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

[Wolfgang Laun]

I see no cogent reason for implementing Identifiable for each fact class.
The hashCode() any object returns should be distinct enough, especially
considering that all the hash codes are munched together, which is
definitely not *guaranteed* to result in different values for *all* rule and
fact set combinations.

But please do as Greg suggested.

BTW, it's sufficient to end with
  return alreadyActivatedRules.add(hash);

-W

On Thu, Oct 8, 2009 at 11:43 PM, Dave Schweisguth <dave at schweisguth.org>wrote:

> 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/<http://schweisguth.org/%7Edave/>|
> | 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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