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https://issues.apache.org/jira/browse/LUCENE-2216?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12801270#action_12801270
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Yonik Seeley commented on LUCENE-2216:
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bq. For what it's worth, I checked the mentioned BitUtil methods - ntz/pop; the
same implementation is included from Java 1.5 upward.
Huh - I didn't realize that Java5 had the same pop impl as I did... it will be
cool if it finally starts using native POPCNT instructions.
As far as ntz, I went though a lot of micro-optimizations and different
implementations before I settled on the one used in BitUtil, so it would be
nice to do some benchmarks to see if it's truly faster now (and also what the
performance difference is for users of JVMs before this optimization was
implemented).
> 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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