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https://issues.apache.org/jira/browse/LUCENE-2216?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12801265#action_12801265
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Dawid Weiss commented on LUCENE-2216:
-------------------------------------

For what it's worth, I checked the mentioned BitUtil methods -- ntz/pop; the 
same implementation is included from Java 1.5 upward. Do you want me to file 
another patch for this, Yonik, or are we leaving this as-is? I'd redirect from 
BitUtil to Long/Integer, deprecate BitUtil methods and replace the places in 
the code where they are used.

> OpenBitSet#hashCode() may return false for identical sets.
> ----------------------------------------------------------
>
>                 Key: LUCENE-2216
>                 URL: https://issues.apache.org/jira/browse/LUCENE-2216
>             Project: Lucene - Java
>          Issue Type: Bug
>          Components: Other
>    Affects Versions: 2.9, 2.9.1, 3.0
>            Reporter: Dawid Weiss
>            Priority: Minor
>         Attachments: LUCENE-2216.patch, openbitset.patch
>
>
> OpenBitSet uses an internal buffer of long variables to store set bits and an 
> additional 'wlen' index that points 
> to the highest used component inside {...@link #bits} buffer.
> Unlike in JDK, the wlen field is not continuously maintained (on clearing 
> bits, for example). This leads to a situation when wlen may point
> far beyond the last set bit. 
> The hashCode implementation iterates over all long components of the bits 
> buffer, rotating the hash even for empty components. This is against the 
> contract of hashCode-equals. The following test case illustrates this:
> {code}
> // initialize two bitsets with different capacity (bits length).
> BitSet bs1 = new BitSet(200);
> BitSet bs2 = new BitSet(64);
> // set the same bit.
> bs1.set(3);
> bs2.set(3);
>         
> // equals returns true (passes).
> assertEquals(bs1, bs2);
> // hashCode returns false (against contract).
> assertEquals(bs1.hashCode(), bs2.hashCode());
> {code}
> Fix and test case attached.

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