Double to Int

  • Author or source: many people, implementation by Andy M00cho
  • Type: pointer cast (round to zero, or ‘trunctate’)
  • Created: 2002-01-17 03:04:41
notes
-Platform independant, literally. You have IEEE FP numbers, this will work, as long as
your not expecting a signed integer back larger than 16bits :)
-Will only work correctly for FP numbers within the range of [-32768.0,32767.0]
-The FPU must be in Double-Precision mode
code
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typedef double lreal;
typedef float  real;
typedef unsigned long uint32;
typedef long int32;

   //2^36 * 1.5, (52-_shiftamt=36) uses limited precision to floor
   //16.16 fixed point representation

const lreal _double2fixmagic = 68719476736.0*1.5;
const int32 _shiftamt        = 16;

#if BigEndian_
        #define iexp_                           0
        #define iman_                           1
#else
        #define iexp_                           1
        #define iman_                           0
#endif //BigEndian_

// Real2Int
inline int32 Real2Int(lreal val)
{
   val= val + _double2fixmagic;
   return ((int32*)&val)[iman_] >> _shiftamt;
}

// Real2Int
inline int32 Real2Int(real val)
{
   return Real2Int ((lreal)val);
}

For the x86 assembler freaks here's the assembler equivalent:

__double2fixmagic    dd 000000000h,042380000h

fld    AFloatingPoint Number
fadd   QWORD PTR __double2fixmagic
fstp   TEMP
movsx  eax,TEMP+2

Comments

www.stereopsis.com/FPU.html credits one Sree Kotay for this code.

On PC this may be faster/easier:

int ftoi(float x)
{
  int res;
  __asm
  {
    fld x
    fistp res
  }
  return res;
}

int dtoi(double x)
{
  return ftoi((float)x);
}