exercise-5-1.hpp

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// ===-- chapter-5/exercise-5-1.hpp ----------------------- -*- C++ -*- --=== //
#ifndef EXERCISE_5_1
#define EXERCISE_5_1
 
#include <boost/static_assert.hpp>
 
#include <boost/mpl/begin_end.hpp>
#include <boost/mpl/deref.hpp>
#include <boost/mpl/equal.hpp>
#include <boost/mpl/greater_equal.hpp>
#include <boost/mpl/push_back.hpp>
#include <boost/mpl/times.hpp>
#include <boost/mpl/vector_c.hpp>
 
 
namespace exercise_5_1 {
 
template <typename S, typename Res, typename Cur>
class double_first_half_impl
{
    // Better to break up the algorithm a bit and make things clearer...
 
    typedef boost::mpl::distance<typename boost::mpl::begin<S>::type, Cur>
    dist_to_begin;
 
    typedef boost::mpl::distance<Cur, typename boost::mpl::end<S>::type>
    dist_to_end;
 
    typedef
    boost::mpl::eval_if<
        boost::mpl::greater_equal<dist_to_begin, dist_to_end>,
        boost::mpl::deref<Cur>,
        boost::mpl::times<
            typename boost::mpl::deref<Cur>::type,
            boost::mpl::int_<2>
        >
    > multiply_or_not;
 
public:
    typedef
    typename
    boost::mpl::eval_if<
        boost::is_same<Cur, typename boost::mpl::end<S>::type>,
        Res,
        double_first_half_impl<
            S,
            typename boost::mpl::push_back<
                Res,
                typename multiply_or_not::type
            >::type,
            typename boost::mpl::next<Cur>::type
        >
    >::type type;
};
 
// Interesting note: double_first_half results in the following (with
// boost::mpl::vector_c<int, 1, 2, 3, 4> as input):
//
//     boost::mpl::v_item<
//         mpl_::integral_c<int, 4>,
//         boost::mpl::v_item<
//             mpl_::integral_c<int, 3>,
//             boost::mpl::v_item<
//                 mpl_::integral_c<int, 4>,
//                 boost::mpl::v_item<
//                     mpl_::integral_c<int, 2>,
//                     boost::mpl::vector_c<int>,
//                 0>,
//             0>,
//         0>,
//     0>
// This is boost::mpl::equal to boost::mpl::vector_c<int, 2, 4, 3, 4>
template <typename S>
struct double_first_half
    : double_first_half_impl<
        S, boost::mpl::vector_c<int>, typename boost::mpl::begin<S>::type>
{ };
 
} // namespace exercise_5_1
 
#endif