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Date: Thu, 21 Jan 2010 20:15:54 +0000 (UTC)
From: "Sean Owen (JIRA)" <jira@apache.org>
To: mahout-dev@lucene.apache.org
Subject: [jira] Commented: (MAHOUT-207) AbstractVector.hashCode() should not
 care about the order of iteration over elements
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    [ https://issues.apache.org/jira/browse/MAHOUT-207?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12803468#action_12803468 ] 

Sean Owen commented on MAHOUT-207:
----------------------------------

Was this committed in some way? it doesn't seem like the patch was. Shall I? I agree it's a bug and would rather have it fixed, then continue discussing better ways to do it, if desired.

> AbstractVector.hashCode() should not care about the order of iteration over elements
> ------------------------------------------------------------------------------------
>
>                 Key: MAHOUT-207
>                 URL: https://issues.apache.org/jira/browse/MAHOUT-207
>             Project: Mahout
>          Issue Type: Bug
>          Components: Math
>    Affects Versions: 0.2
>         Environment: all
>            Reporter: Jake Mannix
>            Assignee: Grant Ingersoll
>             Fix For: 0.3
>
>         Attachments: MAHOUT-207.patch
>
>
> As was discussed in MAHOUT-165, hashCode can be implemented simply like this:
> {code} 
> public int hashCode() {
>     final int prime = 31;
>     int result = prime + ((name == null) ? 0 : name.hashCode());
>     result = prime * result + size();
>     Iterator<Element> iter = iterateNonZero();
>     while (iter.hasNext()) {
>       Element ele = iter.next();
>       long v = Double.doubleToLongBits(ele.get());
>       result += (ele.index() * (int)(v^(v>>32)));
>     }
>     return result;
>   }
> {code}
> which obviates the need to sort the elements in the case of a random access hash-based implementation.  Also, (ele.index() * (int)(v^(v>>32)) ) == 0 when v = Double.doubleToLongBits(0d), which avoids the wrong hashCode() for sparse vectors which have zero elements returned from the iterateNonZero() iterator.

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