[gecode-users] order in calls to status and commit...

[Sébastien Mouthuy]

Le lundi 28 mai 2007 à 18:29 +0200, Guido Tack a écrit :
> Sébastien Mouthuy wrote:
> 
> 
> > Could you tell me what is exactly the order in which the functions
> > status and commit are called ? 
> 
> 
> I presume you mean status and commit of a branching, right?
yes 

> 
> > For me it should be
> > 
> > 
> > call to status with space S. if it returns true, then copy S. let's
> > call
> > the copy Sc. Then call commit() with S and a=0. when we will have
> > backtracked, commit will be called with Sc and a=1.
> 
> 
> In principle, this is how it works. Our search engines however use
> recomputation, so they don't actually make a copy everytime, but
> sometimes use a copy that is higher up in the search tree and
> recompute the needed space from there.
> 
> 
> The exact protocol is:
> 
> 
>  * When you call Space::status, after computing a fixpoint of the
> propagators, the space calls the status method of the installed
> branchings to find the first branching that returns true. If there is
> none, the status is SS_SOLVED, otherwise it's SS_BRANCH.
>  * When you call Space::description, the space asks the branching that
> returned true for a description and returns it
>  * When you call commit, the space tries to find the branching that
> matches the description, and calls its commit method.
> 
> > I ask this question because I post a branching in the commit
> > function of
> > another branching. Then I catch the following exception:
> > terminate called after throwing an instance of
> > 'Gecode::SpaceNoBranching'
> >   what():  Space::commit: Attempt to commit with no branching
> 
> 
> This means that in the commit step, the space didn't find a branching
> that matched the branching description. This could either mean that
> the branching description is bogus, or that your branching really
> hasn't been installed yet.

ok. so the problem would be that I post a Branching B1 and then return a
description d1 for that branching. Then when the recomputation occurs,
the branching B1 is not found and Gecode has no idea what to do with d1.
I verified that but it seems that I post a branching B2 again during
recomputation. This is the same branching as B1 (ie the same class) but
is not a direct copy from B1. I use the general constructor (ie the one
we use when defining CSP's). 

Could that be the problem ? Should B1 and B2 be two copies of a comman
instance of the branching ? I know descriptions and branchings use id's.
could it be the problem ?
> 
> 
> Are you using the standard, built-in search engines? If yes, please
> check if the problem still persists with c_d=1 and a_d=100000 as
> parameters passed to e.g. DFS (switching off recomputation). If the
> problem goes away using these options, there's either something wrong
> in our Space::commit logic, or in the way you create and use
> BranchingDescs.

yes. the problem disappears when disabling recomputation.

Thanks for your quick replies,

have a good evening,

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