Hi everyone. I hope this isn't some silly misunderstanding but I'm having
trouble figuring out how the precede() constraint works when applied to
SetVarArray.
Imagine I have a set of tasks with dependencies, for example 1->2, 2->3 and
3->4 to keep things simple. I've added the following constraints:
// Array of sets of integers from {1..4}
SetVarArray deps(*this, 4, IntSet::empty, IntSet(1, 4));
// Enforce pairwise disjointedness
sequence(*this, deps);
// Final task must appear
rel(*this, setunion(deps) >= IntSet(IntArgs(1, 4)));
// Task ordering constraints
precede(*this, deps, 1, 2);
precede(*this, deps, 2, 3);
precede(*this, deps, 3, 4);
Now this should produce at least a solution like {{1}, {2}, {3}, {4}}. However
I get this as my first solution in the output:
{{1..4}, {}, {}, {}}
which indicates that the precede() constraint is not behaving as I think it
should.
According to to the documentation the precede(home, x, s, t) version of the
value precedence constraint as applied to set variables says that s is not an
element of x_j and t is an element of x_j, then i < j such that s is an element
of x_i and t is not an element of x_i.
I don't understand how the solution above satisfies the above unless maybe the
i < j is really acting as i <= j. This seems to be the case based on the other
solutions as they're sorted but that could be a side effect of the sequence()
constraint.
Does anyone know what I'm doing wrong here?
Thanks,
Tim.
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