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
35template <class _InIter1, class _InIter2, class _OutIter>
36struct __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.
51template <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.
81template <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 bool __prev_may_be_equal = false;
100
101 while (__first2 != __last2) {
102 _InForwardIter1 __first1_next = std::__lower_bound_onesided<_AlgPolicy>(__first1, __last1, *__first2, __comp);
103 std::swap(__first1_next, __first1);
104 // keeping in mind that a==b iff !(a<b) && !(b<a):
105 // if we can't advance __first1, that means !(*__first1 < *_first2), therefore __may_be_equal==true
106 std::__set_intersection_add_output_if_equal(
107 __first1 == __first1_next, __first1, __first2, __result, __prev_may_be_equal);
108 if (__first1 == __last1)
109 break;
110
111 _InForwardIter2 __first2_next = std::__lower_bound_onesided<_AlgPolicy>(__first2, __last2, *__first1, __comp);
112 std::swap(__first2_next, __first2);
113 std::__set_intersection_add_output_if_equal(
114 __first2 == __first2_next, __first1, __first2, __result, __prev_may_be_equal);
115 }
116 return __set_intersection_result<_InForwardIter1, _InForwardIter2, _OutIter>(
117 _IterOps<_AlgPolicy>::next(std::move(__first1), std::move(__last1)),
118 _IterOps<_AlgPolicy>::next(std::move(__first2), std::move(__last2)),
119 std::move(__result));
120}
121
122// input iterators are not suitable for multipass algorithms, so we stick to the classic single-pass version
123template <class _AlgPolicy,
124 class _Compare,
125 class _InInputIter1,
126 class _Sent1,
127 class _InInputIter2,
128 class _Sent2,
129 class _OutIter>
130[[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI
131_LIBCPP_CONSTEXPR_SINCE_CXX20 __set_intersection_result<_InInputIter1, _InInputIter2, _OutIter>
132__set_intersection(
133 _InInputIter1 __first1,
134 _Sent1 __last1,
135 _InInputIter2 __first2,
136 _Sent2 __last2,
137 _OutIter __result,
138 _Compare&& __comp,
139 std::input_iterator_tag,
140 std::input_iterator_tag) {
141 while (__first1 != __last1 && __first2 != __last2) {
142 if (__comp(*__first1, *__first2))
143 ++__first1;
144 else {
145 if (!__comp(*__first2, *__first1)) {
146 *__result = *__first1;
147 ++__result;
148 ++__first1;
149 }
150 ++__first2;
151 }
152 }
153
154 return __set_intersection_result<_InInputIter1, _InInputIter2, _OutIter>(
155 _IterOps<_AlgPolicy>::next(std::move(__first1), std::move(__last1)),
156 _IterOps<_AlgPolicy>::next(std::move(__first2), std::move(__last2)),
157 std::move(__result));
158}
159
160template <class _AlgPolicy, class _Compare, class _InIter1, class _Sent1, class _InIter2, class _Sent2, class _OutIter>
161[[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI
162_LIBCPP_CONSTEXPR_SINCE_CXX20 __set_intersection_result<_InIter1, _InIter2, _OutIter>
163__set_intersection(
164 _InIter1 __first1, _Sent1 __last1, _InIter2 __first2, _Sent2 __last2, _OutIter __result, _Compare&& __comp) {
165 return std::__set_intersection<_AlgPolicy>(
166 std::move(__first1),
167 std::move(__last1),
168 std::move(__first2),
169 std::move(__last2),
170 std::move(__result),
171 std::forward<_Compare>(__comp),
172 typename std::_IterOps<_AlgPolicy>::template __iterator_category<_InIter1>(),
173 typename std::_IterOps<_AlgPolicy>::template __iterator_category<_InIter2>());
174}
175
176template <class _InputIterator1, class _InputIterator2, class _OutputIterator, class _Compare>
177inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _OutputIterator set_intersection(
178 _InputIterator1 __first1,
179 _InputIterator1 __last1,
180 _InputIterator2 __first2,
181 _InputIterator2 __last2,
182 _OutputIterator __result,
183 _Compare __comp) {
184 return std::__set_intersection<_ClassicAlgPolicy, __comp_ref_type<_Compare> >(
185 std::move(__first1),
186 std::move(__last1),
187 std::move(__first2),
188 std::move(__last2),
189 std::move(__result),
190 __comp)
191 .__out_;
192}
193
194template <class _InputIterator1, class _InputIterator2, class _OutputIterator>
195inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _OutputIterator set_intersection(
196 _InputIterator1 __first1,
197 _InputIterator1 __last1,
198 _InputIterator2 __first2,
199 _InputIterator2 __last2,
200 _OutputIterator __result) {
201 return std::__set_intersection<_ClassicAlgPolicy>(
202 std::move(__first1),
203 std::move(__last1),
204 std::move(__first2),
205 std::move(__last2),
206 std::move(__result),
207 __less<>())
208 .__out_;
209}
210
211_LIBCPP_END_NAMESPACE_STD
212
213_LIBCPP_POP_MACROS
214
215#endif // _LIBCPP___ALGORITHM_SET_INTERSECTION_H
216