// -*- c++ -*-
// C++11 20.4
#pragma once

#include <cstddef>
#include <utility>

namespace std {
  //
  // tuple
  //

  template <class... Types>
  class tuple;

  template <class T0, class... TN>
  class tuple<T0, TN...>
  {
    T0 car_;
    tuple<TN...> cdr_;

    template<size_t, typename...>
    friend struct __get_impl;

  public:
    constexpr tuple() : car_(), cdr_() { }

    explicit tuple(const T0& car, const TN&... cdr)
      : car_(car), cdr_(tuple<TN...>(cdr...)) { }

    template<class UType0, class... UTypes>
    explicit tuple(UType0 &&car, UTypes&&... cdr)
      : car_(std::forward<UType0>(car)),
        cdr_(tuple<TN...>(std::forward<UTypes>(cdr)...)) { }

    tuple(const tuple&) = default;
    tuple(tuple&&) = default;

    tuple& operator=(const tuple&) = default;
    tuple& operator=(tuple&&) = default;

    void swap(tuple& rhs)
    {
      swap(car_, rhs.car_);
      swap(cdr_, rhs.cdr_);
    }
  };

  template <>
  class tuple<>
  {
  public:
    constexpr tuple() { }
    void swap(tuple& rhs) { }
  };

  //
  // tuple_element
  //

  template <size_t I, class... Types>
  class tuple_element;

  template <class T0, class... TN>
  class tuple_element<0, tuple<T0, TN...> > {
  public:
    typedef T0 type;
  };

  template <size_t I, class T0, class... TN>
  class tuple_element<I, tuple<T0, TN...> >
    : public tuple_element<I - 1, tuple<TN...> > { };

  //
  // get
  //

  template <size_t I, class... Types>
  struct __get_impl;

  template <class T0, class... TN>
  struct __get_impl<0, T0, TN...>
  {
    static T0 &get(tuple<T0, TN...>& t) noexcept
    {
      return t.car_;
    }
  };

  template <size_t I, class T0, class... TN>
  struct __get_impl<I, T0, TN...>
  {
    static typename tuple_element<I, tuple<T0, TN...> >::type&
    get(tuple<T0, TN...>& t) noexcept
    {
      return __get_impl<I - 1, TN...>::get(t.cdr_);
    }
  };

  template <size_t I, class... Types>
  typename tuple_element<I, tuple<Types...> >::type& get(tuple<Types...>& t)
    noexcept
  {
    return __get_impl<I, Types...>::get(t);
  }

  template <size_t I, class... Types>
  typename tuple_element<I, tuple<Types...> >::type&& get(tuple<Types...>&& t)
    noexcept
  {
    return std::forward<typename tuple_element<I, tuple<Types...> >
                        ::type&&>(get<I>(t));
  }
}