exercise-4-3.hpp

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// ===-- chapter-4/exercise-4-3.hpp ----------------------- -*- C++ -*- --=== //
#ifndef EXERCISE_4_3
#define EXERCISE_4_3
 
#include <boost/static_assert.hpp>
 
#include <boost/mpl/apply.hpp>
#include <boost/mpl/if.hpp>
#include <boost/mpl/greater.hpp>
#include <boost/mpl/identity.hpp>
#include <boost/mpl/minus.hpp>
#include <boost/mpl/multiplies.hpp>
#include <boost/mpl/not_equal_to.hpp>
#include <boost/mpl/plus.hpp>
 
#include <boost/mpl/placeholders.hpp>
using namespace boost::mpl::placeholders;
 
// Convenience used throughout the book's examples...
namespace mpl = boost::mpl;
 
 
namespace exercise_4_3 {
 
template <typename N, typename Predicate>
struct next_if
    : mpl::eval_if<
        typename mpl::apply<Predicate, N>::type, // Why do I need ::type here?
        mpl::next<N>,
        mpl::identity<N>
    >::type
{ };
 
template <typename N1, typename N2>
struct formula
    : mpl::eval_if<
        mpl::not_equal_to<N1,N2>,
        mpl::eval_if<
            mpl::greater<N1,N2>,
            mpl::minus<N1,N2>,
            mpl::identity<N1> // Could skip identity...
        >,
        mpl::plus<
            N1,
            mpl::multiplies< N1, mpl::int_<2> >
        >
    >::type
{ };
 
} // namespace exercise_4_3
 
#endif // EXERCISE_4_3