Re: [boost] [Random] bug in uniform_int or uniform_smallint

From: Jens Maurer (Jens.Maurer_at_[hidden])
Date: 2002-03-11 15:33:47

Douglas Gregor wrote:
> 
> This code in uniform_int:
>  120     if(_brange / _range > 4 /* quantization_cutoff */ ) {
>  121       // the new range is vastly smaller than the source range,
>  122       // so quantization effects are not relevant
>  123       return uniform_smallint<base_type,result_type>(_rng, _min, _max)();
>  124     } else {
> 
> will make uniform_smallint do very bad things when _max-_min+1 is large
> enough to make 32*_range*_range (where _range = _max-_min+1 at line 95 of
> uniform_smallint.hpp) overflow and become negative. Then we get stuck in the
> following loop forever (uniform_smallint.hpp):
> 
>   95     const base_result r = 32*_range*_range;
>   96     for(; r_base >= r; _factor *= 2)
>   97       r_base /= 2;
> 
> I don't know enough about this to know if uniform_int should take care of
> this (and not deflect to uniform_smallint if it would overflow), or if
> uniform_smallint should deal with this overflow itself.

Indeed.  uniform_smallint<> should cope with the input.  I've fixed it in the
CVS.

Thanks for the report.

Jens Maurer