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https://issues.apache.org/jira/browse/LUCENE-10610?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17553131#comment-17553131
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Uwe Schindler commented on LUCENE-10610:
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I looked at the code again:
- Automaton class has no equals and no hashCode
- RunAutomaton has equals and hashCode
I don't want to refactor or decide if equals or hashCode is needed. I would
just make the already existing hashCode bug free. hashCode should take the same
fields to calculate the hashcode that are also used by equals. This would make
query cache work fine, that's all needed.
I do not think we need to discuss if equals/hashCode ensures that two
automatons are semantically equal (describe state machine with same behaviour).
For query cache we only need to make sure that a query thats created with the
same input has the RunAutomaton. We don't need to cache cases where the
automaton looks different because the regex was different but functionally same.
> RunAutomaton#hashCode() can easily cause hash collision for different
> Automatons
> --------------------------------------------------------------------------------
>
> Key: LUCENE-10610
> URL: https://issues.apache.org/jira/browse/LUCENE-10610
> Project: Lucene - Core
> Issue Type: Bug
> Reporter: Tomoko Uchida
> Priority: Minor
>
> Current RunAutomaton#hashCode() is:
> {code:java}
> @Override
> public int hashCode() {
> final int prime = 31;
> int result = 1;
> result = prime * result + alphabetSize;
> result = prime * result + points.length;
> result = prime * result + size;
> return result;
> }
> {code}
> Since it does not take account of the contents of the {{points}} array, this
> returns the same value for different automatons when their alphabet size and
> state size are the same.
> For example, this test code passes.
> {code:java}
> public void testHashCode() throws IOException {
> PrefixQuery q1 = new PrefixQuery(new Term("field", "aba"));
> PrefixQuery q2 = new PrefixQuery(new Term("field", "fee"));
> assert q1.compiled.runAutomaton.hashCode() ==
> q2.compiled.runAutomaton.hashCode();
> }
> {code}
> I suspect this is a bug?
> Note that I think it's not a serious one; all callers of this {{hashCode()}}
> take account of additional information when calculating their own hash value,
> it seems there is no substantial impact on higher-level APIs.
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