[libcxx] r185343 - Implement n3658 - Compile-time integer sequences

[Howard Hinnant]

On Jul 1, 2013, at 1:43 PM, Richard Smith <richard at metafoo.co.uk> wrote:

> On Mon, Jul 1, 2013 at 9:26 AM, Marshall Clow <mclow at qualcomm.com> wrote:
>> Author: marshall
>> Date: Mon Jul  1 11:26:55 2013
>> New Revision: 185343
>> 
>> URL: http://llvm.org/viewvc/llvm-project?rev=185343&view=rev
>> Log:
>> Implement n3658 - Compile-time integer sequences
>> 
>> +#if _LIBCPP_STD_VER > 11
>> +
>> +template<class _Tp, _Tp... _Ip>
>> +struct integer_sequence
>> +{
>> +    typedef _Tp value_type;
>> +    static_assert( is_integral<_Tp>::value,
>> +                  "std::integer_sequence can only be instantiated with an integral type" );
>> +    static
>> +    _LIBCPP_INLINE_VISIBILITY
>> +    constexpr
>> +    size_t
>> +    size() noexcept { return sizeof...(_Ip); }
>> +};
>> +
>> +template<size_t... _Ip>
>> +    using index_sequence = integer_sequence<size_t, _Ip...>;
>> +
>> +template <class _Tp, _Tp _Sp, _Tp _Ep, class _IntSequence>
>> +struct __make_integer_sequence_unchecked;
>> +
>> +template <class _Tp, _Tp _Sp, _Tp _Ep, _Tp ..._Indices>
>> +struct __make_integer_sequence_unchecked<_Tp, _Sp, _Ep,
>> +                                         integer_sequence<_Tp, _Indices...>>
>> +{
>> +    typedef typename __make_integer_sequence_unchecked
>> +                     <
>> +                        _Tp, _Sp+1, _Ep,
>> +                        integer_sequence<_Tp, _Indices..., _Sp>
> 
> This is O(n^2). You can do it in O(n log n):
> 
> template<typename T, T N, T Parity> struct __make_integer_sequence;
> template<typename T, T N> using make_integer_sequence =
> __make_integer_sequence<T, N, N % 2>::type;
> 
> template<typename T> struct __make_integer_sequence<T, 0, 0> { typedef
> integer_sequence<T> type; };
> template<typename T> struct __make_integer_sequence<T, 1, 1> { typedef
> integer_sequence<T, 0> type; };
> 
> template<typename T, T ...Extra> struct __make_integer_sequence_impl;
> template<typename T, T ...N, T ...Extra> struct
> __make_integer_sequence_impl<integer_sequence<T, N...>, Extra...> {
>  typedef integer_sequence<T, N..., sizeof...(N) + N..., Extra...> type;
> };
> 
> template<typename T, T N> struct __make_integer_sequence<T, N, 0> {
>  typedef __make_integer_sequence_impl<make_integer_sequence<T, N /
> 2>>::type type;
> };
> template<typename T, T N> struct __make_integer_sequence<T, N, 1> {
>  typedef __make_integer_sequence_impl<make_integer_sequence<T, N /
> 2>, N>::type type;
> };

This is an interesting suggestion, thanks.  I think it has the potential to make make_integer_sequence much more robust against exceeding recursive template instantiation limits.  However as suggested, the above doesn't compile.  After working with it for a while I got it to compile, but then it didn't give the desired results.

Howard

> 
>> +                     >::type type;
>> +};
>> +
>> +template <class _Tp, _Tp _Ep, _Tp ..._Indices>
>> +struct __make_integer_sequence_unchecked<_Tp, _Ep, _Ep,
>> +                                         integer_sequence<_Tp, _Indices...>>
>> +{
>> +    typedef integer_sequence<_Tp, _Indices...> type;
>> +};
>> +
>> +template <class _Tp, _Tp _Ep>
>> +struct __make_integer_sequence
>> +{
>> +    static_assert(is_integral<_Tp>::value,
>> +                  "std::make_integer_sequence can only be instantiated with an integral type" );
>> +    static_assert(0 <= _Ep, "std::make_integer_sequence input shall not be negative");
>> +    typedef typename __make_integer_sequence_unchecked
>> +                     <
>> +                        _Tp, 0, _Ep, integer_sequence<_Tp>
>> +                     >::type type;
>> +};
>> +
>> +template<class _Tp, _Tp _Np>
>> +    using make_integer_sequence = typename __make_integer_sequence<_Tp, _Np>::type;
>> +
>> +template<size_t _Np>
>> +    using make_index_sequence = make_integer_sequence<size_t, _Np>;
>> +
>> +template<class... _Tp>
>> +    using index_sequence_for = make_index_sequence<sizeof...(_Tp)>;
>> +
>> +#endif  // _LIBCPP_STD_VER > 11