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_SET_INTERSECTION_H 10 #define _LIBCPP___ALGORITHM_SET_INTERSECTION_H 11 12 #include <__algorithm/comp.h> 13 #include <__algorithm/comp_ref_type.h> 14 #include <__algorithm/iterator_operations.h> 15 #include <__algorithm/lower_bound.h> 16 #include <__config> 17 #include <__functional/identity.h> 18 #include <__iterator/iterator_traits.h> 19 #include <__iterator/next.h> 20 #include <__type_traits/is_same.h> 21 #include <__utility/exchange.h> 22 #include <__utility/forward.h> 23 #include <__utility/move.h> 24 #include <__utility/swap.h> 25 26 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER) 27 # pragma GCC system_header 28 #endif 29 30 _LIBCPP_PUSH_MACROS 31 #include <__undef_macros> 32 33 _LIBCPP_BEGIN_NAMESPACE_STD 34 35 template <class _InIter1, class _InIter2, class _OutIter> 36 struct __set_intersection_result { 37 _InIter1 __in1_; 38 _InIter2 __in2_; 39 _OutIter __out_; 40 41 // need a constructor as C++03 aggregate init is hard 42 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 43 __set_intersection_result(_InIter1&& __in_iter1, _InIter2&& __in_iter2, _OutIter&& __out_iter) 44 : __in1_(std::move(__in_iter1)), __in2_(std::move(__in_iter2)), __out_(std::move(__out_iter)) {} 45 }; 46 47 // Helper for __set_intersection() with one-sided binary search: populate result and advance input iterators if they 48 // are found to potentially contain the same value in two consecutive calls. This function is very intimately related to 49 // the way it is used and doesn't attempt to abstract that, it's not appropriate for general usage outside of its 50 // context. 51 template <class _InForwardIter1, class _InForwardIter2, class _OutIter> 52 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void __set_intersection_add_output_if_equal( 53 bool __may_be_equal, 54 _InForwardIter1& __first1, 55 _InForwardIter2& __first2, 56 _OutIter& __result, 57 bool& __prev_may_be_equal) { 58 if (__may_be_equal && __prev_may_be_equal) { 59 *__result = *__first1; 60 ++__result; 61 ++__first1; 62 ++__first2; 63 __prev_may_be_equal = false; 64 } else { 65 __prev_may_be_equal = __may_be_equal; 66 } 67 } 68 69 // With forward iterators we can make multiple passes over the data, allowing the use of one-sided binary search to 70 // reduce best-case complexity to log(N). Understanding how we can use binary search and still respect complexity 71 // guarantees is _not_ straightforward: the guarantee is "at most 2*(N+M)-1 comparisons", and one-sided binary search 72 // will necessarily overshoot depending on the position of the needle in the haystack -- for instance, if we're 73 // searching for 3 in (1, 2, 3, 4), we'll check if 3<1, then 3<2, then 3<4, and, finally, 3<3, for a total of 4 74 // comparisons, when linear search would have yielded 3. However, because we won't need to perform the intervening 75 // reciprocal comparisons (ie 1<3, 2<3, 4<3), that extra comparison doesn't run afoul of the guarantee. Additionally, 76 // this type of scenario can only happen for match distances of up to 5 elements, because 2*log2(8) is 6, and we'll 77 // still be worse-off at position 5 of an 8-element set. From then onwards these scenarios can't happen. TL;DR: we'll be 78 // 1 comparison worse-off compared to the classic linear-searching algorithm if matching position 3 of a set with 4 79 // elements, or position 5 if the set has 7 or 8 elements, but we'll never exceed the complexity guarantees from the 80 // standard. 81 template <class _AlgPolicy, 82 class _Compare, 83 class _InForwardIter1, 84 class _Sent1, 85 class _InForwardIter2, 86 class _Sent2, 87 class _OutIter> 88 [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI 89 _LIBCPP_CONSTEXPR_SINCE_CXX20 __set_intersection_result<_InForwardIter1, _InForwardIter2, _OutIter> 90 __set_intersection( 91 _InForwardIter1 __first1, 92 _Sent1 __last1, 93 _InForwardIter2 __first2, 94 _Sent2 __last2, 95 _OutIter __result, 96 _Compare&& __comp, 97 std::forward_iterator_tag, 98 std::forward_iterator_tag) { 99 _LIBCPP_CONSTEXPR std::__identity __proj; 100 bool __prev_may_be_equal = false; 101 102 while (__first2 != __last2) { 103 _InForwardIter1 __first1_next = 104 std::__lower_bound_onesided<_AlgPolicy>(__first1, __last1, *__first2, __comp, __proj); 105 std::swap(__first1_next, __first1); 106 // keeping in mind that a==b iff !