Re: Add three way lower_bound

["François Dumont" <frs.dumont@gmail.com>]

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Any feedback regarding this proposal:
https://gcc.gnu.org/pipermail/libstdc++/2021-June/052821.html


On 23/06/21 22:34, François Dumont wrote:
> Hi
>
> Following the message to propose an alternative lower_bound and the 
> reply to use three way comparison I try to implement this.
>
> Before going further I wonder if this is something possible ?
>
> The purpose of the:
>
> if constexpr (three_way_comparable<>)
>
> is to make sure that we use it only if there is a proper <=> operator 
> defined. Afai understood what is in <compare> we can have the 
> __synth3way for any type as long as < exist. But I think that if <=> 
> is implemented in terms of < then it might be too expensive, the 
> actual lower_bound might already be implemented this way.
>
> My main concerns is of course Standard conformity, could it be ok ?
>
> François
>

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diff --git a/libstdc++-v3/include/bits/stl_algobase.h b/libstdc++-v3/include/bits/stl_algobase.h
index 84a1f9e98f6..7a0c42a4909 100644
--- a/libstdc++-v3/include/bits/stl_algobase.h
+++ b/libstdc++-v3/include/bits/stl_algobase.h
@@ -1472,6 +1472,48 @@ _GLIBCXX_END_NAMESPACE_CONTAINER
       return __first;
     }
 
+#if __cpp_lib_three_way_comparison
+  template<typename _ForwardIterator, typename _Tp, typename _Compare>
+    _GLIBCXX20_CONSTEXPR
+    _ForwardIterator
+    __lower_bound_three_way(_ForwardIterator __first, _ForwardIterator __last,
+			    const _Tp& __val, _Compare __comp)
+    {
+      auto __len = std::distance(__first, __last);
+
+      while (__len > 0)
+	{
+	  auto __half = __len >> 1;
+	  if (__half == 0)
+	    {
+	      if (auto __c = __comp(*__first, __val); __c < 0)
+		return std::next(__first);
+	      return __first;
+	    }
+
+	  _ForwardIterator __prev_mid = __first;
+	  std::advance(__prev_mid, __half - 1);
+	  _ForwardIterator __middle = std::next(__prev_mid);
+	  if (auto __c = __comp(*__middle, __val); __c != 0)
+	    {
+	      if (__c < 0)
+		{
+		  __first = __middle;
+		  ++__first;
+		  __len = __len - __half - 1;
+		}
+	      else
+		__len = __half;
+	    }
+	  else if (__c = __comp(*__prev_mid, __val); __c != 0)
+	    return __middle;
+	  else // __c == 0.
+	    __len = __half - 1;
+	}
+      return __first;
+    }
+#endif
+
   /**
    *  @brief Finds the first position in which @a val could be inserted
    *         without changing the ordering.
@@ -1495,6 +1537,13 @@ _GLIBCXX_END_NAMESPACE_CONTAINER
 	    typename iterator_traits<_ForwardIterator>::value_type, _Tp>)
       __glibcxx_requires_partitioned_lower(__first, __last, __val);
 
+#if __cpp_lib_three_way_comparison
+      if constexpr (three_way_comparable_with<
+		    typename iterator_traits<_ForwardIterator>::reference,
+		    const _Tp&>)
+	return std::__lower_bound_three_way(__first, __last, __val,
+					    compare_three_way{});
+#endif
       return std::__lower_bound(__first, __last, __val,
 				__gnu_cxx::__ops::__iter_less_val());
     }