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Fast rounding

Dec 8, 2009 8:53 PM

Since the lrint function seems to be unavailable in Alchemy's C library - along with all the other rounding functions that ought to be in math.h* - any suggestions on fast rounding of a double to an integer?  Is casting, subtracting, comparing and incrementing the only way?

 

*When I try to use these functions it compiles just fine, but I get "Undefined sym: _rint" (or similar) when I try to use the resulting .swc in Flash.

881 Views   2 Replies   Latest reply: zazzo9, Dec 18, 2009 6:31 AM
 
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  • chocotaco22
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    1. chocotaco22,
    Dec 9, 2009 8:56 AM   in reply to chocotaco22
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    This is made stranger by the fact that ldexp IS available.  Is there anywhere to find out what C library functions are available and which aren't?

     
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  • zazzo9
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    2. zazzo9,
    Dec 18, 2009 6:31 AM   in reply to chocotaco22
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    Simple rounding of a double to an integer:

    int round(double d) { return (int)(d + 0.5); }

     

    I once made this implementation of lrintf, designed to reproduce IEEE behaviour (where .5 is rounded to the nearest even number):

    extern long lrintf(float f)
{
  /* Implements the default IEEE 754-1985 rounding mode */
  long * fp = (void *)&f;
  int sign = (*fp) >> 31;
  int exponent = 23 + 0x7f - (((*fp) >> 23) & 0xff);
  unsigned int fraction = ((*fp) & 0x7fffff) | 0x800000;
  long result = fraction >> exponent;
  -- exponent;
  if (fraction & (1 << exponent)) { // fraction >= 0.5
    if (!(fraction & ~(-1 << exponent))) // fraction == 0.5
      if (!(result & 1)) // result is even
        return sign ? -result : result;
    ++ result;
  }
  return sign ? -result : result;
}

    Those constants (31, 23, 0x7f) are specific to the float datatype.

     

    You can identify what functions are available by passing avm2-libc/lib/avm2-libc.l.bc through llvm-dis and examining the definitions in the resulting .ll file.

     
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