[Boost-users] [numeric::converter] finding the smallest c such that (a/c) + (b/c) < inf

21 Mar 2009

      Hi All,

       template<typename T>
       struct promote{};
       template<>
       struct promote<float> : mpl::identity<double>{};
       template<>
       struct promote<double> : mpl::identity<long double>{};

     template<typename T> //T: float, double
     struct fit_a_plus_b_to_range{
         static T divider(T a, T b){
             static const T mx = math::tools::max_value<T>();
             static const T mi = math::tools::min_value<T>();
             if(a > (mx - b)){
                 typedef typename promote<T>::type prom_t;
                 typedef numeric::converter up_t;
                 typedef numeric::converter down_t;
                 prom_t au = up_t::convert(a);
                 prom_t bu = up_t::convert(b);
                 prom_t mu = up_t::convert(mx);
                 prom_t cu = ((au/mu)+(bu/mu));
		T delta = ...;
                 T c = down_t::convert(cu) + delta;
                 BOOST_ASSERT(!math::isinf((a/c)+(b/c)));
                 return c;
             }else{
                 return (T)(1);
             }
         }
     };

With delta = 0.0 the assertion fails, with delta = 1.0 it works (for a 
few examples). What's the smallest delta value? Is there a better way to 
do this overall?

If this is off topic, let me know...