1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
|
//===----------------------------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
#ifndef _LIBCPP___ALGORITHM_NTH_ELEMENT_H
#define _LIBCPP___ALGORITHM_NTH_ELEMENT_H
#include <__algorithm/comp.h>
#include <__algorithm/comp_ref_type.h>
#include <__algorithm/iterator_operations.h>
#include <__algorithm/sort.h>
#include <__assert>
#include <__config>
#include <__debug_utils/randomize_range.h>
#include <__iterator/iterator_traits.h>
#include <__utility/move.h>
#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
# pragma GCC system_header
#endif
_LIBCPP_BEGIN_NAMESPACE_STD
template<class _Compare, class _RandomAccessIterator>
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 bool
__nth_element_find_guard(_RandomAccessIterator& __i, _RandomAccessIterator& __j,
_RandomAccessIterator __m, _Compare __comp)
{
// manually guard downward moving __j against __i
while (true) {
if (__i == --__j) {
return false;
}
if (__comp(*__j, *__m)) {
return true; // found guard for downward moving __j, now use unguarded partition
}
}
}
template <class _AlgPolicy, class _Compare, class _RandomAccessIterator>
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 void
// NOLINTNEXTLINE(readability-function-cognitive-complexity)
__nth_element(_RandomAccessIterator __first, _RandomAccessIterator __nth, _RandomAccessIterator __last, _Compare __comp)
{
using _Ops = _IterOps<_AlgPolicy>;
// _Compare is known to be a reference type
typedef typename iterator_traits<_RandomAccessIterator>::difference_type difference_type;
const difference_type __limit = 7;
while (true)
{
if (__nth == __last)
return;
difference_type __len = __last - __first;
switch (__len)
{
case 0:
case 1:
return;
case 2:
if (__comp(*--__last, *__first))
_Ops::iter_swap(__first, __last);
return;
case 3:
{
_RandomAccessIterator __m = __first;
std::__sort3<_AlgPolicy, _Compare>(__first, ++__m, --__last, __comp);
return;
}
}
if (__len <= __limit)
{
std::__selection_sort<_AlgPolicy, _Compare>(__first, __last, __comp);
return;
}
// __len > __limit >= 3
_RandomAccessIterator __m = __first + __len/2;
_RandomAccessIterator __lm1 = __last;
unsigned __n_swaps = std::__sort3<_AlgPolicy, _Compare>(__first, __m, --__lm1, __comp);
// *__m is median
// partition [__first, __m) < *__m and *__m <= [__m, __last)
// (this inhibits tossing elements equivalent to __m around unnecessarily)
_RandomAccessIterator __i = __first;
_RandomAccessIterator __j = __lm1;
// j points beyond range to be tested, *__lm1 is known to be <= *__m
// The search going up is known to be guarded but the search coming down isn't.
// Prime the downward search with a guard.
if (!__comp(*__i, *__m)) // if *__first == *__m
{
// *__first == *__m, *__first doesn't go in first part
if (_VSTD::__nth_element_find_guard<_Compare>(__i, __j, __m, __comp)) {
_Ops::iter_swap(__i, __j);
++__n_swaps;
} else {
// *__first == *__m, *__m <= all other elements
// Partition instead into [__first, __i) == *__first and *__first < [__i, __last)
++__i; // __first + 1
__j = __last;
if (!__comp(*__first, *--__j)) { // we need a guard if *__first == *(__last-1)
while (true) {
if (__i == __j) {
return; // [__first, __last) all equivalent elements
} else if (__comp(*__first, *__i)) {
_Ops::iter_swap(__i, __j);
++__n_swaps;
++__i;
break;
}
++__i;
}
}
// [__first, __i) == *__first and *__first < [__j, __last) and __j == __last - 1
if (__i == __j) {
return;
}
while (true) {
while (!__comp(*__first, *__i)) {
++__i;
_LIBCPP_ASSERT_UNCATEGORIZED(
__i != __last,
"Would read out of bounds, does your comparator satisfy the strict-weak ordering requirement?");
}
do {
_LIBCPP_ASSERT_UNCATEGORIZED(
__j != __first,
"Would read out of bounds, does your comparator satisfy the strict-weak ordering requirement?");
--__j;
} while (__comp(*__first, *__j));
if (__i >= __j)
break;
_Ops::iter_swap(__i, __j);
++__n_swaps;
++__i;
}
// [__first, __i) == *__first and *__first < [__i, __last)
// The first part is sorted,
if (__nth < __i) {
return;
}
// __nth_element the second part
// _VSTD::__nth_element<_Compare>(__i, __nth, __last, __comp);
__first = __i;
continue;
}
}
++__i;
// j points beyond range to be tested, *__lm1 is known to be <= *__m
// if not yet partitioned...
