[PATCH 7/8] libstdc++: Directly implement ranges::sample [PR100795]

[Patrick Palka]

On Thu, 26 Jun 2025, Patrick Palka wrote:

> 	PR libstdc++/100795
> 
> libstdc++-v3/ChangeLog:
> 
> 	* include/bits/ranges_algo.h (__sample_fn::operator()):
> 	Reimplement the forward_iterator branch directly.
> 	* testsuite/25_algorithms/sample/constrained.cc (test02):
> 	New test.
> ---
>  libstdc++-v3/include/bits/ranges_algo.h       | 70 +++++++++++++++++--
>  .../25_algorithms/sample/constrained.cc       | 28 ++++++++
>  2 files changed, 91 insertions(+), 7 deletions(-)
> 
> diff --git a/libstdc++-v3/include/bits/ranges_algo.h b/libstdc++-v3/include/bits/ranges_algo.h
> index b12da2af1263..672a0ebce0de 100644
> --- a/libstdc++-v3/include/bits/ranges_algo.h
> +++ b/libstdc++-v3/include/bits/ranges_algo.h
> @@ -1839,14 +1839,70 @@ namespace ranges
>        operator()(_Iter __first, _Sent __last, _Out __out,
>  		 iter_difference_t<_Iter> __n, _Gen&& __g) const
>        {
> +	// FIXME: Correctly handle integer-class difference types.

On second thought maybe we don't need to teach uniform_int_distribution
to handle integer-class difference types.  We could just assert that
__n fits inside a long long and use that as the difference type?  Same
for shuffle.

>  	if constexpr (forward_iterator<_Iter>)
>  	  {
> -	    // FIXME: Forwarding to std::sample here requires computing __lasti
> -	    // which may take linear time.
> -	    auto __lasti = ranges::next(__first, __last);
> -	    return _GLIBCXX_STD_A::
> -	      sample(std::move(__first), std::move(__lasti), std::move(__out),
> -		     __n, std::forward<_Gen>(__g));
> +	    using _Size = iter_difference_t<_Iter>;
> +	    using __distrib_type = uniform_int_distribution<_Size>;
> +	    using __param_type = typename __distrib_type::param_type;
> +	    using _USize = __detail::__make_unsigned_like_t<_Size>;
> +	    using __uc_type
> +	      = common_type_t<typename remove_reference_t<_Gen>::result_type, _USize>;
> +
> +	    if (__first == __last)
> +	      return __out;
> +
> +	    __distrib_type __d{};
> +	    _Size __unsampled_sz = ranges::distance(__first, __last);
> +	    __n = std::min(__n, __unsampled_sz);
> +
> +	    // If possible, we use __gen_two_uniform_ints to efficiently produce
> +	    // two random numbers using a single distribution invocation:
> +
> +	    const __uc_type __urngrange = __g.max() - __g.min();
> +	    if (__urngrange / __uc_type(__unsampled_sz) >= __uc_type(__unsampled_sz))
> +	      // I.e. (__urngrange >= __unsampled_sz * __unsampled_sz) but without
> +	      // wrapping issues.
> +	      {
> +		while (__n != 0 && __unsampled_sz >= 2)
> +		  {
> +		    const pair<_Size, _Size> __p =
> +		      __gen_two_uniform_ints(__unsampled_sz, __unsampled_sz - 1, __g);
> +
> +		    --__unsampled_sz;
> +		    if (__p.first < __n)
> +		      {
> +			*__out = *__first;
> +			++__out;
> +			--__n;
> +		      }
> +
> +		    ++__first;
> +
> +		    if (__n == 0) break;
> +
> +		    --__unsampled_sz;
> +		    if (__p.second < __n)
> +		      {
> +			*__out = *__first;
> +			++__out;
> +			--__n;
> +		      }
> +
> +		    ++__first;
> +		  }
> +	      }
> +
> +	    // The loop above is otherwise equivalent to this one-at-a-time version:
> +
> +	    for (; __n != 0; ++__first)
> +	      if (__d(__g, __param_type{0, --__unsampled_sz}) < __n)
> +		{
> +		  *__out = *__first;
> +		  ++__out;
> +		  --__n;
> +		}
> +	    return __out;
>  	  }
>  	else
>  	  {
> @@ -1867,7 +1923,7 @@ namespace ranges
>  		if (__k < __n)
>  		  __out[__k] = *__first;
>  	      }
> -	    return __out + __sample_sz;
> +	    return __out + iter_difference_t<_Out>(__sample_sz);
>  	  }
>        }
>  
> diff --git a/libstdc++-v3/testsuite/25_algorithms/sample/constrained.cc b/libstdc++-v3/testsuite/25_algorithms/sample/constrained.cc
> index b9945b164903..150e2d2036e0 100644
> --- a/libstdc++-v3/testsuite/25_algorithms/sample/constrained.cc
> +++ b/libstdc++-v3/testsuite/25_algorithms/sample/constrained.cc
> @@ -20,6 +20,7 @@
>  
>  #include <algorithm>
>  #include <random>
> +#include <ranges>
>  #include <testsuite_hooks.h>
>  #include <testsuite_iterators.h>
>  
> @@ -59,9 +60,36 @@ test01()
>      }
>  }
>  
> +void
> +test02()
> +{
> +  // PR libstdc++/100795 - ranges::sample should not use std::sample
> +#if 0 // FIXME: ranges::sample rejects integer-class difference types.
> +#if __SIZEOF_INT128__
> +  auto v = std::views::iota(__int128(0), __int128(20));
> +#else
> +  auto v = std::views::iota(0ll, 20ll);
> +#endif
> +#else
> +  auto v = std::views::iota(0, 20);
> +#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> );
> +
> +  ranges::sample(v, w.begin(), 20, rng);
> +  ranges::sort(w);
> +  VERIFY( ranges::equal(w, v) );
> +}
> +
>  int
>  main()
>  {
>    test01<forward_iterator_wrapper, output_iterator_wrapper>();
>    test01<input_iterator_wrapper, random_access_iterator_wrapper>();
> +  test02();
>  }
> -- 
> 2.50.0.131.gcf6f63ea6b
> 
>