[gecode-users] Advisors : need your advise if worth a look for my propagator! ; )

[Martin Mann]

Hi everybody,

thanks to your answers Mikael and the silent support and mails from 
Daniel Przybilla I have a better understanding of the Advisors, their 
use and functionality.

Now the question appears: is the effort/overhead worth for me or not?

I had a look at Mikaels thesis and the final statement that Advisors are 
mainly advantageous for n-ary constraint and not that useful for e.g. 
binary constraints (correct me if I got it wrong).

Thus, I would like to get your feeling on my constraint, because you are 
the experts on your system! ;)

================
  My problem:
================

I have a binary constraint propagator p(X,Y) that does a very strong but 
expensive filtering on the domains of variables X and Y. Thus I would 
like to delay the application until one of the domain sizes is smaller 
than a certain threshold.

In Gecode 1.3.* I managed like that:
- I subscribed the propagator for domain changes on the two variables
- within the propagate function I checked if the minimal domain size of 
both is smaller than my threshold
- if so I ran the filtering algorithm, otherwise I returned that nothing 
could be propagated (I was lazy and know this is kind of a hack.. ;) )

In my Gecode 3.* reimplementation I want to do it in the best way 
possible. So the question is: Do I stick to the old "hack" or is it 
worth a look to implement Advisors that do the domain size check for me?

=================================
  The CSP itself is quite simple:
=================================

- n variables : X_i with 1 <= i <= n
- one alldiff : distinct( X_1,..,X_n )
- (n-1) binary constraints : p(X_i, X_(i+1)) with 1 <= i < n

(for anybody even more interested in details : p encodes a neighboring 
constraint in a lattice and the whole CSP encodes a self-avoiding-walk 
of length n on a lattice)

=================================

Soo... the again final question: *Is it worth to use an Advisor for my 
domain size check? Or is the hack nasty but most probably faster?*

I know it is hard for you to give a statement without knowledge on the 
propagator's details but an expert guess would be sufficient for me. ;)

Thanks a lot!

Martin


-- 
Martin Mann, Dipl. Bioinf.
Bioinformatics - Inst. of Computer Science
Albert-Ludwigs-University Freiburg
Tel: ++49-761-203-8259
Fax: ++49-761-203-7462
http://www.bioinf.uni-freiburg.de/~mmann/