Re: [FFmpeg-devel] [PATCH 1/2] libavfilter/signature_lookup: fix possible division by zero

[Michael Niedermayer <michael@niedermayer.cc>]

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On Fri, May 24, 2024 at 12:33:11PM +0200, Gerion Entrup wrote:
> Am Dienstag, 7. Mai 2024, 19:46:28 MESZ schrieb Michael Niedermayer:
> > On Mon, May 06, 2024 at 12:30:39AM +0200, Gerion Entrup wrote:
> > > ---
> > >  libavfilter/signature_lookup.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/libavfilter/signature_lookup.c b/libavfilter/signature_lookup.c
> > > index a0ca818a9b..b39a3e225b 100644
> > > --- a/libavfilter/signature_lookup.c
> > > +++ b/libavfilter/signature_lookup.c
> > > @@ -128,7 +128,7 @@ static int get_jaccarddist(SignatureContext *sc, CoarseSignature *first, CoarseS
> > >      int jaccarddist, i, composdist = 0, cwthcount = 0;
> > >      for (i = 0; i < 5; i++) {
> > >          if ((jaccarddist = intersection_word(first->data[i], second->data[i])) > 0) {
> > > -            jaccarddist /= union_word(first->data[i], second->data[i]);
> > > +            jaccarddist /= FFMAX(union_word(first->data[i], second->data[i]), 1);
> > >          }
> > 
> > for which input data does this cause a division by 0 ?
> 
> Sorry for the late answer. I missed your mail somehow.
> union_word counts the amount of one bits that are created when you are "or"ing
> the course signatures. So, when the underlying videos are so different that all
> bits of the created signatures are different, the "or"-operator will always
> return 0 and so also its sum (I have not tested this).

the division only occurs if jaccarddist > 0

basically what iam asking is for which A and B do we have
(A&B) != 0 && (A|B) == 0
or am i misreading the code ?

thx

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Awnsering whenever a program halts or runs forever is
On a turing machine, in general impossible (turings halting problem).
On any real computer, always possible as a real computer has a finite number
of states N, and will either halt in less than N cycles or never halt.

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