[gecode-users] sequence constraint: variable occurring multiply

[Guido Tack]

Hi,

David Rijsman wrote:
> you are absolutely right, this extension is very useful for cyclical rosters. Should not be very hard to enable in the current implementation, basically changing the check at posting the constraint. I see if I can do this and see if this gets accepted?

The question is whether the algorithm is still correctly reporting fixpoints when the variable array contains variables more than once.   I don't know the algorithm well enough to tell from just a quick glance.  Usually, the test suite is pretty good at detecting these cases, so you could simply remove the check in the post function and write a test to see if anything goes wrong.
If shared variables are indeed a problem for the algorithm, you can use unshare to replace all shared variables with fresh variables (linked to the originals by equality constraints).

Cheers,
	Guido

> 
> David 
> 
>>>> 
> From: 	Kish Shen <kisshen at cisco.com>
> To:	<users at gecode.org>
> Date: 	8/19/2010 2:03 AM
> Subject: 	[gecode-users] sequence constraint: variable occurring multiply
> 
> Hi,
> 
> I have another question about the sequence constraint: exception 
> ArumentSame is thrown if a variable occurs more than once. Should this 
> exception be thrown? I can see that it does not make sense for a 
> variable to occur in a subsequence of q variables, but if the 'distance' 
> between the same variable > q, i.e. it cannot occur multiply in the same 
> subsequence, should this be allowed? Can the implemented algorithm deal 
> with this?
> 
> There are useful cases for variables to occur multiply, the simplest is 
> a cycle, e.g. say for the following variables
> 
> x1,x2,x3,x4,x5
> 
> and a cycle would be
> 
> x1,x2,x3,x4,x5,x1,x2,....
> 
> such cycle could be used for a schedule that repeats.
> 
> Cheers,
> 
> Kish
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-- 
Guido Tack, http://people.cs.kuleuven.be/~guido.tack/