[Gecode] Discarding propagators

[Gustavo Gutierrez]

Hi,

Christian Schulte said:
> Hi,
>
> the problem is to call the propagate method directly: this is not
> possible!
> The only interaction that is allowed for a propagator is creation!
>

Actually, what I am doing at propagate method is just a by-pass (i.e
it returns whatever the method implementing the consistency will
return). So the propagate method is never called directly.


> What is not entirely clear to me why do you want to have temporay
> propagators.

Temporary propagators intend to separate the behaviour of a propagator. This
is a design decision. Different algorithms (in terms of complexity) may
enforce the same consistency technique. In this context, the idea of using a
propagator as an auxiliar tool to build up a more sophisticated,
parametrized consistency technique makes sense.

>From a design view, one could write propagators that can be *parametrized*
with other propagators. For instance, a propagator implementing some
consistency technique that uses other low level consistency
technique. The way in which the other (low level) consistency is
used depends entirely on the new propagator.

The propagator I am implementing uses the returned value from a
low level propagator to decide whether to prune domains or not. A failure of
the low level propagator does not imply that the propagator must fail.
Here is when temporary propagators come into play. The temporary
propagator is created only for checking purposes and depending on its
output some
actions are taken. For instance, another low level propagator may be
invoked. That is why i need temporary propagators.

>As far as I understand, you are just using some propagation
> method, that's it. Why not use a method that can be used without creating
> a
> propagator first?

That is another choice. To implement functions enforcing consistency
techniques and to make this functions friends of propagator objects.
I think this could work but in my opinion it is not as clear as the
temporary propagator approach and it does not allow parametrization either.

Thanks in advance,
Gustavo Gutierrez

>
> Christian
>
> --
> Christian Schulte, http://www.imit.kth.se/~schulte/
>
> -----Original Message-----
> From: gecode-bounces at ps.uni-sb.de [mailto:gecode-bounces at ps.uni-sb.de] On
> Behalf Of Gustavo Gutierrez
> Sent: Friday, March 18, 2005 11:00 PM
> To: gecode at ps.uni-sb.de
> Subject: [Gecode] Discarding propagators
>
>
> Hi all,
>
> Few weeks ago i posted a message with subject "design question". There, i
> was trying to implement a propagator that uses other propagators to verify
> some kind of consistency and then decide to prune or not domain values.
> That
> is, the user-imposed propagator calls auxiliar propagators, and using the
> outcome of such propagators, takes certain actions.
>
> In order to do that without fall in space failures i use a copy of
> variables
> and create a new propagator, then call the propagate method directly. This
> new propagator needs to be disabled after the consistency proof (i.e. it
> will not exist as long as ordinary propagators imposed by user). In order
> to
> "simulate" propagator disabling i added a new method that contains a flag.
> When the flag becomes true the propagate method returns ES_SUBSUMMED. My
> hope is that the auxiliar propagator will be removed (in a safe way) from
> the propagation queue at some time after this.
>
> I was able of implementing the "main" propagator following this approach.
> My
> problem is that it works fine in some examples, but for others it raises a
> segfault. Very likely, I have a misassumption regarding the role and
> functioning of the propagation queue. I would apprecciate any comments
> and/or suggestions on this issue.
>
> Thanks in advance,
>
> Gustavo Gutierrez _______________________________________________
> Gecode mailing list
> Gecode at ps.uni-sb.de http://www.ps.uni-sb.de/mailman/listinfo/gecode


--------------------------------------------------------------
 Please avoid sending me Word or PowerPoint attachments.
 See http://www.gnu.org/philosophy/no-word-attachments.html