/*=============================================================================
    Copyright (c) 2001-2011 Joel de Guzman
    Copyright (c) 2006 Dan Marsden

    Distributed under the Boost Software License, Version 1.0. (See accompanying
    file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
==============================================================================*/
#if !defined(FUSION_VALUE_AT_KEY_05052005_0229)
#define FUSION_VALUE_AT_KEY_05052005_0229

#include <boost/fusion/support/config.hpp>
#include <boost/mpl/int.hpp>
#include <boost/mpl/empty_base.hpp>
#include <boost/mpl/if.hpp>
#include <boost/mpl/or.hpp>
#include <boost/fusion/sequence/intrinsic_fwd.hpp>
#include <boost/fusion/sequence/intrinsic/has_key.hpp>
#include <boost/fusion/iterator/value_of_data.hpp>
#include <boost/fusion/algorithm/query/find.hpp>
#include <boost/fusion/support/tag_of.hpp>
#include <boost/fusion/support/category_of.hpp>

namespace boost {
namespace fusion {
// Special tags:
struct sequence_facade_tag;
struct boost_array_tag; // boost::array tag
struct mpl_sequence_tag; // mpl sequence tag
struct std_pair_tag; // std::pair tag

namespace extension {
template <typename Tag>
struct value_at_key_impl
{
    template <typename Seq, typename Key>
    struct apply
        : result_of::value_of_data<
          typename result_of::find<Seq, Key>::type
          >
    {};
};

template <>
struct value_at_key_impl<sequence_facade_tag>
{
    template <typename Sequence, typename Key>
    struct apply : Sequence::template value_at_key<Sequence, Key> {};
};

template <>
struct value_at_key_impl<boost_array_tag>;

template <>
struct value_at_key_impl<mpl_sequence_tag>;

template <>
struct value_at_key_impl<std_pair_tag>;
}

namespace detail {
template <typename Sequence, typename N, typename Tag>
struct value_at_key_impl
    : mpl::if_<
      mpl::or_<
      typename extension::has_key_impl<Tag>::template apply<Sequence, N>
, traits::is_unbounded<Sequence>
>
, typename extension::value_at_key_impl<Tag>::template apply<Sequence, N>
, mpl::empty_base
>::type
{};
}

namespace result_of {
template <typename Sequence, typename N>
struct value_at_key
    : detail::value_at_key_impl<Sequence, N, typename detail::tag_of<Sequence>::type>
{};
}
}
}

#endif
