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From "Yonik Seeley (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (LUCENE-5604) Should we switch BytesRefHash to MurmurHash3?
Date Sun, 13 Apr 2014 14:34:15 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-5604?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13967851#comment-13967851
] 

Yonik Seeley commented on LUCENE-5604:
--------------------------------------

The JVM recognizes pairs of shifts that amount to a rotate and replaces them with an intrinsic.

bq. Initial patch, Lucene tests pass, but solrj doesn't yet compile....

Right - SolrJ does not have lucene dependencies.  Solr also depends on the *exact* hash, so
it can't be tweaked (for example if a variant turns out to be better for lucene indexing).
 Perhaps Lucene should just make a copy of the one it needs (the byte[] version).

> Should we switch BytesRefHash to MurmurHash3?
> ---------------------------------------------
>
>                 Key: LUCENE-5604
>                 URL: https://issues.apache.org/jira/browse/LUCENE-5604
>             Project: Lucene - Core
>          Issue Type: Improvement
>          Components: core/index
>            Reporter: Michael McCandless
>            Assignee: Michael McCandless
>             Fix For: 4.9, 5.0
>
>         Attachments: BytesRefHash.perturb.patch, LUCENE-5604.patch
>
>
> MurmurHash3 has better hashing distribution than the current hash function we use for
BytesRefHash which is a simple multiplicative function with 31 multiplier (same as Java's
String.hashCode, but applied to bytes not chars).  Maybe we should switch ...



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