Thanks a lot for this list!

You are totally right, simply translating the dependencies into SAT constraints 
and feeding them to a solver returns in most cases a very bad, totally useless 
However, nowadays many solvers support solution optimization, i.e., you can 
specify an ordered list of criteria you want to maximize/minimize, exactly like 
you did.
I already implemented the minimal status difference and the minimal 
installation size criteria, and the solutions I get are already acceptable/good.

I wasn't aware of the criteria list you gave (maybe it's in the PMS), so thank 
you very much, this is a big help :)
I have few questions about these criteria:
 - in criteria a., there could be an ambiguity between what I call a package 
(e.g., 'app-editors/nano-2.9.3') and a package group (e.g., 
'app-editors/nano'): is the criteria about package group (i.e., are version 
changes allowed)?
 - similarly in criteria b.: is this criteria valid across versions (i.e., when 
changing version, the USE-flag change should be minimal), across slots (i.e., 
two installed versions of the same package group should have a minimal USE flag 
   If yes, what if package.use specifies very different USE flags for two 
versions of the same package group?
 - does the "prefer new version" criteria go between a. and b.?

b. Similarly, the number of USE-flag changes necessary to achieve this
aim should be minimized.
(You didn't tell whether your solver already supports such changes,
but when it is finished, it definitely should.)

Yes, my solver supports USE-flag, keyword and mask changes (it is currently 
oblivious of licenses, but supporting them should just be a technical/time 
consuming effort).
Due to keywords and mask changes, the "prefer new version" criteria needs to be after 
criteria "less keyword and mask change".
 - just to be sure, the "less keyword and mask change" criteria is at the top 
of the list?

Best regards,
Michael Lienhardt

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