if (__i < __j)
{
// known that *(__i - 1) < *__m
while (true)
{
// __m still guards upward moving __i
while (__comp(*__i, *__m)) {
++__i;
_LIBCPP_ASSERT_UNCATEGORIZED(
__i != __last,
"Would read out of bounds, does your comparator satisfy the strict-weak ordering requirement?");
}
// It is now known that a guard exists for downward moving __j
do {
_LIBCPP_ASSERT_UNCATEGORIZED(
__j != __first,
"Would read out of bounds, does your comparator satisfy the strict-weak ordering requirement?");
--__j;
} while (!__comp(*__j, *__m));
if (__i >= __j)
break;
_Ops::iter_swap(__i, __j);
++__n_swaps;
// It is known that __m != __j
// If __m just moved, follow it
if (__m == __i)
__m = __j;
++__i;
}
}
// [__first, __i) < *__m and *__m <= [__i, __last)
if (__i != __m && __comp(*__m, *__i))
{
_Ops::iter_swap(__i, __m);
++__n_swaps;
}
// [__first, __i) < *__i and *__i <= [__i+1, __last)
if (__nth == __i)
return;
if (__n_swaps == 0)
{
// We were given a perfectly partitioned sequence. Coincidence?
if (__nth < __i)
{
// Check for [__first, __i) already sorted
__j = __m = __first;
while (true) {
if (++__j == __i) {
// [__first, __i) sorted
return;
}
if (__comp(*__j, *__m)) {
// not yet sorted, so sort
break;
}
__m = __j;
}
}
else
{
// Check for [__i, __last) already sorted
__j = __m = __i;
while (true) {
if (++__j == __last) {
// [__i, __last) sorted
return;
}
if (__comp(*__j, *__m)) {
// not yet sorted, so sort
break;
}
__m = __j;
}
}
}
// __nth_element on range containing __nth
if (__nth < __i)
{
// _VSTD::__nth_element<_Compare>(__first, __nth, __i, __comp);
__last = __i;
}
else
{
// _VSTD::__nth_element<_Compare>(__i+1, __nth, __last, __comp);
__first = ++__i;
}
}
}
template <class _AlgPolicy, class _RandomAccessIterator, class _Compare>
inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20
void __nth_element_impl(_RandomAccessIterator __first, _RandomAccessIterator __nth, _RandomAccessIterator __last,
_Compare& __comp) {
if (__nth == __last)
return;
std::__debug_randomize_range<_AlgPolicy>(__first, __last);
std::__nth_element<_AlgPolicy, __comp_ref_type<_Compare> >(__first, __nth, __last, __comp);
std::__debug_randomize_range<_AlgPolicy>(__first, __nth);
if (__nth != __last) {
std::__debug_randomize_range<_AlgPolicy>(++__nth, __last);
}
}
template <class _RandomAccessIterator, class _Compare>
inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20
void nth_element(_RandomAccessIterator __first, _RandomAccessIterator __nth, _RandomAccessIterator __last,
_Compare __comp) {
std::__nth_element_impl<_ClassicAlgPolicy>(std::move(__first), std::move(__nth), std::move(__last), __comp);
}
template <class _RandomAccessIterator>
inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20
void nth_element(_RandomAccessIterator __first, _RandomAccessIterator __nth, _RandomAccessIterator __last) {
std::nth_element(std::move(__first), std::move(__nth), std::move(__last), __less<>());
}
_LIBCPP_END_NAMESPACE_STD
#endif // _LIBCPP___ALGORITHM_NTH_ELEMENT_H
|
