Prolog manual or custom labeling

I am currently writing a solver for a floor planning problem in Prolog and have some issues with the labeling part.

The current problem is my constraints are posted but when I launch the labeling, it takes forever to find a solution. I would like to bring in some heuristics.

My question is, how do I manually label my variables ? I am afraid that after defining a clpfd variable like this :

X in Xinf..Xsup

and constraining it, If I do something like :

    fd_sup(X, Xmax),
    X = Xmax,
...

in my custom label, I won't be using the backtrack ability of Prolog to test the other values of X's domain. Am I wrong ?

Also, is there a smarter way to label my variables than writing custom labeling procedures ? My idea of heuristics would consist in trying extrema of a variable domain alternatively (like max(X), min(X), max(X-1), min(X-1) etc...)

Hope you can help me :)

Solution

First, always try built-in heuristics. ff is often a good strategy.

For custom labeling strategies, it is often easiest to first convert the domain to a list, then reorder the list, and then simply use member/2 to assign the values of the domain using the new order.

A good building black is dom_integers/2, relating a finite CLP(FD) domain to a list of integers:

:- use_module(library(clpfd)).

dom_integers(D, Is) :- phrase(dom_integers_(D), Is).

dom_integers_(I)      --> { integer(I) }, [I].
dom_integers_(L..U)   --> { numlist(L, U, Is) }, Is.
dom_integers_(D1\/D2) --> dom_integers_(D1), dom_integers_(D2).

Your specific strategy is easily expressed on a list of such ordered integers, relating these integers to a second list where the values occur in the order you describe:

outside_in([]) --> [].
outside_in([I]) --> [I].
outside_in([First|Rest0]) --> [First,Last],
        { append(Rest, [Last], Rest0) },
        outside_in(Rest).

Sample query and result:

?- phrase(outside_in([1,2,3,4]), Is).
Is = [1, 4, 2, 3] ;
false.

Combining this with fd_dom/2 and dom_integers/2, we get (bindings for variables other than X omitted):

?- X in 10..20,
   fd_dom(X, Dom),
   dom_integers(Dom, Is0),
   phrase(outside_in(Is0), Is),
   member(X, Is).
X = 10 ;
X = 20 ;
X = 11 ;
X = 19 ;
X = 12 ;
X = 18 ;
etc.

Nondeterminism is preserved by member/2.