integer_sequence.hpp

// template <class T, T... Ints> class integer_sequence;  // in <utility> since C++14

template <size_t... N>
struct IntegerSeq {
  using type = IntegerSeq;
  static constexpr size_t size() { return sizeof...(N); }
};

template <typename S, typename T>
struct ConcatSeq_;
template <typename S, typename T>
using ConcatSeq = typename ConcatSeq_<S, T>::type;
template <size_t... I, size_t... J>
struct ConcatSeq_<IntegerSeq<I...>, IntegerSeq<J...> >
    : IntegerSeq<I..., (sizeof...(I) + J)...> {};

template <size_t N>
struct MakeSeq_;
template <size_t N>
using MakeSeq = typename MakeSeq_<N>::type;
template <>
struct MakeSeq_<0> : IntegerSeq<> {};
template <>
struct MakeSeq_<1> : IntegerSeq<0> {};
template <size_t N>
struct MakeSeq_ : ConcatSeq<MakeSeq<N / 2>, MakeSeq<N - N / 2> > {};