[Maxima] Cache bug in bigfloat %e

Raymond Toy toy.raymond at gmail.com
Tue Dec 8 19:16:59 CST 2009

Dieter Kaiser wrote:
>> >From the testsuite, the hash table has 75 entries, ranging from 56 bits
>> to 1681 bits.
> A lot of entries might be caused, because I have tried to check the
> numerical algorithm of the beta and gamma functions over a greater range
> of values for fpprec.
Yes, and that's good.  I figure the testsuite is probably more than what
a typical user would do.
>> There is another approach that might work, but I suspect there are cases
>> where it would also give incorrect results.  We can continue to use the
>> current algorithm, but if fpprec is too close to the max saved
>> precision, we recompute %e to, say, twice fpprec and round that.
> I think most important is to have a bigfloat arithmetic, which is
> correct and consistent. The time I have implemented the bigfloat
> algorithm for special functions I had no idea that we can get
> inconsistencies because of the bigfloat implementation.
Agreed.  Computations should be repeatable.  I was also surprised about
the inconsistency, and was shocked to learn it was caused by the caching
in fpe.  I've never seen an example of this in maxima.

I'm not happy with the test failure for rtest_gamma 644.   I'm willing
to attribute it to a change in the value of %e, but I don't like it. 
I'm sure there are other cases where we will get different results from
before, but the new results should be repeatable.


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