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
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