[PATCH 5/8] libstdc++: Directly implement ranges::stable_partition [PR100795]

[Jonathan Wakely]

On 26/06/25 22:25 -0400, Patrick Palka wrote:
>	PR libstdc++/100795
>
>libstdc++-v3/ChangeLog:
>
>	* include/bits/ranges_algo.h (__detail::__find_if_not_n): New,
>	based on the stl_algo.h implementation.
>	(__detail::__stable_partition_adaptive): Likewise.
>	(__stable_partition_fn::operator()): Reimplement in terms of
>	the above.
>	* testsuite/25_algorithms/stable_partition/constrained.cc
>	(test03): New test.
>---
> libstdc++-v3/include/bits/ranges_algo.h       | 106 +++++++++++++++++-
> .../stable_partition/constrained.cc           |  26 +++++
> 2 files changed, 127 insertions(+), 5 deletions(-)
>
>diff --git a/libstdc++-v3/include/bits/ranges_algo.h b/libstdc++-v3/include/bits/ranges_algo.h
>index 7dfd4e7ed64c..a9924cd9c49e 100644
>--- a/libstdc++-v3/include/bits/ranges_algo.h
>+++ b/libstdc++-v3/include/bits/ranges_algo.h
>@@ -3133,6 +3133,81 @@ namespace ranges
>   inline constexpr __partition_fn partition{};
>
> #if _GLIBCXX_HOSTED
>+  namespace __detail
>+  {
>+    /// Like find_if_not(), but uses and updates a count of the
>+    /// remaining range length instead of comparing against an end
>+    /// iterator.
>+    template<typename _Iter, typename _Pred, typename _Distance>
>+      constexpr _Iter
>+      __find_if_not_n(_Iter __first, _Distance& __len, _Pred __pred)
>+      {
>+	for (; __len; --__len,  (void) ++__first)
>+	  if (!__pred(*__first))
>+	    break;
>+	return __first;
>+      }
>+
>+    template<typename _Iter, typename _Sent, typename _Pointer,
>+	     typename _Pred, typename _Distance>
>+      _GLIBCXX26_CONSTEXPR
>+      subrange<_Iter>
>+      __stable_partition_adaptive(_Iter __first, _Sent __last,
>+				  _Pred __pred, _Distance __len,
>+				  _Pointer __buffer,
>+				  _Distance __buffer_size)
>+      {
>+	if (__len == 1)
>+	  return {__first, ranges::next(__first, 1)};
>+
>+	if (__len <= __buffer_size)
>+	  {
>+	    _Iter __result1 = __first;
>+	    _Pointer __result2 = __buffer;
>+
>+	    // The precondition guarantees that !__pred(__first), so
>+	    // move that element to the buffer before starting the loop.
>+	    // This ensures that we only call __pred once per element.
>+	    *__result2 = ranges::iter_move(__first);
>+	    ++__result2;
>+	    ++__first;
>+	    for (; __first != __last; ++__first)
>+	      if (__pred(*__first))
>+		{
>+		  *__result1 = ranges::iter_move(__first);
>+		  ++__result1;
>+		}
>+	      else
>+		{
>+		  *__result2 = ranges::iter_move(__first);
>+		  ++__result2;
>+		}
>+
>+	    ranges::move(__buffer, __result2, __result1);
>+	    return {__result1, __first};
>+	  }
>+
>+	_Iter __middle = __first;
>+	ranges::advance(__middle, __len / 2);
>+	_Iter __left_split
>+	  = __detail::__stable_partition_adaptive(__first, __middle, __pred,
>+						  __len / 2, __buffer,
>+						  __buffer_size).begin();
>+
>+	// Advance past true-predicate values to satisfy this
>+	// function's preconditions.
>+	_Distance __right_len = __len - __len / 2;
>+	_Iter __right_split = __detail::__find_if_not_n(__middle, __right_len, __pred);
>+
>+	if (__right_len)
>+	  __right_split
>+	    = __detail::__stable_partition_adaptive(__right_split, __last, __pred,
>+						    __right_len, __buffer, __buffer_size).begin();
>+
>+	return ranges::rotate(__left_split, __middle, __right_split);
>+      }
>+  } // namespace __detail
>+
>   struct __stable_partition_fn
>   {
>     template<bidirectional_iterator _Iter, sentinel_for<_Iter> _Sent,
>@@ -3144,11 +3219,32 @@ namespace ranges
>       operator()(_Iter __first, _Sent __last,
> 		 _Pred __pred, _Proj __proj = {}) const
>       {
>-	auto __lasti = ranges::next(__first, __last);
>-	auto __middle
>-	  = std::stable_partition(std::move(__first), __lasti,
>-				  __detail::__make_pred_proj(__pred, __proj));
>-	return {std::move(__middle), std::move(__lasti)};
>+	auto __pred_proj = __detail::__make_pred_proj(__pred, __proj);
>+	__first = ranges::find_if_not(__first, __last, __pred_proj);

