Ticket #12580: sub_from_pow2.cpp

#include <iostream>
#include <boost/multiprecision/cpp_bin_float.hpp>

typedef boost::multiprecision::number<boost::multiprecision::cpp_bin_float<128, boost::multiprecision::backends::digit_base_2> > boost_float128_t;
typedef boost::multiprecision::number<boost::multiprecision::cpp_bin_float<256, boost::multiprecision::backends::digit_base_2> > boost_float256_t;

int main(int , char** )
{
  boost_float128_t a = 1;
  boost_float128_t b = ldexp(boost_float128_t(0.99), -128);
  boost_float128_t c = a - b;
  boost_float128_t d = boost_float128_t(boost_float256_t(a) - boost_float256_t(b));

  std::cout
    << std::setprecision(40)
    << std::scientific
    << "a = " << a << "\n"
    << "b = " << b << "\n"
    << "c = " << c << " (a - b)\n"
    << "d = " << d << " (a - b) calculated with 256-bit precision.\n"
    ;

  if (c == d)
    std::cout << "o.k\n";
  else
    std::cout << "fail.\n";
  return 0;
}