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

On Sat, 14 Sep, 2024 at 11:09, Ilya Zakharevich wrote:

> The function exponent(x) gives a very coarse value of log2(x).  The
> function log(x)/log(2) gives a very fine value of log2(x) (but is
> slow, especially if x has high precision).  But sometimes I prefer to
> know log2(x) with an intermediate precision; in fact, any sufficiently
> smooth monotonic function which coincides with log2 on powers of 2
> would be enough!
>
> In particular, exponent(x) + mantissa12(x) - 1 would be OK.  (Here
> mantissa12() is the (top word of) mantissa-scaled-to-[1,2].)  It
> should be very cheap to calculate.  It may be beneficial to have two
> flavors, returning a double, or a 1-word PARI float.
>

 expofp(x)=my(e=exponent(x));e+(precision(x,19)>>e-1);

     Works only on floating point x's.

         Loïc