New feature idea: Eligible Items for recommendations

[Anatoliy Kats]

Hi all,

I have a use case for Taste where we should use all available items for 
computation of user similarity, but only some items are eligible to be 
recommended.  The reason is that items expire:  goods get discontinued, 
for example.  The fact that a user bought something now discontinued is 
important for finding similar users, but we cannot recommend it.  One 
way of solving this is using mahout to generate recommendations, and 
then post-process it to remove ineligible items.  But this can run into 
performance issues if the set of ineligible items is large.  Also, the 
evaluation framework is not designed to handle this kind of a restriction.

I can contribute a patch, but since I am new to the project, I need some 
help with the software engineering from the regular contributors.  How 
would I design the necessary classes?  One idea I considered was using 
the existing tools for candidate item strategy.  This has two 
downsides:  you can only use one candidate item strategy at a time, so 
having an eligible set would make it impossible to have a candidate item 
strategy.  Also, the evaluator doesn't take the candidate item strategy 
into account.  So, I suppose I could try writing an 
EligibleSetCandidateItemStrategy that takes another 
CandidateItemStrategy and applies it as well, and I can add a new 
evaluator that takes eligible sets into account.  I don't know the 
thinking behind the design, so I don't know if this is a good idea or 
not.  Can we agree on a design before I get started?

Cheers,

Anatoliy