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