libcxx/include/__cxx03/__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 <__cxx03/__algorithm/comp.h>
  13 #include <__cxx03/__algorithm/comp_ref_type.h>
  14 #include <__cxx03/__algorithm/iterator_operations.h>
  15 #include <__cxx03/__algorithm/lower_bound.h>
  16 #include <__cxx03/__config>
  17 #include <__cxx03/__functional/identity.h>
  18 #include <__cxx03/__iterator/iterator_traits.h>
  19 #include <__cxx03/__iterator/next.h>
  20 #include <__cxx03/__type_traits/is_same.h>
  21 #include <__cxx03/__utility/exchange.h>
  22 #include <__cxx03/__utility/move.h>
  23 #include <__cxx03/__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 <__cxx03/__undef_macros>
  31
  32 _LIBCPP_BEGIN_NAMESPACE_STD
  33
  34 template <class _InIter1, class _InIter2, class _OutIter>
  35 struct __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.
  50 template <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.
  80 template <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
 125 template <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
 162 template <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
 178 template <class _InputIterator1, class _InputIterator2, class _OutputIterator, class _Compare>
 179 inline _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
 196 template <class _InputIterator1, class _InputIterator2, class _OutputIterator>
 197 inline _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