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https://issues.apache.org/jira/browse/LUCENE-2216?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12801263#action_12801263
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Dawid Weiss commented on LUCENE-2216:
-------------------------------------
Chances of this happening are really slim (this would probably be a single
inlined read as soon as the compilation takes place, but you're right in the
general case. I am not arguing changing the object in hashCode is good -- my
argument is that ideally it should be fixed elsewhere (as in my previous
suggestion -- either updating wlen every time the tail changes, or make
explicit changes to the documentation that inform about suboptimal performance
for zero-tailed sets).
> 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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