nth_element.h revision 1.1.1.1 
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 <__config>
13#include <__algorithm/comp.h>
14#include <__algorithm/comp_ref_type.h>
15#include <__algorithm/sort.h>
16#include <__iterator/iterator_traits.h>
17#include <__utility/swap.h>
18
19#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
20#pragma GCC system_header
21#endif
22
23_LIBCPP_PUSH_MACROS
24#include <__undef_macros>
25
26_LIBCPP_BEGIN_NAMESPACE_STD
27
28template<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
44template <class _Compare, class _RandomAccessIterator>
45_LIBCPP_CONSTEXPR_AFTER_CXX11 void
46__nth_element(_RandomAccessIterator __first, _RandomAccessIterator __nth, _RandomAccessIterator __last, _Compare __comp)
47{
48    // _Compare is known to be a reference type
49    typedef typename iterator_traits<_RandomAccessIterator>::difference_type difference_type;
50    const difference_type __limit = 7;
51    while (true)
52    {
53        if (__nth == __last)
54            return;
55        difference_type __len = __last - __first;
56        switch (__len)
57        {
58        case 0:
59        case 1:
60            return;
61        case 2:
62            if (__comp(*--__last, *__first))
63                swap(*__first, *__last);
64            return;
65        case 3:
66            {
67            _RandomAccessIterator __m = __first;
68            _VSTD::__sort3<_Compare>(__first, ++__m, --__last, __comp);
69            return;
70            }
71        }
72        if (__len <= __limit)
73        {
74            _VSTD::__selection_sort<_Compare>(__first, __last, __comp);
75            return;
76        }
77        // __len > __limit >= 3
78        _RandomAccessIterator __m = __first + __len/2;
79        _RandomAccessIterator __lm1 = __last;
80        unsigned __n_swaps = _VSTD::__sort3<_Compare>(__first, __m, --__lm1, __comp);
81        // *__m is median
82        // partition [__first, __m) < *__m and *__m <= [__m, __last)
83        // (this inhibits tossing elements equivalent to __m around unnecessarily)
84        _RandomAccessIterator __i = __first;
85        _RandomAccessIterator __j = __lm1;
86        // j points beyond range to be tested, *__lm1 is known to be <= *__m
87        // The search going up is known to be guarded but the search coming down isn't.
88        // Prime the downward search with a guard.
89        if (!__comp(*__i, *__m))  // if *__first == *__m
90        {
91            // *__first == *__m, *__first doesn't go in first part
92            if (_VSTD::__nth_element_find_guard<_Compare>(__i, __j, __m, __comp)) {
93                swap(*__i, *__j);
94                ++__n_swaps;
95            } else {
96                // *__first == *__m, *__m <= all other elements
97                // Partition instead into [__first, __i) == *__first and *__first < [__i, __last)
98                ++__i;  // __first + 1
99                __j = __last;
100                if (!__comp(*__first, *--__j)) {  // we need a guard if *__first == *(__last-1)
101                    while (true) {
102                        if (__i == __j) {
103                            return;  // [__first, __last) all equivalent elements
104                        } else if (__comp(*__first, *__i)) {
105                            swap(*__i, *__j);
106                            ++__n_swaps;
107                            ++__i;
108                            break;
109                        }
110                        ++__i;
111                    }
112                }
113                // [__first, __i) == *__first and *__first < [__j, __last) and __j == __last - 1
114                if (__i == __j) {
115                    return;
116                }
117                while (true) {
118                    while (!__comp(*__first, *__i))
119                        ++__i;
120                    while (__comp(*__first, *--__j))
121                        ;
122                    if (__i >= __j)
123                        break;
124                    swap(*__i, *__j);
125                    ++__n_swaps;
126                    ++__i;
127                }
128                // [__first, __i) == *__first and *__first < [__i, __last)
129                // The first part is sorted,
130                if (__nth < __i) {
131                    return;
132                }
133                // __nth_element the second part
134                // _VSTD::__nth_element<_Compare>(__i, __nth, __last, __comp);
135                __first = __i;
136                continue;
137            }
138        }
139        ++__i;
140        // j points beyond range to be tested, *__lm1 is known to be <= *__m
141        // if not yet partitioned...
142        if (__i < __j)
143        {
144            // known that *(__i - 1) < *__m
145            while (true)
146            {
147                // __m still guards upward moving __i
148                while (__comp(*__i, *__m))
149                    ++__i;
150                // It is now known that a guard exists for downward moving __j
151                while (!__comp(*--__j, *__m))
152                    ;
153                if (__i >= __j)
154                    break;
155                swap(*__i, *__j);
156                ++__n_swaps;
157                // It is known that __m != __j
158                // If __m just moved, follow it
159                if (__m == __i)
160                    __m = __j;
161                ++__i;
162            }
163        }
164        // [__first, __i) < *__m and *__m <= [__i, __last)
165        if (__i != __m && __comp(*__m, *__i))
166        {
167            swap(*__i, *__m);
168            ++__n_swaps;
169        }
170        // [__first, __i) < *__i and *__i <= [__i+1, __last)
171        if (__nth == __i)
172            return;
173        if (__n_swaps == 0)
174        {
175            // We were given a perfectly partitioned sequence.  Coincidence?
176            if (__nth < __i)
177            {
178                // Check for [__first, __i) already sorted
179                __j = __m = __first;
180                while (true) {
181                    if (++__j == __i) {
182                        // [__first, __i) sorted
183                        return;
184                    }
185                    if (__comp(*__j, *__m)) {
186                        // not yet sorted, so sort
187                        break;
188                    }
189                    __m = __j;
190                }
191            }
192            else
193            {
194                // Check for [__i, __last) already sorted
195                __j = __m = __i;
196                while (true) {
197                    if (++__j == __last) {
198                        // [__i, __last) sorted
199                        return;
200                    }
201                    if (__comp(*__j, *__m)) {
202                        // not yet sorted, so sort
203                        break;
204                    }
205                    __m = __j;
206                }
207            }
208        }
209        // __nth_element on range containing __nth
210        if (__nth < __i)
211        {
212            // _VSTD::__nth_element<_Compare>(__first, __nth, __i, __comp);
213            __last = __i;
214        }
215        else
216        {
217            // _VSTD::__nth_element<_Compare>(__i+1, __nth, __last, __comp);
218            __first = ++__i;
219        }
220    }
221}
222
223template <class _RandomAccessIterator, class _Compare>
224inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
225void
226nth_element(_RandomAccessIterator __first, _RandomAccessIterator __nth, _RandomAccessIterator __last, _Compare __comp)
227{
228    typedef typename __comp_ref_type<_Compare>::type _Comp_ref;
229    _VSTD::__nth_element<_Comp_ref>(__first, __nth, __last, __comp);
230}
231
232template <class _RandomAccessIterator>
233inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
234void
235nth_element(_RandomAccessIterator __first, _RandomAccessIterator __nth, _RandomAccessIterator __last)
236{
237    _VSTD::nth_element(__first, __nth, __last, __less<typename iterator_traits<_RandomAccessIterator>::value_type>());
238}
239
240_LIBCPP_END_NAMESPACE_STD
241
242_LIBCPP_POP_MACROS
243
244#endif // _LIBCPP___ALGORITHM_NTH_ELEMENT_H
245