[gecode-users] Correct ExecStatus to return from propagate?

[Guido Tack]

Hi!

> Im a bit confused as to when exactly i should return the various
> ExecStatus from my (n-ary) propagator.
>
> I understand that ES_FAILED means that the propagation failed, ie it
> signals a dead-end, so i return that whenever i completely empty a
> domain.

Exactly.

> I also understand ES_SUBSUMED which i return when my constraint is
> entailed by the domains.

Yes.

> However, ES_OK, ES_FIX and ES_NOFIX im not sure if i understand
> correctly. If i have pruned all i can unless another propagator
> removes more values (without reducing all domains to size 1 or
> failing), what should i return? Also, should i return something
> specific if i didnt prune any values?

You never return ES_OK, it is only used for the propagator post  
functions, not for the propagate function itself. Your intuition  
about FIX and NOFIX is basically correct: If you can guarantee that  
after propagation, your propagator is at a fixpoint, i.e. running it  
immediately again will not result in any pruning, you may return  
ES_FIX. In all other cases, return ES_NOFIX. So it is always safe to  
return NOFIX, but potentially less efficient than possible.

Not pruning anything is not a special case: if you know the  
propagator is at a fixpoint, return ES_FIX, otherwise ES_NOFIX.

Christian has some slides dealing with fixpoints of propagators  
(slide 40):
http://www.ict.kth.se/courses/2G1515/notes/2G1515-L05-1.pdf

> Finally if all domains are reduced to size 1, should i return
> ES_SUBSUMED, or something else?

You should return ES_SUBSUMED. This will guarantee that the  
propagator is removed and its memory reclaimed. Our test suite, e.g.,  
checks for all propagators that they are subsumed when all variables  
are assigned.

> On a related note, does a variable automatically switch to being
> assigned when its domain is reduced to a single value?

Yes, you don't have to do anything about that. Actually, a variable  
doesn't have a flag or anything that says that its assigned - it just  
checks (in the case of integer variables) whether min=max.

Cheers,
	Guido

-- 
Guido Tack
Programming Systems Lab, Saarland University, Germany
http://www.ps.uni-sb.de/~tack



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