Malcolm Ryan wrote:
I want to make a constraint which represents the order of three values
around a ring.
Eg: if we have a,b,c \in {1,2,3,4} then I want to represent the
constraint:
clockwise(a,b,c) == (a < b < c) or (b < c < a) or (c < a < b)
I can construct this directly using BExprs, but the use of 'or' means
that it doesn't propagate very well. Eg:
[...]
Will give a = {1,4}, b = [2..3], c = [2..3] when it should be able to
deduce that b = 2, c = 3. Is there any other way to represent this so
that the propagation works better?
You can't get the full pruning directly (you'd need something like
constructive disjunction for that). Something you might want to add
is a distinct constraint over all variables (although it wouldn't do
the pruning in your example, it would add some pruning in other
cases). Or, if the domains are always dense (at most one value more
than variables, that is), you could use a+1=b instead of a<b. You
could also encode the constraint into a TupleSet and use extensional,
which would probably result in a really big table but give you domain
consistency.
Cheers,
Guido
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