join.hh

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#pragma once

#include <vcsn/misc/type_traits.hh>

namespace vcsn
{
  namespace detail
  {

    template <typename V1, typename V2,
              typename Enable = void>
    struct join_impl
    {};

    template <typename V1, typename V2>
    typename join_impl<V1, V2>::type
    join_(V1 v1, V2 v2, int)
    {
      return join_impl<V1, V2>::join(v1, v2);
    }

    template <typename V1, typename V2>
    typename join_impl<V1, V2>::type
    join_(V2 v2, V1 v1, long)
    {
      return join_impl<V1, V2>::join(v1, v2);
    }
  }

  template <typename ValueSet>
  auto
  join(const ValueSet& vs)
    -> ValueSet
  {
    return vs;
  }

  template <typename ValueSet1, typename ValueSet2>
  auto
  join(ValueSet1 vs1, ValueSet2 vs2)
    -> decltype(detail::join_(vs1, vs2, 0))
  {
    return detail::join_(vs1, vs2, 0);
  }

  template <typename ValueSet1, typename ValueSet2, typename ValueSet3,
            typename... VSs>
  auto
  join(ValueSet1 vs1, ValueSet2 vs2, ValueSet3 vs3, VSs&&... vs)
    -> decltype(join(join(vs1, vs2), vs3, std::forward<VSs>(vs)...))
  {
    return join(join(vs1, vs2), vs3, std::forward<VSs>(vs)...);
  }

  template <typename... ValueSets>
  using join_t = decltype(join(std::declval<ValueSets>()...));
}

#define VCSN_JOIN_SIMPLE(Lhs, Rhs)              \
      \
  template <>                                   \
  struct join_impl<Lhs, Rhs>                    \
  {                                             \
    using type = Rhs;                           \
    static type join(Lhs, Rhs)                  \
    {                                           \
      return {};                                \
    }                                           \
  }