[gecode-users] Unary and Cumulative constraints

[Guido Tack]

David Rijsman wrote:

> Hi,
> 
> First of all compliments for another very nice version. I am looking at the new scheduling constraints introduced in version 4.3.0, and I trying to understand why you have decided the following for some of the new cumulative and unary constraints:
> 
> "The propagator does not enforce s = p + e, this constraint has to be posted in addition to ensure consistency of the task bounds."
> 
> I can not figure out what a use case would be where you do not want s = p + e enforced. If there are use cases why is it enforced in some of the constraints? 

The reason is that you can have the same task on several resources at once, which would cause an overhead if each resource constraint in addition propagated s=p+e.  Currently, the interface is a bit inconsistent, as the cumulatives constraint does propagate s=p+e.

We plan to add modeling support for scheduling constraints in the future (I'm working on it), where you would declare task variables that encapsulate the s=p+e propagation.  In fact, there would be one propagator for all the simple temporal constraints (such as task A is before task B, which translates to e(A) <= s(B) ), which gives strictly stronger propagation than the individual constraints.
The resource constraints can then be posted in terms of these task variables.

Cheers,
	Guido

-- 
Guido Tack, http://people.cs.kuleuven.be/~guido.tack/