Re: A missing (?) function: exponentfp(x)

[Ilya Zakharevich <nospam-abuse@ilyaz.org>]

On Thu, Sep 19, 2024 at 12:30:24PM +0200, Aurel Page wrote:
> > Unfortunately, the complex branch of dbllog2() seems to be majorly
> > wrong…
> > 
> > Using flog2() wrapper defined earlier in this thread:
> > 
> >        	       flog2delta(x)=flog2(x)-log(abs(x))/log(2);
> >    (03:01) gp > flog2delta(1+0.0008*I)
> >    %284 = -4.6166226535255913560320062458846938056 E-7
> > 
> > It seems that somebody wanted to write a Taylor series of order
> > BYTES_IN_LONG/4 (in 4**(-abs(x-y))/2?), but stopped at writing the 0th
> > term only…

> I was quite puzzled by the complex branch too. But in any case when adding
> other types we would apply the same recursion for t_COMPLEX as for
> exponent(), i.e. maximum of result for real and imaginary parts.

I think complex numbers are sufficiently different from vectors and
polynomials to “continue to support log2(abs(x)) semantic”.

As far as fixing the bug goes, “if only I knew a cheap way to do
todbl(x/y) with precision 2^-43”, or do mantissa2dbl(x), then it would
be easy to deal with it.

Thanks,
Ilya