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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