1 files changed, 118 insertions, 4 deletions

diff --git a/contrib/llvm-project/libcxx/include/__algorithm/set_intersection.h b/contrib/llvm-project/libcxx/include/__algorithm/set_intersection.h
index 73d888d1b038..bb0d86cd0f58 100644
--- a/contrib/llvm-project/libcxx/include/__algorithm/set_intersection.h
+++ b/contrib/llvm-project/libcxx/include/__algorithm/set_intersection.h
@@ -12,10 +12,15 @@
 #include <__algorithm/comp.h>
 #include <__algorithm/comp_ref_type.h>
 #include <__algorithm/iterator_operations.h>
+#include <__algorithm/lower_bound.h>
 #include <__config>
+#include <__functional/identity.h>
 #include <__iterator/iterator_traits.h>
 #include <__iterator/next.h>
+#include <__type_traits/is_same.h>
+#include <__utility/exchange.h>
 #include <__utility/move.h>
+#include <__utility/swap.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
@@ -38,10 +43,103 @@ struct __set_intersection_result {
       : __in1_(std::move(__in_iter1)), __in2_(std::move(__in_iter2)), __out_(std::move(__out_iter)) {}
 };
 
-template <class _AlgPolicy, class _Compare, class _InIter1, class _Sent1, class _InIter2, class _Sent2, class _OutIter>
-_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 __set_intersection_result<_InIter1, _InIter2, _OutIter>
+// Helper for __set_intersection() with one-sided binary search: populate result and advance input iterators if they
+// are found to potentially contain the same value in two consecutive calls. This function is very intimately related to
+// the way it is used and doesn't attempt to abstract that, it's not appropriate for general usage outside of its
+// context.
+template <class _InForwardIter1, class _InForwardIter2, class _OutIter>
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void __set_intersection_add_output_if_equal(
+    bool __may_be_equal,
+    _InForwardIter1& __first1,
+    _InForwardIter2& __first2,
+    _OutIter& __result,
+    bool& __prev_may_be_equal) {
+  if (__may_be_equal && __prev_may_be_equal) {
+    *__result = *__first1;
+    ++__result;
+    ++__first1;
+    ++__first2;
+    __prev_may_be_equal = false;
+  } else {
+    __prev_may_be_equal = __may_be_equal;
+  }
+}
+
+// With forward iterators we can make multiple passes over the data, allowing the use of one-sided binary search to
+// reduce best-case complexity to log(N). Understanding how we can use binary search and still respect complexity
+// guarantees is _not_ straightforward: the guarantee is "at most 2*(N+M)-1 comparisons", and one-sided binary search
+// will necessarily overshoot depending on the position of the needle in the haystack -- for instance, if we're
+// 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
+// comparisons, when linear search would have yielded 3. However, because we won't need to perform the intervening
+// reciprocal comparisons (ie 1<3, 2<3, 4<3), that extra comparison doesn't run afoul of the guarantee. Additionally,
+// this type of scenario can only happen for match distances of up to 5 elements, because 2*log2(8) is 6, and we'll
+// 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
+// 1 comparison worse-off compared to the classic linear-searching algorithm if matching position 3 of a set with 4
+// elements, or position 5 if the set has 7 or 8 elements, but we'll never exceed the complexity guarantees from the
+// standard.
+template <class _AlgPolicy,
+          class _Compare,
+          class _InForwardIter1,
+          class _Sent1,
+          class _InForwardIter2,
+          class _Sent2,
+          class _OutIter>
+_LIBCPP_NODISCARD _LIBCPP_HIDE_FROM_ABI
+_LIBCPP_CONSTEXPR_SINCE_CXX20 __set_intersection_result<_InForwardIter1, _InForwardIter2, _OutIter>
 __set_intersection(
-    _InIter1 __first1, _Sent1 __last1, _InIter2 __first2, _Sent2 __last2, _OutIter __result, _Compare&& __comp) {
+    _InForwardIter1 __first1,
+    _Sent1 __last1,
+    _InForwardIter2 __first2,
+    _Sent2 __last2,
+    _OutIter __result,
+    _Compare&& __comp,
+    std::forward_iterator_tag,
+    std::forward_iterator_tag) {
+  _LIBCPP_CONSTEXPR std::__identity __proj;
+  bool __prev_may_be_equal = false;
+
+  while (__first2 != __last2) {
+    _InForwardIter1 __first1_next =
+        std::__lower_bound_onesided<_AlgPolicy>(__first1, __last1, *__first2, __comp, __proj);
+    std::swap(__first1_next, __first1);
+    // keeping in mind that a==b iff !(a<b) && !(b<a):
+    // if we can't advance __first1, that means !(*__first1 < *_first2), therefore __may_be_equal==true
+    std::__set_intersection_add_output_if_equal(
+        __first1 == __first1_next, __first1, __first2, __result, __prev_may_be_equal);
+    if (__first1 == __last1)
+      break;
+
+    _InForwardIter2 __first2_next =
+        std::__lower_bound_onesided<_AlgPolicy>(__first2, __last2, *__first1, __comp, __proj);
+    std::swap(__first2_next, __first2);
+    std::__set_intersection_add_output_if_equal(
+        __first2 == __first2_next, __first1, __first2, __result, __prev_may_be_equal);
+  }
+  return __set_intersection_result<_InForwardIter1, _InForwardIter2, _OutIter>(
+      _IterOps<_AlgPolicy>::next(std::move(__first1), std::move(__last1)),
+      _IterOps<_AlgPolicy>::next(std::move(__first2), std::move(__last2)),
+      std::move(__result));
+}
+
+// input iterators are not suitable for multipass algorithms, so we stick to the classic single-pass version
+template <class _AlgPolicy,
+          class _Compare,
+          class _InInputIter1,
+          class _Sent1,
+          class _InInputIter2,
+          class _Sent2,
+          class _OutIter>
+_LIBCPP_NODISCARD _LIBCPP_HIDE_FROM_ABI
+_LIBCPP_CONSTEXPR_SINCE_CXX20 __set_intersection_result<_InInputIter1, _InInputIter2, _OutIter>
+__set_intersection(
+    _InInputIter1 __first1,
+    _Sent1 __last1,
+    _InInputIter2 __first2,
+    _Sent2 __last2,
+    _OutIter __result,
+    _Compare&& __comp,
+    std::input_iterator_tag,
+    std::input_iterator_tag) {
   while (__first1 != __last1 && __first2 != __last2) {
     if (__comp(*__first1, *__first2))
       ++__first1;
@@ -55,12 +153,28 @@ __set_intersection(
     }
   }
 
