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_NTH_ELEMENT_H
10 #define _LIBCPP___ALGORITHM_NTH_ELEMENT_H
11 
12 #include <__algorithm/comp.h>
13 #include <__algorithm/comp_ref_type.h>
14 #include <__algorithm/iterator_operations.h>
15 #include <__algorithm/sort.h>
16 #include <__config>
17 #include <__debug>
18 #include <__debug_utils/randomize_range.h>
19 #include <__iterator/iterator_traits.h>
20 #include <__utility/move.h>
21 
22 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
23 #  pragma GCC system_header
24 #endif
25 
26 _LIBCPP_BEGIN_NAMESPACE_STD
27 
28 template<class _Compare, class _RandomAccessIterator>
29 _LIBCPP_CONSTEXPR_AFTER_CXX11 bool
30 __nth_element_find_guard(_RandomAccessIterator& __i, _RandomAccessIterator& __j,
31                          _RandomAccessIterator __m, _Compare __comp)
32 {
33     // manually guard downward moving __j against __i
34     while (true) {
35         if (__i == --__j) {
36             return false;
37         }
38         if (__comp(*__j, *__m)) {
39             return true;  // found guard for downward moving __j, now use unguarded partition
40         }
41     }
42 }
43 
44 template <class _AlgPolicy, class _Compare, class _RandomAccessIterator>
45 _LIBCPP_CONSTEXPR_AFTER_CXX11 void
46 __nth_element(_RandomAccessIterator __first, _RandomAccessIterator __nth, _RandomAccessIterator __last, _Compare __comp)
47 {
48     using _Ops = _IterOps<_AlgPolicy>;
49 
50     // _Compare is known to be a reference type
51     typedef typename iterator_traits<_RandomAccessIterator>::difference_type difference_type;
52     const difference_type __limit = 7;
53     while (true)
54     {
55         if (__nth == __last)
56             return;
57         difference_type __len = __last - __first;
58         switch (__len)
59         {
60         case 0:
61         case 1:
62             return;
63         case 2:
64             if (__comp(*--__last, *__first))
65                 _Ops::iter_swap(__first, __last);
66             return;
67         case 3:
68             {
69             _RandomAccessIterator __m = __first;
70             std::__sort3<_AlgPolicy, _Compare>(__first, ++__m, --__last, __comp);
71             return;
72             }
73         }
74         if (__len <= __limit)
75         {
76             std::__selection_sort<_AlgPolicy, _Compare>(__first, __last, __comp);
77             return;
78         }
79         // __len > __limit >= 3
80         _RandomAccessIterator __m = __first + __len/2;
81         _RandomAccessIterator __lm1 = __last;
82         unsigned __n_swaps = std::__sort3<_AlgPolicy, _Compare>(__first, __m, --__lm1, __comp);
83         // *__m is median
84         // partition [__first, __m) < *__m and *__m <= [__m, __last)
85         // (this inhibits tossing elements equivalent to __m around unnecessarily)
86         _RandomAccessIterator __i = __first;
87         _RandomAccessIterator __j = __lm1;
88         // j points beyond range to be tested, *__lm1 is known to be <= *__m
89         // The search going up is known to be guarded but the search coming down isn't.
90         // Prime the downward search with a guard.
91         if (!__comp(*__i, *__m))  // if *__first == *__m
92         {
93             // *__first == *__m, *__first doesn't go in first part
94             if (_VSTD::__nth_element_find_guard<_Compare>(__i, __j, __m, __comp)) {
95                 _Ops::iter_swap(__i, __j);
96                 ++__n_swaps;
97             } else {
98                 // *__first == *__m, *__m <= all other elements
99                 // Partition instead into [__first, __i) == *__first and *__first < [__i, __last)
100                 ++__i;  // __first + 1
101                 __j = __last;
102                 if (!__comp(*__first, *--__j)) {  // we need a guard if *__first == *(__last-1)
103                     while (true) {
104                         if (__i == __j) {
105                             return;  // [__first, __last) all equivalent elements
106                         } else if (__comp(*__first, *__i)) {
107                             _Ops::iter_swap(__i, __j);
108                             ++__n_swaps;
109                             ++__i;
110                             break;
111                         }
112                         ++__i;
113                     }
114                 }
115                 // [__first, __i) == *__first and *__first < [__j, __last) and __j == __last - 1
116                 if (__i == __j) {
117                     return;
118                 }
119                 while (true) {
120                     while (!__comp(*__first, *__i))
121                         ++__i;
122                     while (__comp(*__first, *--__j))
123                         ;
124                     if (__i >= __j)
125                         break;
126                     _Ops::iter_swap(__i, __j);
127                     ++__n_swaps;
128                     ++__i;
129                 }
130                 // [__first, __i) == *__first and *__first < [__i, __last)
131                 // The first part is sorted,
132                 if (__nth < __i) {
133                     return;
134                 }
135                 // __nth_element the second part
136                 // _VSTD::__nth_element<_Compare>(__i, __nth, __last, __comp);
137                 __first = __i;
138                 continue;
139             }
140         }
141         ++__i;
142         // j points beyond range to be tested, *__lm1 is known to be <= *__m
143         // if not yet partitioned...
