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I'm building a student schedule generator and I need a way of producing more than one solution. Is there some way to save off feasible scores or scores of Xhard/Ysoft?

I need to be able to output more than one potential schedule, that way the student will have a choice for one schedule over the other if for whatever reason they don't want the "best" schedule (maybe they don't like one of the professors, maybe they don't want an 8am class, whatever)

My original idea was to save off all feasible solutions using the bestSolutionChanged event listener. The problem with this, is that once it finds a 0hard/0soft score, it ignores all scores after that, including scores that are equal.

Ideally I'd like to save off all scores of 0hard/-3soft or better, but just being able to save any feasible scores or force optaplanner to look for a new best score would be useful as well.

Liam Ward
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    I found this post from a few years ago: https://stackoverflow.com/questions/30945343/returning-multiple-solutions-to-a-scheduling-issue-with-optaplanner and it sounds like you have to hack BestSolutionRecaller to get that sort of behaviour, but I can't seem to find any examples of how this is implemented. – Liam Ward May 13 '19 at 17:03
  • It's also related to Pareto optimization, as you don't just want all those solutions, but just the pareto front. – Geoffrey De Smet May 14 '19 at 13:48
  • So extending BestSolutionRecaller is currently the only way of solving this issue? If so, I have what might be kind of a dumb or obvious question: how do I actually tell the solver to use my extended BestSolutionRecaller class? Is there something in the configuration file I need to change or otherwise? I really appreciate the help. I'm mostly asking because all of forums I've seen on the topic are from 2015 or earlier, and I want to make sure no way has been implemented for this yet before I go too deep. – Liam Ward May 14 '19 at 17:55

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This is not a solution, but an analysis of the problem:

Hacking the BestSolutionRecaller is obviously not just a big pain, it's also behaviour we don't want to encourage as it makes upgrading to newer version an even bigger pain. So don't expect us to solve this by adding an easy way to configure that in the solver config any time soon. That being said, a solution for this common problem is clearly needed.

When a new best solution is found, it is planning cloned (see docs for definition) from the working solution (the internal solution in OptaPlanner). This allow us to remember that new best solution as the working solution solution changes. That also means the BestSolutionChangedEvents gets a plannng clone and can safely ship it to another thread, for example to marshal it to a client (presuming any ProblemFactChanges you create do copies instead of alterations), without being corrupted by the solver thread that modifies the working solution.

New best solution imply that workingScore > bestScore. The moment it instead does workingScore >= bestScore, we need far more planning clones (which are a bit CPU expensive), but we could then just send out BestSolutionChangedEvents for that too, if and only if a flag is enabled of course, because most users (unlike yourself) don't want this behaviour.

One proposal is to create a separate BestSolutionChangedOrSameEvent, next to the BestSolutionChangedEvent. This might not be ideal, because we need to be able to detect whether or not someone needs those extra planning clones.

Another proposal is to just have a flag in the <solver> config that switches from > to >= behavior for BestSolutionChangedEvent.

Please create a jira (see "get help" on webpage) and link it it here, or create a support ticket (also see "get help" on webpage).

Geoffrey De Smet
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    Thanks so much for the response! It means a lot that you're so active in helping with issues the community comes up with. Here's the link to the jira issue: https://issues.jboss.org/browse/PLANNER-1492 – Liam Ward May 15 '19 at 14:27