[gecode-users] Interfacing GeCode to ECLiPSe

[Kish Shen]

Hi Christian and Guido,

Thanks for your replies!

Christian Schulte wrote:
> Hi Kish,
> 
> As it comes to the search interface: I have been discussing with other
> LP-minded people and from these discussions I do recall the following design
> outline:
> 
> Search:
>  (1) Eclipse tells Gecode to create a choicepoint
>  (2) Eclipse tells Gecode to backtrack until the next choicepoint (or some
> choicepoint if you want it to be
>      more powerful than that)
> 
> Interfacing:
>  (3) Eclipse tells Gecode about variable modifications
>  (4) Eclipse tells Gecode to create new variables and propagators
>  (5) Eclipse instructs Gecode to do propagation and queries Gecode about
> changes to variable domains 
> 
> Then, a starting point for implementing backtracking would be to just use
> cloning to make it work (that should be really use but inefficient). Then,
> in a second step one could use a hybrid of recomputation and cloning (just
> replay what has been done by (3) and (4) and redo propagation). For a
> realistic interface Gecode must use recomputation.

After Guido's last post, I remember some of our (you, Joachim Schimpf 
and I)  discussion about this during CP a couple of years ago.

My thinking is that for ECLiPSe (and I suspect for other Prolog-like 
languages), it is difficult to be explicit about the creation of a 
choicepoint, so potentially any changes ECLiPSe tells GeCode about 
(poinys 3,4) are possible choicepoints.

After reading Guido's e-mail (but before seeing yours), I was thinking 
of maintaining a chronological list of the `events' that are sent to 
GeCode, with a counter incremented for each event. I think this 
corresponds to the `choicepoint' you mentioned. So ECLiPSe can tell 
GeCode to `backtrack' (or more properly recreate) the GeCode state when 
counter = N.

Can this be done with GeCode with the existing API, without the ECLiPSe 
side maintaining the list of events?

3) and 4) are needed for the backbox interface, because to keep close 
ECLiPSe's native finite domain solvers' semantics, propagation happens 
when a constraint is posted.

So if I use the Reflection interface, I need to be able to incrementally 
add variables and propagators.

> 
> I think this kind of interface would be quite useful also for other
> languages/systems based on backtracking. Provided that:
>  - you come up with a naïve implementation that is not Eclipse specific
> (say, there will be one part that is Gecode specific and then a separate
> part that is Eclipse specific, and the Gecode part can be used without
> Eclipse)
>  - you put the Gecode-specific part under the MIT license
>

 From our perspective, we'd like the ECLiPSe interface to be as general 
as possible, i.e. ideally we can plug in another constraint solver 
engine other than GeCode with ease.

I think your points 1) and 2) are not solver specific, and indeed can be 
used by
other backtracking languages. I guess this will be the `ECLiPSe 
specific' part,
as well as the actual 'events' themselves -- ideally I'd like the events 
to be non-GeCode specific.

Then there is the part of getting GeCode to recreate the state at a 
choicepoint/counter. This is probably specific to GeCode (or at least a 
recomputation system). As I asked earlier, can you do this with the 
GeCode APU, and if not, should the code for this be put into GeCode, 
rather than be on the `ECLiPSe side'?

ECLiPSe is now under a "Cisco style" Mozilla license (I am not a lawyer, 
but I believe Cisco style means that Cisco was substituted for Mozilla 
in the license, i.e. Cisco has the right to change the licensing terms 
etc.). I guess the question of license will only kick in if some of the 
GeCode specific code needs to be kept on the ECLiPSe side (i.e. 
distributed as part of ECLiPSe). I don't know if the MIT license is 
compatible with Mozilla license (i.e. can you have a distribution with 
code that has both licenses). I do recall the lawyers telling me that 
the Mozilla license is not compatible with GPL, and we had to remove 
some GPL licensed code from the ECLiPSe distribution.

Cheers,

Kish



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