[jira] [Created] (LUCENE-6687) MLT term frequency calculation bug

["Marko Bonaci (JIRA)" <ji...@apache.org>]

Marko Bonaci created LUCENE-6687:
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             Summary: MLT term frequency calculation bug
                 Key: LUCENE-6687
                 URL: https://issues.apache.org/jira/browse/LUCENE-6687
             Project: Lucene - Core
          Issue Type: Bug
          Components: core/query/scoring, core/queryparser
    Affects Versions: 5.2.1, Trunk
         Environment: OS X v10.10.4; Solr 5.2.1
            Reporter: Marko Bonaci


In {{org.apache.lucene.queries.mlt.MoreLikeThis}}, there's a method {{retrieveTerms}} that receives a {{Map}} of fields, i.e. a document basically, but it doesn't have to be an existing doc.

There are 2 for loops, one inside the other, which both loop through the same set of fields.
That effectively doubles the term frequency for all the terms from fields that we provide in MLT QP {{qf}} parameter. 
It basically goes two times over the list of fields and accumulates the term frequencies from all fields into {{termFreqMap}}.

The private method {{retrieveTerms}} is only called from one public method, the version of overloaded method {{like}} that receives a Map: so that private class member {{fieldNames}} is always derived from {{retrieveTerms}}'s argument {{fields}}.
 
Uh, I don't understand what I wrote myself, but that basically means that, by the time {{retrieveTerms}} method gets called, its parameter fields and private member {{fieldNames}} always contain the same list of fields.

Here's the proof:
These are the final results of the calculation:


And this is the actual {{thread_id:TID0009}} document, where those values were derived from (from fields {{title_mlt}} and {{pagetext_mlt}}):

Now, let's further test this hypothesis by seeing MLT QP in action from the AdminUI.
Let's try to find docs that are More Like doc {{TID0009}}. 
Here's the interesting part, the query:

{{q={!mlt qf=pagetext_mlt,title_mlt mintf=14 mindf=2 minwl=3 maxwl=15}TID0009}}

We just saw, in the last image above, that the term accumulator appears {{7}} times in {{TID0009}} doc, but the {{accumulator}}'s TF was calculated as {{14}}.
By using {{mintf=14}}, we say that, when calculating similarity, we don't want to consider terms that appear less than 14 times (when terms from fields {{title_mlt}} and {{pagetext_mlt}} are merged together) in {{TID0009}}.
I added the term accumulator in only one other document ({{TID0004}}), where it appears only once, in the field {{title_mlt}}. 


Let's see what happens when we use {{mintf=15}}:


Bug, no?




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