On 09/07/2016 09:20 PM, Andrew Borodin wrote:
Well, arithmetics is too fragile.

This version of float packing with arithmetical packaging
static float
pack_float(float actualValue, int realm)
double max,min;
max = FLT_MAX / ( 8 >> realm );
min = FLT_MAX / ( 16 >> realm );
if( realm == 0 )
min = 0;
/* squeeze the actual value between min and max */
return ( min + (actualValue * ( max - min ) / FLT_MAX));
Inserts are the same as of bithacked pack_float, but selects are 5 times slower.
When we are trying to pack value linearly into range we loose too much bits.

That's why I suggested scaling it by the new value's volume and/or edge-sum. I was hoping that the old and new values are roughly of the same magnitude, so that it would work out. I guess not.

But we could probably something like the above too, if we use logarithmic or exponential, rather than linear, packing. Something like:

static float
pack_float(float actualValue, int realm)
  double  val;

  val = sqrt(sqrt(actualValue));

  if (realm == 0)
    return actualvalue;
  if (realm == 1)
    return actualValue * sqrt(sqrt(FLT_MAX));
  if (realm == 2)
    return actualValue * sqrt(FLT_MAX);

Unfortunately, sqrt(x) isn't very cheap.

- Heikki

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