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From luc <...@spaceroots.org>
Subject Re: [math] Is overflow dealt with properly in Dfp?
Date Wed, 11 Apr 2012 08:16:53 GMT
On Wed, 11 Apr 2012 06:23:56 +0200, Sébastien Brisard wrote:
> Hi,

Hi Sébastien,

> I'm currently working on MATH-753, and the underlying accuracy 
> issues.
> I therefore implemented the Gamma function for Dfps, and came accross
> a few strange things. For example, althoug the javadoc states that 
> the
> maximum exponent is 32768, there is no systematic check on this 
> limit.
> I suppose overflow should happen in the following case
> {code}
>     DfpField field = new DfpField(40);
>     final Dfp x = field.getOne().power10K(Integer.MAX_VALUE);
>     System.out.println(x);
>     System.out.println(x.isInfinite());
> {code}
>
> Instead, what happens is
> {console}
> 1.000000000000000000000000000000000000e-4
> false
> {console}
>
> The surprising answer comes from the fact that power10K actually
> increments its integer argument. I do not think the above printout is
> the expected behavior, am I wrong? I think this is potentially a vast
> issue, because a quick check seems to indicate that "this.nans ==
> INFINITE" is often checked in class Dfp, but I don't think the
> statement "this.nans = INFINITE" ever occurs, which would mean that
> overflow is never dealt with properly. Again, I'm probably wrong, I'd
> like to have the feeling of someone more used to this class than I 
> am.

I obviously agree this result is wrong ...

Is William (the creator of both Dfp and FastMath) still around to deal 
with this issue ?

Luc

>
> Sébastien
>
>
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