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From "Jake Mannix (JIRA)" <j...@apache.org>
Subject [jira] Commented: (MAHOUT-207) AbstractVector.hashCode() should not care about the order of iteration over elements
Date Mon, 23 Nov 2009 21:15:39 GMT

    [ https://issues.apache.org/jira/browse/MAHOUT-207?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12781625#action_12781625
] 

Jake Mannix commented on MAHOUT-207:
------------------------------------

bq. We really need a more exhaustive equals/hashCode compatibility test suite in here though.

Ah, I see that we actually do have a lot of equivalence/equals testing in VectorTest, but
we somehow missed this case.

> 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: Matrix
>    Affects Versions: 0.2
>         Environment: all
>            Reporter: Jake Mannix
>             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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