Hello John,

On Wed, Apr 02, 2025 at 02:52:39PM -0400, j@bitminer.ca wrote:

> Hello, I think you left out the best part:
> 
>         switch (unit) {
>         case 'f':
>                 myres *= 65536.0;
>                 break;
>     ...
>     <snip>
>     ...
>         case 'M':
>                 myres *= 24.0 / 100.0;
>                 break;
>         default:
>                 break;
>         }
>         if (res != NULL)
>                 *res = myres;    <--- ***here***
> 
> The whole point seems to be to calculate an integer,
> returned in *res.

That is completely correct.
The point is that the roff(7) language allows users to specify
arbitrary floating point numbers and these can be scaled with
so-called scaling units - but the end result is indeed an integer.

> Some programmer

That was me.  :-)

> who might have been less inclined to write scaled
> integer arithmetic code

Before i wrote the double arithmetic code, that code did use integer
arithmetic.  I had written that integer arithmetic code several years
earlier.

> decided, instead, to adopt doubles.

I deemed the change necessary because the roff(7) input syntax
supports decimal fractions.

Admittedly, it might be possible to parse 23.999 as *two* integers
(23 and 999), scale both parts separately, do some kind of carry,
and finally add both results, all using integer arithmetics.
I admit i did not consider that possibility when switching this
code to floating point, but it does not feel like a simple solution.

> OK, fine, but I think an implied goal here is to generate an exact
> *rounded* result.  Which is necessary because as often seen doubles
> don't calculate integer values very reliably.
> 
> While your fix is obviously valid I think the fix could also have
> been
> 
>     *res = trunc(myres + 0.5);
> 
> which eliminates the 23.999999 problem in the usual floating-point
> way.

No, that would be incorrect behaviour.
If myres is 23.9 or 23.99 or even 23.999999, then roff(7) semantics
requires that *res must become 23, but your rounding would make it 24.

Yours,
  Ingo