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From Sébastien Brisard <sebastien.bris...@m4x.org>
Subject Re: [math] Is overflow dealt with properly in Dfp?
Date Fri, 13 Apr 2012 15:30:05 GMT
Hello,
>
>> 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 ?
>
I'm afraid he probably is not... I feel like walking on eggs on this
issue, because I'm not yet fully familiar with the Dfp class. If
everyone agrees, I'll wait a little bit before I file a bug report.
Sébastien


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