-  return __set_intersection_result<_InIter1, _InIter2, _OutIter>(
+  return __set_intersection_result<_InInputIter1, _InInputIter2, _OutIter>(
       _IterOps<_AlgPolicy>::next(std::move(__first1), std::move(__last1)),
       _IterOps<_AlgPolicy>::next(std::move(__first2), std::move(__last2)),
       std::move(__result));
 }
 
+template <class _AlgPolicy, class _Compare, class _InIter1, class _Sent1, class _InIter2, class _Sent2, class _OutIter>
+_LIBCPP_NODISCARD _LIBCPP_HIDE_FROM_ABI
+_LIBCPP_CONSTEXPR_SINCE_CXX20 __set_intersection_result<_InIter1, _InIter2, _OutIter>
+__set_intersection(
+    _InIter1 __first1, _Sent1 __last1, _InIter2 __first2, _Sent2 __last2, _OutIter __result, _Compare&& __comp) {
+  return std::__set_intersection<_AlgPolicy>(
+      std::move(__first1),
+      std::move(__last1),
+      std::move(__first2),
+      std::move(__last2),
+      std::move(__result),
+      std::forward<_Compare>(__comp),
+      typename std::_IterOps<_AlgPolicy>::template __iterator_category<_InIter1>(),
+      typename std::_IterOps<_AlgPolicy>::template __iterator_category<_InIter2>());
+}
+
 template <class _InputIterator1, class _InputIterator2, class _OutputIterator, class _Compare>
 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 _OutputIterator set_intersection(
     _InputIterator1 __first1,