Re: [musl] DTOA: question about rendering / code pointer

["alice" <alice@ayaya.dev>]

On Tue May 16, 2023 at 9:36 PM CEST, Rich Felker wrote:
> On Tue, May 16, 2023 at 08:49:11PM +0200, newbie nullzwei wrote:
> > ( apologize - re-tweet - 'plain text', hope that will work better | 
> > apologize - re-tweet - now 'subscribed' - got the impression my
> > mails didn't make it through to the list while not subscribed? ) 
> >  
> > hello @ all, 
> >  
> > gnumeric uses musl dtoa for rendering, and a self constructed 
> > 'brute force' concept to find 'shortet round tripping' figures mostly 
> > similar to https://www.exploringbinary.com/the-shortest-decimal-string-that-round-trips-may-not-be-the-nearest/ 
> >  
> > we face the same issue as mentioned there, some powers of two 
> > miss to get the shortest 'round tripping' string, but have assigned 
> > another one digit longer string. Forcing to one digit less produces a 
> > string one decimal off in last position, and not tripping back to 
> > the original double. 
> >  
> > Example: 0x1p-44 ( 2^-44 ) is rendered to 
> > 
> > 5.6843418860808015E-14 when allowed 17 digits, for 
> > 16 digits it switches to 5.68434188608080**1**E-14, which is 
> > not too bad as the 'exact' decimal weight of the binary representative 
> > is ~5.684341886080801486969E-14, thus undershot to ~15, but! 
> > it points to a different double, and 5.68434188608080**2**E-14 
> > would be a better choice as it round trips to the originating double 
> > value. Affected ~46 integral powers of two in doubles, many more 
> > with long doubles.   
> >  
> > Is there any hope musl could change that? provide it as an option? 
> > Or can anyone give a code pointer or nearer explanation to enable 
> > us to patch it for our 'exotic' use? 
>
> musl always, and very intentionally, performs correct rounding
> according to the current rounding direction for all conversions
> to/from decimal. Getting the shortest decimal string that round-trips
> is a somewhat different problem, and not one libc has any API for. But
> if what you want is shortest string which round-trips, I think there
> are faster approaches than what musl does; ensuring exactness like we
> do is more work.

dragonbox/schubfach is probably exactly what you're looking for, for which
a (c++) reference implementation is at
https://github.com/jk-jeon/dragonbox

afaik libreoffice now uses this for the same usecase(?).

>
> Rich