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From "stack (JIRA)" <j...@apache.org>
Subject [jira] Updated: (HBASE-1136) HashFunction inadvertently destroys some randomness
Date Mon, 19 Jan 2009 21:26:59 GMT

     [ https://issues.apache.org/jira/browse/HBASE-1136?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

stack updated HBASE-1136:
-------------------------

       Resolution: Fixed
    Fix Version/s: 0.20.0
           Status: Resolved  (was: Patch Available)

Committed to TRUNK.  Thanks for the patch Jonathan.

Please open a new patch against Hadoop with a patch for src/core/org/apache/hadoop/util/bloom/HashFunction.java.
 HashFunction was moved there by HADOOP-3063 (We'll move to using the hadoop versions in next
hbase).  I can commit it after letting it air for a while in case objection.

> HashFunction inadvertently destroys some randomness
> ---------------------------------------------------
>
>                 Key: HBASE-1136
>                 URL: https://issues.apache.org/jira/browse/HBASE-1136
>             Project: Hadoop HBase
>          Issue Type: Bug
>            Reporter: Jonathan Ellis
>             Fix For: 0.20.0
>
>         Attachments: hash.patch
>
>
> the code
>       for (int i = 0, initval = 0; i < nbHash; i++) {
>         initval = result[i] = Math.abs(hashFunction.hash(b, initval) % maxValue);
>       }
> restricts initval for the next hash to the [0, maxValue) range of the hash indexes returned.
 This is suboptimal, particularly for larger nbHash and smaller maxValue.  Instead, use:
>       for (int i = 0, initval = 0; i < nbHash; i++) {
>         initval = hashFunction.hash(b, initval);
>         result[i] = Math.abs(initval) % maxValue;
>       }

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