ut0tuple.h

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/** @file include/ut0tuple.h
  std::tuple helper utilities. */
 
#ifndef ut0tuple_h
#define ut0tuple_h
 
#include <tuple>
#include <utility>
 
// std::index_sequence can only generate a sequence of indices starting from 0.
// This utility, index_sequence_with_offset, enables us to generate a
// sequence of indices with the offset.
//
// E.g.
//  index_sequence_with_offset_t<10, std::make_index_sequence<5>{}>
//  generates a sequence of {10, 11, 12, 13, 14} integers.
template <std::size_t N, typename Seq>
struct index_sequence_with_offset;
 
template <std::size_t N, std::size_t... Ints>
struct index_sequence_with_offset<N, std::index_sequence<Ints...>> {
  using type = std::index_sequence<Ints + N...>;
};
 
// Helper alias which saves us from typing ::type
template <std::size_t N, typename Seq>
using index_sequence_with_offset_t =
    typename index_sequence_with_offset<N, Seq>::type;
 
namespace detail {
template <typename Tuple, size_t... Is>
constexpr auto select_from_tuple_impl(Tuple &&t, std::index_sequence<Is...>) {
  return std::make_tuple(std::get<Is>(t)...);
}
}  // namespace detail
 
// Utility function which returns a new tuple which is a [Begin, End> subset of
// given tuple.
// E.g.
//  auto t = select_from_tuple<1, 3>(make_tuple(1, 2, 3, 4, 5));
//  assert(t == make_tuple(2, 3));
template <size_t Begin, size_t End, typename Tuple>
constexpr auto select_from_tuple(Tuple &&t) {
  return detail::select_from_tuple_impl(
      std::forward<Tuple>(t),
      index_sequence_with_offset_t<Begin,
                                   std::make_index_sequence<End - Begin>>{});
}
 
#endif /* ut0tuple_h */