#include <stdint.h>
#include <stdio.h>
#include <cmath>
#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_quadfloat_t;

double my_convert_to_double(const boost_quadfloat_t& x)
{
  if (isnormal(x)) {
    if (x.backend().exponent() < -1023+1) {
      // subnormal or zero
      if (x.backend().exponent() < -1023 - 52) {
        return x.backend().sign() ? -0.0 : 0.0; // underflow
      }
      double adj =  x.backend().sign() ? -DBL_MIN : DBL_MIN;
      boost_quadfloat_t tmp = x + adj;
      return tmp.convert_to<double>() - adj;
    }
  }
  return x.convert_to<double>();
}

int main(int , char** )
{
  boost_quadfloat_t b = ldexp(boost_quadfloat_t(0x8000000000000bffull), -63 - 1023);
  double d0 = b.convert_to<double>();
  double d1 = std::nexttoward(d0, d0 < b ? DBL_MAX : -DBL_MAX);

  std::cout
    << std::setprecision(21)
    << b  << " = b\n"
    << d0 << " = d0 = b.convert_to<double>()\n"
    << d1 << " = d1 = std::nexttoward(d0, ...)\n"
    << abs(d0 - b) << " = abs(d0 - b)\n"
    << abs(d1 - b) << " = abs(d1 - b)\n"
    << ((abs(d1 - b) < abs(d0 - b)) ? "d0 is not the closest representable double!!!\n" : "o.k.\n")
    ;

  d0 = my_convert_to_double(b);
  d1 = std::nexttoward(d0, d0 < b ? DBL_MAX : -DBL_MAX);
  std::cout
    << d0 << " = d0 = my_convert_to_double(b)\n"
    << d1 << " = d1 = std::nexttoward(d0, ...)\n"
    << abs(d0 - b) << " = abs(d0 - b)\n"
    << abs(d1 - b) << " = abs(d1 - b)\n"
    << ((abs(d1 - b) < abs(d0 - b)) ? "d0 is not the closest representable double!!!\n" : "o.k.\n")
    ;

  return 0;
}