Hi Christian, sorry for taking long to respond. I read the manual and tried to modify the engines to do what I wanted. Unfortunately, I was not succesfull...
My first attempt was to run BAB until completion with the long steps (adding chunks of constraints in the constrain() function). I was then hoping that I should be able to modify the last solution returned by BAB (by setting a parameter in the corresponding object, lastSolution->setLargeChunks(false), to influence the behavior of the constrain function in the future), and create and run a new BAB with this solution object as initial object. (something like BAB<...> e(lastSolution, options) ). Although it does run, I get the feeling that it starts the search from the beginning and does not benefit from the search done so far. I then tried to understand how to modify the parallel engine code, but it was beyond my capabilities... Thanks, Dirk On Tue, Apr 18, 2017 at 1:52 PM, Christian Schulte <cschu...@kth.se> wrote: > Hi Dirk, > > > > Yes, I see your motivation but the Gecode engines cannot do that. Have you > ever had a look at the part in MPG about search? Building a BAB engine with > this additional feature might be not that hard… > > > > Good luck > > Christian > > > > -- > > Christian Schulte, www.gecode.org/~schulte > > Professor of Computer Science, KTH, cschu...@kth.se > > Expert Researcher, RISE SICS, christian.schu...@ri.se > > > > *From:* Dirk Obermann [mailto:dirk.oberman...@gmail.com] > *Sent:* Wednesday, April 12, 2017 22:28 > *To:* cschu...@kth.se > *Cc:* users@gecode.org > *Subject:* Re: [gecode-users] Adding constraints to model in <RBS>: is > there any example code? > > > > Thanks, Christian. > > > > You were, of course, correct. BAB does the job and my code is now much > more elegant than before. It also works well to combine with RBS. > > > > However, my earlier code was sometimes faster. The reason is that I could > take "large steps", adding constraints in chunks until the search failed. I > could then go back to the last feasible model and add constraints one by > one instead of in chunks. > > Is something similar possible in the gecode search engines? What I would > like do is the following: I run BAB until fail, and then "reset the state > of the engine" to where it was when it produced the last feasible solution > ("before my last call to the constrain function", so to speak). I would > then re-run the search from there (with another behavior of the constrain > function). > > Thanks, > > Dirk > > > > On Tue, Apr 11, 2017 at 1:49 PM, Christian Schulte <cschu...@kth.se> > wrote: > > Hi, > > > > First, I am not so sure that your idea works in principle: you say that > the problem is that you spend a lot of time to find a first feasible > solution, especially after adding additional constraints. However, that > would not change with restarts: after all, you would restart the search and > would have to do the same search in order to find the first feasible > solution. The only benefit I could see is adding no-goods from restarts. > > > > Anyway, what you want to do (I guess) is that you accumulate additional > constraints which each restart. This you can only do by adding constraints > to the master (remember, the slave will always be initialized fresh from > the master for each restart). LNS is different as you want try different > constraints each time you restart, hence you have to do it on the slave. > > > > Hence, you will have to reimplement the master function, there you just > add the additional constraints. > > > > Have you thought about using BAB? There, each time you find a solution you > can add new constraints based on that solution _*without*_ restarting > search. Sounds more like it to me. > > > > Cheers > > Christian > > > > -- > > Christian Schulte, www.gecode.org/~schulte > > Professor of Computer Science, KTH, cschu...@kth.se > > Expert Researcher, RISE SICS, christian.schu...@ri.se > > > > *From:* users-boun...@gecode.org [mailto:users-boun...@gecode.org] *On > Behalf Of *Dirk Obermann > *Sent:* Tuesday, April 11, 2017 13:34 > *To:* users@gecode.org > *Subject:* [gecode-users] Adding constraints to model in <RBS>: is there > any example code? > > > > Hello - > > > > I am trying to do program up a search engine that performs sequential > refinement by adding more and more constraints to a model. In the first > run, I am happy with a solution at all, but then I want to add increasingly > complex constraints until there is no more solution (or the search engine > times out). > > > > Now, I first run a model with a single constraint. If DFS can find a > solution, then I run a completely new model with two constraints and from > scratch. If this model is feasible, then I run the model with three > constraints, etc. > > > > In principle, this solution works, but it is a very inefficient: a lot of > time is spent generating the first feasible solution when restarting the > search (especially as the problem becomes more constrained). > > > > It seems to me that this should be doable with RBS or a variation of the > LDS engine, as described in the MPG. But there are very few examples to > follow (the closest I found was this question posted on stack overflow > http://stackoverflow.com/questions/34182681/how-to- > change-the-variable-value-ordering-in-gecodes-restart-based-search but > unfortunately it does not contain any answer). I simply cannot understand > how to do it right. In particular, where do I post additional constraint on > the model (in the master space, the slave space or elsewhere)? > > > > Any leads on how to do it, or links to code that adds constraints or > changes branching order in RBS would be appreciated! > > > > Cheers, > > > > Dirk > > >
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