[gecode-users] Gecode terminates on incomplete solution

[Philipp Keck]

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?

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?


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