LIBCPP_INLINE_VISIBILITY
53 _SampleIterator __sample(_PopulationIterator __first,
54                          _PopulationIterator __last, _SampleIterator __output_iter,
55                          _Distance __n,
56                          _UniformRandomNumberGenerator& __g,
57                          forward_iterator_tag) {
58   _Distance __unsampled_sz = _VSTD::distance(__first, __last);
59   for (__n = _VSTD::min(__n, __unsampled_sz); __n != 0; ++__first) {
60     _Distance __r = uniform_int_distribution<_Distance>(0, --__unsampled_sz)(__g);
61     if (__r < __n) {
62       *__output_iter++ = *__first;
63       --__n;
64     }
65   }
66   return __output_iter;
67 }
68 
69 template <class _PopulationIterator, class _SampleIterator, class _Distance,
70           class _UniformRandomNumberGenerator>
71 _LIBCPP_INLINE_VISIBILITY
72 _SampleIterator __sample(_PopulationIterator __first,
73                          _PopulationIterator __last, _SampleIterator __output_iter,
74                          _Distance __n, _UniformRandomNumberGenerator& __g) {
75   typedef typename iterator_traits<_PopulationIterator>::iterator_category
76         _PopCategory;
77   typedef typename iterator_traits<_PopulationIterator>::difference_type
78         _Difference;
79   static_assert(__is_cpp17_forward_iterator<_PopulationIterator>::value ||
80                 __is_cpp17_random_access_iterator<_SampleIterator>::value,
81                 "SampleIterator must meet the requirements of RandomAccessIterator");
82   typedef typename common_type<_Distance, _Difference>::type _CommonType;
83   _LIBCPP_ASSERT(__n >= 0, "N must be a positive number.");
84   return _VSTD::__sample(
85       __first, __last, __output_iter, _CommonType(__n),
86       __g, _PopCategory());
87 }
88 
89 #if _LIBCPP_STD_VER > 14
90 template <class _PopulationIterator, class _SampleIterator, class _Distance,
91           class _UniformRandomNumberGenerator>
92 inline _LIBCPP_INLINE_VISIBILITY
93 _SampleIterator sample(_PopulationIterator __first,
94                        _PopulationIterator __last, _SampleIterator __output_iter,
95                        _Distance __n, _UniformRandomNumberGenerator&& __g) {
96     return _VSTD::__sample(__first, __last, __output_iter, __n, __g);
97 }
98 #endif // _LIBCPP_STD_VER > 14
99 
100 _LIBCPP_END_NAMESPACE_STD
101 
102 _LIBCPP_POP_MACROS
103 
104 #endif // _LIBCPP___ALGORITHM_SAMPLE_H
105