Re: [PATCH] libstdc++: Implement P2474R2 changes to views::take/drop

[Jonathan Wakely <jwakely.gcc@gmail.com>]

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On Wed, 19 Oct 2022, 17:47 Patrick Palka via Libstdc++, <
libstdc++@gcc.gnu.org> wrote:

> Tested on x86_64-pc-linux-gnu, does this look OK for trunk?
>

OK



> libstdc++-v3/ChangeLog:
>
>         * include/std/ranges (views::__detail::__is_repeat_view): Define
>         and later define a partial specialization.
>         (views::__detail::__take_of_repeat_view): Declare and later define.
>         (views::__detail::__drop_of_repeat_view): Likewise.
>         (views::_Take::operator()): Return a repeat_view if the argument
>         is a repeat_view as per P2474R2.
>         (views::_Drop::operator()): Likewise.
>         (repeat_view): Befriend __take/drop_of_repeat_view.
>         * testsuite/std/ranges/repeat/1.cc (test04): New test.
> ---
>  libstdc++-v3/include/std/ranges               | 56 +++++++++++++++++++
>  libstdc++-v3/testsuite/std/ranges/repeat/1.cc | 33 +++++++++++
>  2 files changed, 89 insertions(+)
>
> diff --git a/libstdc++-v3/include/std/ranges
> b/libstdc++-v3/include/std/ranges
> index b29264931cc..6de08244bd1 100644
> --- a/libstdc++-v3/include/std/ranges
> +++ b/libstdc++-v3/include/std/ranges
> @@ -2257,6 +2257,13 @@ namespace views::__adaptor
>        template<typename _Winc, typename _Bound>
>         inline constexpr bool __is_iota_view<iota_view<_Winc, _Bound>> =
> true;
>
> +      template<typename _Range>
> +       inline constexpr bool __is_repeat_view = false;
> +
> +      template<typename _Range>
> +       constexpr auto
> +       __take_of_repeat_view(_Range&&, range_difference_t<_Range>); //
> defined later
> +
>        template<typename _Range, typename _Dp>
>         concept __can_take_view
>           = requires { take_view(std::declval<_Range>(),
> std::declval<_Dp>()); };
> @@ -2291,6 +2298,8 @@ namespace views::__adaptor
>               else
>                 return iota_view(*__begin, *__end);
>             }
> +         else if constexpr (__detail::__is_repeat_view<_Tp>)
> +           return
> __detail::__take_of_repeat_view(std::forward<_Range>(__r), __n);
>           else
>             return take_view(std::forward<_Range>(__r), __n);
>         }
> @@ -2527,6 +2536,10 @@ namespace views::__adaptor
>    {
>      namespace __detail
>      {
> +      template<typename _Range>
> +       constexpr auto
> +       __drop_of_repeat_view(_Range&&, range_difference_t<_Range>); //
> defined later
> +
>        template<typename _Range, typename _Dp>
>         concept __can_drop_view
>           = requires { drop_view(std::declval<_Range>(),
> std::declval<_Dp>()); };
> @@ -2568,6 +2581,8 @@ namespace views::__adaptor
>               else
>                 return _Tp(__begin, __end);
>             }
> +         else if constexpr (__detail::__is_repeat_view<_Tp>)
> +           return
> __detail::__drop_of_repeat_view(std::forward<_Range>(__r), __n);
>           else
>             return drop_view(std::forward<_Range>(__r), __n);
>         }
> @@ -7367,6 +7382,14 @@ namespace views::__adaptor
>
>      struct _Iterator;
>
> +    template<typename _Range>
> +      friend constexpr auto
> +      views::__detail::__take_of_repeat_view(_Range&&,
> range_difference_t<_Range>);
> +
> +    template<typename _Range>
> +      friend constexpr auto
> +      views::__detail::__drop_of_repeat_view(_Range&&,
> range_difference_t<_Range>);
> +
>    public:
>      repeat_view() requires default_initializable<_Tp> = default;
>
> @@ -7540,6 +7563,9 @@ namespace views::__adaptor
>    {
>      namespace __detail
>      {
> +      template<typename _Tp, typename _Bound>
> +       inline constexpr bool __is_repeat_view<repeat_view<_Tp, _Bound>> =
> true;
> +
>        template<typename _Tp>
>         concept __can_repeat_view
>           = requires { repeat_view(std::declval<_Tp>()); };
> @@ -7565,6 +7591,36 @@ namespace views::__adaptor
>      };
>
>      inline constexpr _Repeat repeat;
> +
> +    namespace __detail
> +    {
> +      template<typename _Range>
> +       constexpr auto
> +       __take_of_repeat_view(_Range&& __r, range_difference_t<_Range> __n)
> +       {
> +         using _Tp = remove_cvref_t<_Range>;
> +         static_assert(__is_repeat_view<_Tp>);
> +         if constexpr (sized_range<_Tp>)
> +           return views::repeat(*__r._M_value,
> std::min(ranges::distance(__r), __n));
> +         else
> +           return views::repeat(*__r._M_value, __n);
> +       }
> +
> +      template<typename _Range>
> +       constexpr auto
> +       __drop_of_repeat_view(_Range&& __r, range_difference_t<_Range> __n)
> +       {
> +         using _Tp = remove_cvref_t<_Range>;
> +         static_assert(__is_repeat_view<_Tp>);
> +         if constexpr (sized_range<_Tp>)
> +           {
> +             auto __sz = ranges::distance(__r);
> +             return views::repeat(*__r._M_value, __sz - std::min(__sz,
> __n));
> +           }
> +         else
> +           return __r;
> +       }
> +    }
>    }
>
>    template<input_range _Vp>
> diff --git a/libstdc++-v3/testsuite/std/ranges/repeat/1.cc
> b/libstdc++-v3/testsuite/std/ranges/repeat/1.cc
> index 3698ed12c14..542c8614f88 100644
> --- a/libstdc++-v3/testsuite/std/ranges/repeat/1.cc
> +++ b/libstdc++-v3/testsuite/std/ranges/repeat/1.cc
> @@ -84,10 +84,43 @@ test03()
>    return true;
>  }
>
> +constexpr bool
> +test04()
> +{
> +  // Verify P2472R2 changes to views::take/drop.
> +  auto r = views::repeat(42);
> +
> +  auto rt = r | views::take(10);
> +  static_assert(views::__detail::__is_repeat_view<decltype(rt)>);
> +  VERIFY( ranges::equal(rt, views::repeat(42, 10)) );
> +
> +  auto rd = r | views::drop(10);
> +  static_assert(std::same_as<decltype(rd), decltype(r)>);
> +
> +  auto br = views::repeat(42, 37);
> +
> +  auto brt = br | views::take(10);
> +  static_assert(std::same_as<decltype(brt), decltype(br)>);
> +  VERIFY( ranges::equal(brt, views::repeat(42, 10)) );
> +
> +  auto brt100 = br | views::take(100);
> +  VERIFY( ranges::equal(brt100, br) );
> +
> +  auto brd = br | views::drop(10);
> +  static_assert(std::same_as<decltype(brd), decltype(br)>);
> +  VERIFY( ranges::equal(brd, views::repeat(42, 27)) );
> +
> +  auto brd100 = br | views::drop(100);
> +  VERIFY( ranges::empty(brd100) );
> +
> +  return true;
> +}
> +
>  int
>  main()
>  {
>    static_assert(test01());
>    static_assert(test02());
>    static_assert(test03());
> +  static_assert(test04());
>  }
> --
> 2.38.1.119.g9c32cfb49c
>
>