[Gecode] Space consumption of set variables

[Guido Tack]

Hi all.

I've tried to improve the memory consumption of set variables a bit. The first 
thing was to implement doubly linked lists for the domains the way Christian 
did it for the integers, using just one pointer for both directions. This 
saves 1.6-4.4% (measured using the peak memory returned by the search 
engines).
My second try was to get rid of half of the modification events and 
propagation conditions, by making all bounds events imply a cardinality 
event. This saves quite a lot of memory, between 4% and 8.7%. My conclusion 
is that additional propagation conditions are expensive, and should only be 
used if a lot of propagators can really make use of the stronger information. 
This does not seem to be the case with cardinality PCs for set constraints.

Cheers,
	Guido

-- 
Guido Tack
Programming Systems Lab, Saarland University, Saarbrücken
http://www.ps.uni-sb.de/~tack