[FFmpeg-devel] [PATCH 2/3] avutil/opt: Add AV_OPT_TYPE_UINT64

[Michael Niedermayer]

On Sun, Nov 20, 2016 at 08:56:15AM -0500, Ronald S. Bultje wrote:
> Hi,
> 
> On Sun, Nov 20, 2016 at 6:57 AM, Michael Niedermayer <michael at niedermayer.cc
> > wrote:
> 
> > @@ -131,6 +132,20 @@ static int write_number(void *obj, const AVOption *o,
> > void *dst, double num, int
> >          if (intnum == 1 && d == (double)INT64_MAX) *(int64_t *)dst =
> > INT64_MAX;
> >          else                                       *(int64_t *)dst =
> > llrint(d) * intnum;
> >          break;}
> > +    case AV_OPT_TYPE_UINT64:{
> > +        double d = num / den;
> > +        // We must special case uint64_t here as llrint() does not
> > support values
> > +        // outside the int64_t range and there is no portable function
> > which does
> > +        // "INT64_MAX + 1ULL" is used as it is representable exactly as
> > IEEE double
> > +        // while INT64_MAX is not
> > +        if (intnum == 1 && d == (double)UINT64_MAX) {
> > +            *(int64_t *)dst = UINT64_MAX;
> > +        } else if (o->max > INT64_MAX + 1ULL && d > INT64_MAX + 1ULL) {
> > +            *(uint64_t *)dst = (llrint(d - (INT64_MAX + 1ULL)) +
> > (INT64_MAX + 1ULL))*intnum;
> > +        } else {
> > +            *(int64_t *)dst = llrint(d) * intnum;
> > +        }
> > +        break;}
> >      case AV_OPT_TYPE_FLOAT:
> >          *(float *)dst = num * intnum / den;
> >          break;
> 
> 
> For the stupid, like me: what does this do? More specifically, this seems
> an integer codepath, but there is a double in there. Why?

write_number() has a num/den double argument. If this is used to
set a uint64_t to UINT64_MAX things fail as double cannot exactly
represent UINT64_MAX.  (the closest it can represent is "UINT64_MAX + 1"
So it needs to be handled as a special case. Otherwise it turns into 0


[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

No human being will ever know the Truth, for even if they happen to say it
by chance, they would not even known they had done so. -- Xenophanes
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