libcxx/include/__algorithm/set_intersection.h

   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