[gecode-users] Gecode terminates on incomplete solution

[Max Ostrowski]

Hi,

On 05/18/2014 06:38 PM, Philipp Keck wrote:
> Hi,
> 
> I am new to Gecode and also to Constraint Programming. While my first
> problem may be a general CP question (so off-topic here? In that case
> ignore it please.), my second problem is Gecode-specific.
> 
> My first problem is that Gecode either finds a solution immediately
> (<1ms) and without any backtracking (peak-depth equals explored
> nodes-1), or takes longer than I want to wait (at least 2 hours). This
> problem might be related to my problem instances, because or-tools shows
> exactly the same behaviour. My problem instances have 1,000 to 10,000
> boolean variables and 400 to 2,500 constraints. There are both large and
> small instances that can be solved within milliseconds, and both large
> and small instances that take a long time. For those that take more than
> 2 hours, I still am sure that there is a solution, because Gurobi finds
> one when I formulate the same problem as an LP.
> How could I improve the running time of Gecode here? Does adding more
> constraints make finding (any feasible) solution faster (because the
> search space is more restricted and therefore the search tree is
> smaller) or slower (because there are simply less solutions)? Should I
> add redundant constraints?

If adding more constraints make the search faster or not can not be said
in general.
Adding redundant constraints is always worse a try, if you can improve
propagation strength with it.
> 
> For a set of boolean variables, if I want at most one of them to be 1,
> is it better to add "a+b+c+... <= 1" using a linear-Constraint, or
> should I use "a!=b", "a!=c", "b!=c", ... instead? Unfortunately I have
> difficulties finding out the answers by experimenting because Gecode
> runs for such long times. Is there a way to get some output during the
> computation?

The first thing you can try is using global cosntraints that exactly fit
your needs.
In this case, allDifferent or atmost, etc...
They usually perform much better than linear constraints.
(It may be the case that Gecode automatically detects this in your case
and replaces the linear constraints with the global constraints. )

Furthermore, your problem sounds like a PB(Pseudo-Boolean) problem.
So maybe you should try a PB solver.

Best,
Max

> 
> My second problem is a weird behaviour of Gecode terminating with some
> variables remaining unassigned:
> My models have the following structure: I have boolean variables only
> and two kinds of constraints. The first kind of constraints requires
> exactly c variables out of a certain subset to be 1, e.g. "a+c+d+x+y =
> 3" or "a+b = 1". The second kind allows at most one variable to be 1,
> e.g. "a+b+r <= 1". That's all.
> As mentioned above, Gecode either terminates immediately, or takes a
> very long time. However, I have a particular problem instances that
> makes Gecode terminate and report "solutions: 1" without having all the
> variables set. So there are variables that still have [0..1] instead of
> a specific value and val() throws ValOfUnassignedVar. Why does Gecode
> terminate before it has a solution?
> I call it like this:
> branch(*this, allVariablesArgs, INT_VAR_NONE(), INT_VAL_MIN());
> Using this call on the other hand makes Gecode compute forever (i.e.
> more than 2 hours):
> branch(*this, allVariablesArgs, INT_VAR_RND(someRnd),
> INT_VAL_RND(someOtherRnd));
> 
> The problem might be related to how I implemented the copy function. I
> have my variables inside a std::map<std::string, BoolVar> *and* inside a
> BoolVarArgs and clone them like this:
> variablesMap = s.variablesMap;
> allVariablesArgs = BoolVarArgs(s.allVariablesArgs);
> I don't know if that's the correct way to go. In particular, my copy
> implementation never uses the share-parameter.
> After reading the thread "Integer Variable Randomization" on this
> mailing list, I also tried setting -c-d and -a-d to high values, but
> still the copy-method was called many times.
> 
> Thank you for any help!
> Philipp
> 
> Platform: Windows 7 x64, Visual Studio 2013 x64, Gecode 4.2.1
> 
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