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From S├ębastien Brisard <sebastien.bris...@m4x.org>
Subject [math] Is overflow dealt with properly in Dfp?
Date Wed, 11 Apr 2012 04:23:56 GMT
Hi,
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.

S├ębastien


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