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https://issues.apache.org/jira/browse/LUCENE-3220?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13060680#comment-13060680
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Robert Muir commented on LUCENE-3220:
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Hi David: I had some ideas on stats to simplify some of these sims:
# I think we can use an easier way to compute average document length:
sumTotalTermFreq() / maxDoc(). This way the average is 'exact' and not skewed
by index-time-boosts, smallfloat quantization, or anything like that.
# To support pivoted unique normalization like lnu.ltc, I think we can solve
this in a similar way: add sumDocFreq(), which is just a single long, and
divide this by maxDoc. this gives us avg # of unique terms. I think terrier
might have a similar stat (#postings or #pointers or something)?
so i think this could make for nice simplifications: especially for switching
norms completely over to docvalues: we should be able to do #1 immediately
right now, change the way we compute avgdoclen for e.g. BM25 and DFR.
then in a separate issue i could revert this norm summation stuff to make the
docvalues integration simpler, and open a new issue for sumDocFreq()
> Implement various ranking models as Similarities
> ------------------------------------------------
>
> Key: LUCENE-3220
> URL: https://issues.apache.org/jira/browse/LUCENE-3220
> Project: Lucene - Java
> Issue Type: Sub-task
> Components: core/search
> Affects Versions: flexscoring branch
> Reporter: David Mark Nemeskey
> Assignee: David Mark Nemeskey
> Labels: gsoc
> Attachments: LUCENE-3220.patch, LUCENE-3220.patch, LUCENE-3220.patch,
> LUCENE-3220.patch, LUCENE-3220.patch, LUCENE-3220.patch, LUCENE-3220.patch,
> LUCENE-3220.patch, LUCENE-3220.patch, LUCENE-3220.patch, LUCENE-3220.patch
>
> Original Estimate: 336h
> Remaining Estimate: 336h
>
> With [LUCENE-3174|https://issues.apache.org/jira/browse/LUCENE-3174] done, we
> can finally work on implementing the standard ranking models. Currently DFR,
> BM25 and LM are on the menu.
> TODO:
> * {{EasyStats}}: contains all statistics that might be relevant for a
> ranking algorithm
> * {{EasySimilarity}}: the ancestor of all the other similarities. Hides the
> DocScorers and as much implementation detail as possible
> * _BM25_: the current "mock" implementation might be OK
> * _LM_
> * _DFR_
> Done:
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