[rules-users] Better way to run each rule once?

[Dave Schweisguth]

Greetings fellow Droolers,

Each of our rules modifies the fact it matches. We'd like to run each of
those rules exactly once, not reactivating them when a fact changes. I see
from the archives that I'm not the first person to discover that no-loop is
too weak and lock-on-activate too strong for my purposes.

I implemented our requirement with an AgendaListener that remembers hashes
of rules + facts. This works but requires each fact to implement an
interface with a method that returns the hash, and means TWO casts each time
I examine a FactHandle. The whole AgendaFilter is below so you can see what
I mean.

Can anyone suggest a better way? I'm looking more for a better approach
altogether than I am for critique of the implementation of my current
approach, although the latter would not be unwelcome.

private static class Once
    implements AgendaFilter {

    private final Set<Integer> alreadyActivatedRules = new HashSet<Integer>();

    public boolean accept(Activation activation) {
	int hash = activation.getRule().getName().hashCode();
	for (FactHandle handle: activation.getFactHandles()) {
	    Object object = ((DefaultFactHandle) handle).getObject();
	    hash *= 31;
	    if (object instanceof Identifiable) {
		hash += ((Identifiable) object).getIdentity();
	    } else if (object instanceof String) {
		// We get here when a rule uses from on a string collection
		hash += object.hashCode();
	    } else {
		throw new IllegalStateException(
		    "Don't know what to do with fact class "
			+ object.getClass() + ", value " + object);
	    }
	}
	boolean accept = !alreadyActivatedRules.contains(hash);
	alreadyActivatedRules.add(hash);
	 return accept;
    }

}

Thanks & cheers,

-- 
| Dave Schweisguth                           http://schweisguth.org/~dave/ |
| Home: dave at schweisguth.org            Work: http://www.nileguide.com/ |
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