[eclipse-clp-users] Query reformulation to achieve better propagation

[Panagiotis S. <ta...@di...>]

Hello Everybody,

After having loaded libraries ic and ic_global, we give the query:

?- N = 4, Count = 0, length(L, N), L #:: 1..N, 
ic_global:alldifferent(L), L = [X1,X2,X3,X4], (X2 #> X1) + (X3 #> 
max([X1,X2])) + (X4 #> max([X1,X2,X3])) #= Count.

N = 4
Count = 0
L = [X1{1 .. 4}, X2{1 .. 4}, X3{1 .. 4}, X4{1 .. 4}]
X1 = X1{1 .. 4}
X2 = X2{1 .. 4}
X3 = X3{1 .. 4}
X4 = X4{1 .. 4}
There are 6 delayed goals.
Yes (0.00s cpu)

As we may see, from the query we can infer that X1 = 4. Is there
any alternative way to pose the query in order to get this result?
Note that Count might be anything between 0 and 3 (if we assign it
the value 3, we get the answer X1 = 1, X2 = 2, X3 = 3, X4 = 4,
as we might expect). But what if Count is less than 3? How could
we get the most propagation possible?

Also, note that this is a simplified version of a more general program,
where N might be anything and the constraint on the sum of constraints
is constructed recursively.

Regards,
Panagiotis