144         if (__i < __j)
145         {
146             // known that *(__i - 1) < *__m
147             while (true)
148             {
149                 // __m still guards upward moving __i
150                 while (__comp(*__i, *__m))
151                     ++__i;
152                 // It is now known that a guard exists for downward moving __j
153                 while (!__comp(*--__j, *__m))
154                     ;
155                 if (__i >= __j)
156                     break;
157                 _Ops::iter_swap(__i, __j);
158                 ++__n_swaps;
159                 // It is known that __m != __j
160                 // If __m just moved, follow it
161                 if (__m == __i)
162                     __m = __j;
163                 ++__i;
164             }
165         }
166         // [__first, __i) < *__m and *__m <= [__i, __last)
167         if (__i != __m && __comp(*__m, *__i))
168         {
169             _Ops::iter_swap(__i, __m);
170             ++__n_swaps;
171         }
172         // [__first, __i) < *__i and *__i <= [__i+1, __last)
173         if (__nth == __i)
174             return;
175         if (__n_swaps == 0)
176         {
177             // We were given a perfectly partitioned sequence.  Coincidence?
178             if (__nth < __i)
179             {
180                 // Check for [__first, __i) already sorted
181                 __j = __m = __first;
182                 while (true) {
183                     if (++__j == __i) {
184                         // [__first, __i) sorted
185                         return;
186                     }
187                     if (__comp(*__j, *__m)) {
188                         // not yet sorted, so sort
189                         break;
190                     }
191                     __m = __j;
192                 }
193             }
194             else
195             {
196                 // Check for [__i, __last) already sorted
197                 __j = __m = __i;
198                 while (true) {
199                     if (++__j == __last) {
200                         // [__i, __last) sorted
201                         return;
202                     }
203                     if (__comp(*__j, *__m)) {
204                         // not yet sorted, so sort
205                         break;
206                     }
207                     __m = __j;
208                 }
209             }
210         }
211         // __nth_element on range containing __nth
212         if (__nth < __i)
213         {
214             // _VSTD::__nth_element<_Compare>(__first, __nth, __i, __comp);
215             __last = __i;
216         }
217         else
218         {
219             // _VSTD::__nth_element<_Compare>(__i+1, __nth, __last, __comp);
220             __first = ++__i;
221         }
222     }
223 }
224 
225 template <class _AlgPolicy, class _RandomAccessIterator, class _Compare>
226 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX17
227 void __nth_element_impl(_RandomAccessIterator __first, _RandomAccessIterator __nth, _RandomAccessIterator __last,
228                         _Compare& __comp) {
229   if (__nth == __last)
230     return;
231 
232   std::__debug_randomize_range<_AlgPolicy>(__first, __last);
233 
234   using _Comp_ref = typename __comp_ref_type<_Compare>::type;
235   std::__nth_element<_AlgPolicy, _Comp_ref>(__first, __nth, __last, __comp);
236 
237   std::__debug_randomize_range<_AlgPolicy>(__first, __nth);
238   if (__nth != __last) {
239     std::__debug_randomize_range<_AlgPolicy>(++__nth, __last);
240   }
241 }
242 
243 template <class _RandomAccessIterator, class _Compare>
244 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX17
245 void nth_element(_RandomAccessIterator __first, _RandomAccessIterator __nth, _RandomAccessIterator __last,
246                  _Compare __comp) {
247   std::__nth_element_impl<_ClassicAlgPolicy>(std::move(__first), std::move(__nth), std::move(__last), __comp);
248 }
249 
250 template <class _RandomAccessIterator>
251 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX17
252 void nth_element(_RandomAccessIterator __first, _RandomAccessIterator __nth, _RandomAccessIterator __last) {
253   std::nth_element(std::move(__first), std::move(__nth), std::move(__last), __less<typename
254       iterator_traits<_RandomAccessIterator>::value_type>());
255 }
256 
257 _LIBCPP_END_NAMESPACE_STD
258 
259 #endif // _LIBCPP___ALGORITHM_NTH_ELEMENT_H
260