Zampelli Stephane wrote:
> Suppose one has a non-monotonic propagator P, because the propagator
> uses an approximation (of a NP-Hard problem) of the pruning.
> How can Gecode accomodate of this situation?
> Does this mean that recomputation cannot be used?
Recomputation will almost certainly fail (i.e., segfault or throw an
exception) if non-monotonic propagators are present.
> Is the result correct if the recomputation is not used (full copying,
> c_d=1)?
Yes.
> What about executing the propag P after the fixpoint of all other
> monotonic constraints?
That looks like a solution in principle, but won't work in practice.
The problem is that the space is not going to be stable (i.e. at a
fixpoint) after running P. Spaces in Gecode can only be copied at
fixpoints. You could iterate the normal fixpoint computation and the
invocation of P until you reach a mutual fixpoint. Still, this is not
guaranteed to work with batch recomputation, where only one fixpoint
is computed for each backtrack.
We have thought a bit about how to accomodate non-monotonic
propagators, but it's really not easy.
Cheers,
Guido
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