(a<b) && !(b<a): 107 // if we can't advance __first1, that means !(*__first1 < *_first2), therefore __may_be_equal==true 108 std::__set_intersection_add_output_if_equal( 109 __first1 == __first1_next, __first1, __first2, __result, __prev_may_be_equal); 110 if (__first1 == __last1) 111 break; 112 113 _InForwardIter2 __first2_next = 114 std::__lower_bound_onesided<_AlgPolicy>(__first2, __last2, *__first1, __comp, __proj); 115 std::swap(__first2_next, __first2); 116 std::__set_intersection_add_output_if_equal( 117 __first2 == __first2_next, __first1, __first2, __result, __prev_may_be_equal); 118 } 119 return __set_intersection_result<_InForwardIter1, _InForwardIter2, _OutIter>( 120 _IterOps<_AlgPolicy>::next(std::move(__first1), std::move(__last1)), 121 _IterOps<_AlgPolicy>::next(std::move(__first2), std::move(__last2)), 122 std::move(__result)); 123 } 124 125 // input iterators are not suitable for multipass algorithms, so we stick to the classic single-pass version 126 template <class _AlgPolicy, 127 class _Compare, 128 class _InInputIter1, 129 class _Sent1, 130 class _InInputIter2, 131 class _Sent2, 132 class _OutIter> 133 [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI 134 _LIBCPP_CONSTEXPR_SINCE_CXX20 __set_intersection_result<_InInputIter1, _InInputIter2, _OutIter> 135 __set_intersection( 136 _InInputIter1 __first1, 137 _Sent1 __last1, 138 _InInputIter2 __first2, 139 _Sent2 __last2, 140 _OutIter __result, 141 _Compare&& __comp, 142 std::input_iterator_tag, 143 std::input_iterator_tag) { 144 while (__first1 != __last1 && __first2 != __last2) { 145 if (__comp(*__first1, *__first2)) 146 ++__first1; 147 else { 148 if (!__comp(*__first2, *__first1)) { 149 *__result = *__first1; 150 ++__result; 151 ++__first1; 152 } 153 ++__first2; 154 } 155 } 156 157 return __set_intersection_result<_InInputIter1, _InInputIter2, _OutIter>( 158 _IterOps<_AlgPolicy>::next(std::move(__first1), std::move(__last1)), 159 _IterOps<_AlgPolicy>::next(std::move(__first2), std::move(__last2)), 160 std::move(__result)); 161 } 162 163 template <class _AlgPolicy, class _Compare, class _InIter1, class _Sent1, class _InIter2, class _Sent2, class _OutIter> 164 [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI 165 _LIBCPP_CONSTEXPR_SINCE_CXX20 __set_intersection_result<_InIter1, _InIter2, _OutIter> 166 __set_intersection( 167 _InIter1 __first1, _Sent1 __last1, _InIter2 __first2, _Sent2 __last2, _OutIter __result, _Compare&& __comp) { 168 return std::__set_intersection<_AlgPolicy>( 169 std::move(__first1), 170 std::move(__last1), 171 std::move(__first2), 172 std::move(__last2), 173 std::move(__result), 174 std::forward<_Compare>(__comp), 175 typename std::_IterOps<_AlgPolicy>::template __iterator_category<_InIter1>(), 176 typename std::_IterOps<_AlgPolicy>::template __iterator_category<_InIter2>()); 177 } 178 179 template <class _InputIterator1, class _InputIterator2, class _OutputIterator, class _Compare> 180 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _OutputIterator set_intersection( 181 _InputIterator1 __first1, 182 _InputIterator1 __last1, 183 _InputIterator2 __first2, 184 _InputIterator2 __last2, 185 _OutputIterator __result, 186 _Compare __comp) { 187 return std::__set_intersection<_ClassicAlgPolicy, __comp_ref_type<_Compare> >( 188 std::move(__first1), 189 std::move(__last1), 190 std::move(__first2), 191 std::move(__last2), 192 std::move(__result), 193 __comp) 194 .__out_; 195 } 196 197 template <class _InputIterator1, class _InputIterator2, class _OutputIterator> 198 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _OutputIterator set_intersection( 199 _InputIterator1 __first1, 200 _InputIterator1 __last1, 201 _InputIterator2 __first2, 202 _InputIterator2 __last2, 203 _OutputIterator __result) { 204 return std::__set_intersection<_ClassicAlgPolicy>( 205 std::move(__first1), 206 std::move(__last1), 207 std::move(__first2), 208 std::move(__last2), 209 std::move(__result), 210 __less<>()) 211 .__out_; 212 } 213 214 _LIBCPP_END_NAMESPACE_STD 215 216 _LIBCPP_POP_MACROS 217 218 #endif // _LIBCPP___ALGORITHM_SET_INTERSECTION_H 219