Thank you for the detailed answer! All clear now...
2011.06.05. 18:33 keltezéssel, Ernest Friedman-Hill Ãrta:
Lots of questions here, some explicit, some implicit. First I'll
answer the one about why you saw an activation compared to only one
other.
Imagine you have a sorted partial deck of cards -- say you have a 5,
8, 10, Queen and King, in that order, top to bottom. The cards have
the important property that they can be sorted into a definite order,
and that order is stable. No matter how many times you check, the 8
will always belong after the 5. Furthermore, it's a transitive
relationship: if the 10 comes after the 8, and the 8 comes after the
5, then the 10 comes after the 5. Remember that -- we'll come back to
it later.
Now imagine you're picking up cards one at a time and inserting them
into the correct place in the deck. If you pick up a 4, what would you
do? You'd see the 5 on top, realize that it's larger than the 4, and
lay the 4 on top. You've only compared the 4 to one other card.
If you picked up a Jack, however, you might cut the deck in half and
compare the card with the middle cards, searching around a little bit
until you found the right spot. The important point is that no matter
what, the number of necessary comparisons is always vastly less than
the number of cards.
Jess doesn't actually use a fully-sorted array for the agenda. It uses
a "priority queue" (which you can Google to learn more about). A
priority queue is a very clever data structure which (1) always knows
which item is "first", (2) lets you insert new items quickly, and (3)
lets you remove items quickly. So Jess always knows which activation
is next to fire, but it doesn't need to have all the other ones sorted
completely into firing order; they're only partially ordered. This
keeps the number of comparisons to a minimum.
Now, to return to the point about the stable, transitive ordering:
your compare() function doesn't behave that way, and that's bad
because Jess assumes that it does. If we call that function with the
same arguments again and again, we'll get different answers. That
means it doesn't provide a random ordering, but rather *no* ordering.
What you want is a function whose result for any two activations is
unpredictable but stable and transitive. You could achieve this by
creating a map of activation/Integer pairs. The first time compare()
sees a given activation as an argument, it could generate a random
number and store it in that map. Then the comparisons would just be
comparing those random numbers from the map. By keeping the random
numbers and reusing them for all comparisons, you'd achieve both
necessary properties.
On Jun 5, 2011, at 7:03 AM, Rajcsányi Vilmos wrote:
Hi!
I would like to write a conflict resolution strategy that orders
(all) activations into a completely random order.
I've written a simple class implementing the Strategy interface,
looks like this:
public final class RandomActivation implements Strategy, Serializable
{
private static final long serialVersionUID = 1L;
private static final String mName = "RandomActivation";
private final Random mGenerator;
public RandomActivation()
{
mGenerator = new Random();
}
public final int compare( final Activation pAct1, final Activation
pAct2 )
{
int result;
final double r1 = mGenerator.nextDouble();
final double r2 = mGenerator.nextDouble();
if ( r1 < r2 )
{
result = 1;
}
else if ( r1 > r2 )
{
result = -1;
}
else
{
result = 0;
}
return result;
}
public final String getName()
{
return mName;
}
}
(I don't know whether the random numbers are really necessary, as
I've read in a former message that returning 0 would
result an indeterministic order of the activations, anyway my problem
has nothing to do with this.)
I've noticed that this strategy doesn't behave as I have expected.
I watched how many times the compare method was invoked after a new
activation was placed to the agenda,
and I don't understand why there are such activations
that aren't compared to every other activations. Isn't this the way
the conflict resolution should work?
Compare every activations to every others?
For example there was such activation that was compared to only one
other activation... Why is that?
Or am I misunderstanding something? Please explain me how and how
many times
the compare method should be invoked, and how can I reach a complete
random reordering of the agenda.
Thanks.
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Ernest Friedman-Hill
Informatics & Decision Sciences, Sandia National Laboratories
PO Box 969, MS 9012, Livermore, CA 94550
http://www.jessrules.com
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