Does this end up going through another layer of
invoke(pred, invoke(proj, *i)) inside ranges::find_if_not?
Hopeuflly with the recent _Pred_proj changes that will get inlined,
but is there any reason to not just use:

	__first = ranges::find_if_not(__first, __last, __pred, __proj);

here, and then use __pred_proj for the __stable_partition_adaptive
calls below?

>+
>+	if (__first == __last)
>+	  return {__first, __first};
>+
>+	using _DistanceType = iter_difference_t<_Iter>;
>+	const _DistanceType __len = ranges::distance(__first, __last);
>+
>+#if __glibcxx_constexpr_algorithms >= 202306L // >= C++26
>+	if consteval {
>+	  // Simulate a _Temporary_buffer of length 1:
>+	  iter_value_t<_Iter> __buf = ranges::iter_move(__first);
>+	  *__first = std::move(__buf);
>+	  return __detail::__stable_partition_adaptive(__first, __last,
>+						       __pred_proj,
>+						       __len, &__buf,
>+						       _DistanceType(1));
>+	}
>+#endif
>+
>+	_Temporary_buffer<_Iter, iter_value_t<_Iter>> __buf(__first, ptrdiff_t(__len));
>+	return __detail::__stable_partition_adaptive(__first, __last,
>+						     __pred_proj,
>+						     __len, __buf.begin(),
>+						     _DistanceType(__buf.size()));
>       }
>
>     template<bidirectional_range _Range, typename _Proj = identity,
>diff --git a/libstdc++-v3/testsuite/25_algorithms/stable_partition/constrained.cc b/libstdc++-v3/testsuite/25_algorithms/stable_partition/constrained.cc
>index fc11c6439f9c..4dc267873ae2 100644
>--- a/libstdc++-v3/testsuite/25_algorithms/stable_partition/constrained.cc
>+++ b/libstdc++-v3/testsuite/25_algorithms/stable_partition/constrained.cc
>@@ -21,6 +21,7 @@
> // { dg-require-effective-target hosted }
>
> #include <algorithm>
>+#include <ranges>
> #include <testsuite_hooks.h>
> #include <testsuite_iterators.h>
>
>@@ -70,9 +71,34 @@ test02()
>     }
> }
>
>+void
>+test03()
>+{
>+  // PR libstdc++/100795 - ranges::stable_partition should not use
>+  // std::stable_partition directly
>+#if __SIZEOF_INT128__
>+  auto v = std::views::iota(__int128(0), __int128(20));
>+#else
>+  auto v = std::views::iota(0ll, 20ll);
>+#endif
>+
>+  int storage[20] = {2,5,4,3,1,6,7,9,10,8,11,14,12,13,15,16,18,0,19,17};
>+  auto w = v | std::views::transform([&](auto i) -> int& { return storage[i]; });
>+  using type = decltype(w);
>+  using cat = std::iterator_traits<std::ranges::iterator_t<type>>::iterator_category;
>+  static_assert( std::same_as<cat, std::output_iterator_tag> );
>+  static_assert( std::ranges::random_access_range<type> );
>+
>+  auto pred = [] (int a) { return a%2==0; };
>+  ranges::stable_partition(w, pred);
>+  VERIFY( ranges::all_of(w.begin(), w.begin() + 10, pred) );
>+  VERIFY( ranges::none_of(w.begin() + 10, w.end(), pred) );
>+}
>+
> int
> main()
> {
>   test01();
>   test02();
>+  test03();
> }
>-- 
>2.50.0.131.gcf6f63ea6b
>
>