| //===----------------------------------------------------------------------===// |
| // |
| // 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 <__config> |
| #include <__algorithm/comp.h> |
| #include <__algorithm/comp_ref_type.h> |
| #include <__algorithm/sort.h> |
| #include <__iterator/iterator_traits.h> |
| #include <__utility/swap.h> |
| |
| #if defined(_LIBCPP_DEBUG_RANDOMIZE_UNSPECIFIED_STABILITY) |
| # include <__algorithm/shuffle.h> |
| #endif |
| |
| #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER) |
| #pragma GCC system_header |
| #endif |
| |
| _LIBCPP_BEGIN_NAMESPACE_STD |
| |
| template<class _Compare, class _RandomAccessIterator> |
| _LIBCPP_CONSTEXPR_AFTER_CXX11 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 _Compare, class _RandomAccessIterator> |
| _LIBCPP_CONSTEXPR_AFTER_CXX11 void |
| __nth_element(_RandomAccessIterator __first, _RandomAccessIterator __nth, _RandomAccessIterator __last, _Compare __comp) |
| { |
| // _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)) |
| swap(*__first, *__last); |
| return; |
| case 3: |
| { |
| _RandomAccessIterator __m = __first; |
| _VSTD::__sort3<_Compare>(__first, ++__m, --__last, __comp); |
| return; |
| } |
| } |
| if (__len <= __limit) |
| { |
| _VSTD::__selection_sort<_Compare>(__first, __last, __comp); |
| return; |
| } |
| // __len > __limit >= 3 |
| _RandomAccessIterator __m = __first + __len/2; |
| _RandomAccessIterator __lm1 = __last; |
| unsigned __n_swaps = _VSTD::__sort3<_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)) { |
| 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)) { |
| 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; |
| while (__comp(*__first, *--__j)) |
| ; |
| if (__i >= __j) |
| break; |
| 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; |
| // It is now known that a guard exists for downward moving __j |
| while (!__comp(*--__j, *__m)) |
| ; |
| if (__i >= __j) |
| break; |
| 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)) |
| { |
| 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 _RandomAccessIterator, class _Compare> |
| inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17 |
| void |
| nth_element(_RandomAccessIterator __first, _RandomAccessIterator __nth, _RandomAccessIterator __last, _Compare __comp) |
| { |
| _LIBCPP_DEBUG_RANDOMIZE_RANGE(__first, __last); |
| typedef typename __comp_ref_type<_Compare>::type _Comp_ref; |
| _VSTD::__nth_element<_Comp_ref>(__first, __nth, __last, __comp); |
| _LIBCPP_DEBUG_RANDOMIZE_RANGE(__first, __nth); |
| if (__nth != __last) { |
| _LIBCPP_DEBUG_RANDOMIZE_RANGE(++__nth, __last); |
| } |
| } |
| |
| template <class _RandomAccessIterator> |
| inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17 |
| void |
| nth_element(_RandomAccessIterator __first, _RandomAccessIterator __nth, _RandomAccessIterator __last) |
| { |
| _VSTD::nth_element(__first, __nth, __last, __less<typename iterator_traits<_RandomAccessIterator>::value_type>()); |
| } |
| |
| _LIBCPP_END_NAMESPACE_STD |
| |
| #endif // _LIBCPP___ALGORITHM_NTH_ELEMENT_H |
