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

[Greg Barton]

Could you give some sample rules and the nature of the problem you're solving?  Maybe you don't need to update all of the time with a different approach.



----- Original Message ----
From: Dave Schweisguth <dave at schweisguth.org>
To: rules-users at lists.jboss.org
Sent: Thu, October 8, 2009 4:43:34 PM
Subject: [rules-users] Better way to run each rule once?

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/ |
| For compliance with the NJ Right to Know Act: Contents partially unknown |
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