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https://issues.apache.org/jira/browse/LUCENE-1261?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12586880#action_12586880
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Karl Wettin commented on LUCENE-1261:
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Hi John,
see LUCENE-1260
karl
> Impossible to use custom norm encoding/decoding
> -----------------------------------------------
>
> Key: LUCENE-1261
> URL: https://issues.apache.org/jira/browse/LUCENE-1261
> Project: Lucene - Java
> Issue Type: Bug
> Components: Query/Scoring
> Affects Versions: 2.3.1
> Environment: All
> Reporter: John Adams
>
> Although it is possible to override methods encodeNorm and decodeNorm in a
> custom Similarity class, these methods are not actually used by the query
> processing and scoring functions, not by the indexing functions. The relevant
> Lucene classes all call "Similarity.decodeNorm" rather than
> "similarity.decodeNorm", i.e. the norm encoding/decoding is fixed to use that
> of the base Similarity class. Also index writing classes such as
> DocumentWriter use "Similarity.decodeNorm" rather than
> "similarity.decodeNorm", so we are stuck with the 3 bit mantissa encoding
> implemented by SmallFloat.floatToByte315 and SmallFloat.byte315ToFloat.
> This is very restrictive and annoying, since in practice many users would
> prefer an encoding that allows finer distinctions for boost and normalisation
> factors close to 1.0. For example. SmallFloat.floatToByte52 uses 5 bits of
> mantissa, and this would be of great help in distinguishing much better
> between subtly different lengthNorms and FieldBoost/DocumentBoost values.
> It hsould be easy to fix this by changing all instances of
> "Similarity.decodeNorm" and "Similarity.encodeNorm" to
> "similarity.decodeNorm" and "similarity.encodeNorm" in the Lucene code (there
> are only a few of each).
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