Nieminen Juha <war### [at] sarakerttunencstutfi> wrote...
> Nathan Kopp <Nat### [at] koppcom> wrote:
> : *   PORTABILITY WARNING:  this function REQUIRES IEEE single precision
> : floating
> : *   point format to work.
>
>   Why?

Later in the code you'll run across these two cryptic operations and
comments that describe them.

First,

  /*
   * This hex operation does a floor to next lower power of 2, by clearing
   * all of the mantissa bits.  Works only on IEEE single precision floats
   */
  convert.f = maxdel;
  convert.l &= 0xff800000;
  bsized = (DBL) convert.f;

And later,
  /*
   * This magic hex operation extracts the exponent, which gives us an
   * integer number suitable for labelling a range of a power of 2.  In IEEE
   * format, value = pow(2,exponent-127). Therefore, if our index is, say,
   * 129, then the item has a maximum extent of (2 to the (129-127)), or
   * about 4 space units.
   */
  convert.f = (float) bsized;
  base_id.Size = (convert.l & 0x7f800000) >> 23;


The variable "convert" is defined as follows:
  union
  {
    float f;
    long l;
  }
  convert;


A portable version of these operations could be written, using logrithms and
fractional powers, but it would be significantly slower.  I suggest #ifdef
blocks so that VAX and other non-IEEE-compliant systems will be able to run
this code, even if it does have to run slowly.

-Nathan

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