Re: [B.A.T.M.A.N.] [RFC 05/10] batman-adv: add bat_metric_is_similar API function

Antonio Quartulli
2013-05-29T14:28:51Z

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On Wed, May 29, 2013 at 04:16:57PM +0200, Simon Wunderlich wrote:
> On Wed, May 29, 2013 at 12:20:45AM +0200, Antonio Quartulli wrote:
> > From: Antonio Quartulli <antonio@open-mesh.com>
> >
> > Each routing protocol has its own metric semantic and
> > therefore is the protocol itself the only component able to
> > compare two metrics to check similarity similarity.
> >
> > This new API allows each routing protocol to implement its
> > own logic and make the external code protocol agnostic.
> >
> > Signed-off-by: Antonio Quartulli <antonio@open-mesh.com>
> > ---
> > bat_iv_ogm.c | 7 +++++++
> > main.c | 3 ++-
> > main.h | 6 ++++++
> > types.h | 3 +++
> > 4 files changed, 18 insertions(+), 1 deletion(-)
> >
> > diff --git a/bat_iv_ogm.c b/bat_iv_ogm.c
> > index abf4cd3..18c9ae8 100644
> > --- a/bat_iv_ogm.c
> > +++ b/bat_iv_ogm.c
> > @@ -1441,6 +1441,12 @@ static uint32_t batadv_iv_ogm_metric_get(struct batadv_neigh_node *neigh_node)
> > return neigh_node->bat_iv.tq_avg;
> > }
> >
> > +static bool batadv_iv_ogm_metric_is_similar(uint32_t metric,
> > + uint32_t new_metric)
> > +{
> > + return (metric - new_metric < BATADV_TQ_SIMILARITY_THRESHOLD);
>
> You might want to use abs(metric - new_metric) here, otherwise is_similar(a, b) output
> might differ from is_similar(b, a).

Mh..imho the name of the function is bad because this has been done on purpose.

The idea is that we want to see if 'b' is at least
as good as 'a', therefore what we want to check if is b is greater than
'a - threshold' only.

Imagine that 'b' is greater than (a + threshold), for me the function has to
return true, because the metric b is at least as good as a, but if I introduce
the abs() the function would return false.

Example:

a=190
b=240
threshold=20

a - b = -50 < 20 => b is at least as good as a!

using abs:

abs(a - b) = 50 < 20 => NO! b is worse than a....and this is not true.

this situation can happen in the code because usually 'a' will represents some
kind of current metric and this is not supposed to be the best ever (maybe we
still have to switch to a new best).

I hope I made it clear.

For me "similar" was something like: "ok, metric b comes from a link that can be
used where we now have the link represented by a".

Cheers,

--
Antonio Quartulli

..each of us alone is worth nothing..
Ernesto "Che" Guevara

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