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From "Simon Willnauer (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (LUCENE-3957) Document precision requirements of setBoost calls
Date Fri, 06 Apr 2012 09:53:23 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-3957?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13248215#comment-13248215
] 

Simon Willnauer commented on LUCENE-3957:
-----------------------------------------

bq. I don't think we should put docs there in those classes:
+1 thats a very long standing impl detail. If it is not on the wiki in the FAQ we should add
it or maybe add a pitfalls section to the wiki.
                
> Document precision requirements of setBoost calls
> -------------------------------------------------
>
>                 Key: LUCENE-3957
>                 URL: https://issues.apache.org/jira/browse/LUCENE-3957
>             Project: Lucene - Java
>          Issue Type: Improvement
>          Components: general/javadocs
>    Affects Versions: 3.5
>            Reporter: Jordi Salvat i Alabart
>
> The behaviour of index-time boosts seems pretty erratic (e.g. a boost of 8.0 produces
the exact same score as a boost of 9.0) until you become aware that these factors end up encoded
in a single byte, with a three-bit mantissa. This consumed a whole day of research for us,
and I still believe we were lucky to spot it, given how deeply dug into the code & documentation
this information is.
> I suggest adding a small note to the JavaDoc of setBoost methods in Document, Fieldable,
FieldInvertState, and possibly AbstractField, Field, and NumericField.
> Suggested text:
> "Note that all index-time boost values end up encoded using Similarity.encodeNormValue,
with a 3-bit mantissa -- so differences in the boost value of less than 25% may easily be
rounded away."

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