[gecode-users] Adding variables incrementally

[Mikael Zayenz Lagerkvist]

On Fri, Apr 25, 2008 at 5:28 AM, Malcolm Ryan <malcolmr at cse.unsw.edu.au> wrote:
> In order to save memory, I've been trying to turn my planning code
>  into a two stage in which the first stage only creates and constrains
>  a certain critical subset of variables. When all the first-stage
>  variables have been assigned, additional variables are created and a
>  second stage of search occurs. The two stages are not completely
>  independent however, and there may be failures in the second stage
>  which require backtracking into the first. This should save memeory,
>  since the second-stage variables don't have to be carried around
>  during the first-stage search.
>
>  I was hoping that I could do this simply by adding new variables and
>  constraints in the copy constructor at the appropriate moment, but it
>  seems that this does not work.
</snip>
>  Is there any other way to do a two-stage search like this?

While the copy-constructor is not the right place to add constraints
and variables, there is a more fundamental problem with your scheme.
When all the variables are assigned, all branchings will be finished,
and thus the Space will be solved (status returns SS_SOLVED). The
search will therefore stop the search and return the current solution.

You could program you addition of variables and constraints as a
branching, but since you want to do this when all the original
variables are assigned, then I would suggest the following scheme.
    FirstStageSpace first = new FirstStageSpace();
    DFSIterator<FirstStageSpace> outerDFS =
        new DFSIterator<FirstStageSpace>(first);
    while (outerDFS.hasNext()) {
        FirstStageSpace firstsolution = outerDFS.next();
        SecondStageSpace second =
            new SecondStageSpace(firstsolution);
        DFSIterator<FirstStageSpace> innerDFS =
            new DFSIterator<SecondStageSpace>(second);
        if (innerDFS.hasNext()) {
            System.out.println(innerDFS.next()); break;
        }
    }
I've not tested the above code, but I think the idea should be clear.

Cheers,
Mikael

-- 
Mikael Zayenz Lagerkvist, http://www.ict.kth.se/~zayenz/