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

    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_AT_IMPL_07172005_0726)
#define FUSION_AT_IMPL_07172005_0726

#include <boost/fusion/support/config.hpp>
#include <boost/fusion/support/detail/access.hpp>
#include <boost/type_traits/is_const.hpp>
#include <boost/type_traits/add_const.hpp>
#include <boost/mpl/if.hpp>
#include <boost/mpl/bool.hpp>

namespace boost {
namespace fusion {
namespace detail {
template <typename Cons>
struct cons_deref
{
    typedef typename Cons::car_type type;
};

template <typename Cons, int I>
struct cons_advance
{
    typedef typename
    cons_advance<Cons, I-1>::type::cdr_type
    type;
};

template <typename Cons>
struct cons_advance<Cons, 0>
{
    typedef Cons type;
};

template <typename Cons>
struct cons_advance<Cons, 1>
{
    typedef typename Cons::cdr_type type;
};

template <typename Cons>
struct cons_advance<Cons, 2>
{
#if BOOST_WORKAROUND(BOOST_MSVC, > 1400) // VC8 and above
    typedef typename Cons::cdr_type::cdr_type type;
#else
    typedef typename Cons::cdr_type _a;
    typedef typename _a::cdr_type type;
#endif
};

template <typename Cons>
struct cons_advance<Cons, 3>
{
#if BOOST_WORKAROUND(BOOST_MSVC, > 1400) // VC8 and above
    typedef typename Cons::cdr_type::cdr_type::cdr_type type;
#else
    typedef typename Cons::cdr_type _a;
    typedef typename _a::cdr_type _b;
    typedef typename _b::cdr_type type;
#endif
};

template <typename Cons>
struct cons_advance<Cons, 4>
{
#if BOOST_WORKAROUND(BOOST_MSVC, > 1400) // VC8 and above
    typedef typename Cons::cdr_type::cdr_type::cdr_type::cdr_type type;
#else
    typedef typename Cons::cdr_type _a;
    typedef typename _a::cdr_type _b;
    typedef typename _b::cdr_type _c;
    typedef typename _c::cdr_type type;
#endif
};
}

struct cons_tag;

namespace extension {
template <typename Tag>
struct at_impl;

template <>
struct at_impl<cons_tag>
{
    template <typename Sequence, typename N>
    struct apply
    {
        typedef typename detail::cons_deref<
        typename detail::cons_advance<Sequence, N::value>::type>::type
        element;

        typedef typename
        mpl::if_<
        is_const<Sequence>
        , typename detail::cref_result<element>::type
        , typename detail::ref_result<element>::type
        >::type
        type;

        template <typename Cons, int N2>
        BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
        static type
        call(Cons& s, mpl::int_<N2>)
        {
            return call(s.cdr, mpl::int_<N2-1>());
        }

        template <typename Cons>
        BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
        static type
        call(Cons& s, mpl::int_<0>)
        {
            return s.car;
        }

        BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
        static type
        call(Sequence& s)
        {
            return call(s, mpl::int_<N::value>());
        }
    };
};
}
}
}

#endif
