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