Stefan Pohl created LUCENE-4546:
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Summary: SorterTemplate.quicksort incorrect
Key: LUCENE-4546
URL: https://issues.apache.org/jira/browse/LUCENE-4546
Project: Lucene - Core
Issue Type: Bug
Components: core/other
Affects Versions: 4.0, 3.6.1, 4.1
Reporter: Stefan Pohl
Fix For: 4.1, 4.0, 3.6.1
On trying to use the very useful o.a.l.utils.SorterTemplate, I stumbled upon
inconsistent sorting behaviour, of course, only a randomized test caught this;)
Because SorterTemplate.quicksort is used in several places in the code
(directly BytesRefList, ArrayUtil, BytesRefHash, CollectionUtil and
transitively index and search), I'm a bit puzzled that this either hasn't been
caught by another higher-level test or that neither my test nor my
understanding of an insufficiency in the code is valid;)
If the former holds and given that the same code is released in 3.6 and 4.0,
this might even be a more critical issue requiring a higher priority than
'major'.
So, can a second pair of eyes please have a timely look at the attached test
and patch?
Basically the current quicksort implementation seems to assume that luckily
always the median is chosen as pivot element by grabbing the mid element, not
handling the case where the initially chosen pivot ends up not in the middle.
Hope this and the test helps to understand the issue.
Reproducible, currently failing test and a patch attached.
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