Ticket #12527: af_p3f.cpp

#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;

float my_convert_to_float(const boost_quadfloat_t& x)
{
  if (isnormal(x)) {
    if (x.backend().exponent() < -127+1) {
      // subnormal or zero
      if (x.backend().exponent() < -127 - 23) {
        return x.backend().sign() ? -0.0f : 0.0f; // underflow
      }
      float adj =  x.backend().sign() ? -FLT_MIN : FLT_MIN;
      boost_quadfloat_t tmp = x + adj;
      return tmp.convert_to<float>() - adj;
    }
  }
  return x.convert_to<float>();
}

int main(int , char** )
{
  boost_quadfloat_t b = ldexp(boost_quadfloat_t(0x8000017f), -31 - 127);
  float f0 = b.convert_to<float>();
  float f1 = std::nexttoward(f0, f0 < b ? FLT_MAX : -FLT_MAX);

  std::cout
    << std::setprecision(11)
    << b  << " = b\n"
    << f0 << " = f0 = b.convert_to<float>()\n"
    << f1 << " = f1 = std::nexttoward (f0, ...)\n"
    << abs(f0 - b) << " = abs(f0 - b)\n"
    << abs(f1 - b) << " = abs(f1 - b)\n"
    << ((abs(f1 - b) < abs(f0 - b)) ? "f0 is not the closest representable float!!!\n" : "o.k.\n")
    ;

  f0 = my_convert_to_float(b);
  f1 = std::nexttoward (f0, f0 < b ? FLT_MAX : -FLT_MAX);
  std::cout
    << f0 << " = f0 = my_convert_to_float(b)\n"
    << f1 << " = f1 = std::nexttoward (f0, ...)\n"
    << abs(f0 - b) << " = abs(f0 - b)\n"
    << abs(f1 - b) << " = abs(f1 - b)\n"
    << ((abs(f1 - b) < abs(f0 - b)) ? "f0 is not the closest representable float!!!\n" : "o.k.\n")
    ;

  return 0;
}