[gecode-users] Clockwise constraint

[Malcolm Ryan]

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:

         BExpr beLessAB = new BExpr(a, IntRelType.IRT_LE, b);
         BExpr beLessBC = new BExpr(b, IntRelType.IRT_LE, c);
         BExpr beLessCA = new BExpr(c, IntRelType.IRT_LE, a);

         BExpr beABC = new BExpr(beLessAB).and(beLessBC);
         BExpr beBCA = new BExpr(beLessBC).and(beLessCA);
         BExpr beCAB = new BExpr(beLessCA).and(beLessAB);

         BExpr clockwise = beABC.or(beBCA).or(beCAB);

         Gecode.post(test, clockwise);
         Gecode.rel(test, test.a, IntRelType.IRT_NE, 2);
         Gecode.rel(test, test.a, IntRelType.IRT_NE, 3);
         Gecode.rel(test, test.b, IntRelType.IRT_NE, 1);
         Gecode.rel(test, test.b, IntRelType.IRT_NE, 4);
         Gecode.rel(test, test.c, IntRelType.IRT_NE, 1);
         Gecode.rel(test, test.c, IntRelType.IRT_NE, 4);

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?

Malcolm