LMSimilarity Formula

[Shayan A Tabrizi <sh...@gmail.com>]

Dear All,

It seems that LMDirichletSimilarity only calculates "score" method if the
term occurs in the document. Otherwise, in line 389 of BooleanWeight
(Lucene 6.2.0) subScorer becomes null, and thus the clause is not added to
the optional list in order to be scored.

However, in the original formula of LM (
http://www.stat.uchicago.edu/~lafferty/pdf/smooth-tois.pdf, formula 6), we
have "n log a_d" (n is the number of query terms). Therefore, even for the
query terms not present in the document a "log a_d" must be added to the
final score.

But the implementation of LMDirichletSimilarity adds "log a_d" to the score
in the "score" method, and therefore it is only added to the final score
for the query terms present in the document.

This can worsen the retrieval results compared to the correct formula. I
tried to correct this for myself but because of the plenty of "final"
methods and classes, I was not successful. Please, check the problem and
solve it if approved, and also please tell me how I can correct it before a
new release is published.

Best Regards,
Shayan A. Tabrizi