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https://issues.apache.org/jira/browse/LUCENE-2970?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13007488#comment-13007488
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Michael McCandless commented on LUCENE-2970:
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Patch looks correct to me!
The algo you impl'd is the same one described in Cormen, Leiserson, Rivest
Algorithms book, as a side effect of doing a depth-first walk through the DFA.
Their description of DFS colors the nodes -- white is unvisited, black is
visited, gray is "being visited" (ie on my current path). A DFA then has a
cycle if every you recurse and find a gray node.
In your patch, the combination of "path" and "visited" maps to these colors,
and you detect a cycle when path is set and visited is not.
Maybe rename the test-only isFiniteSimple to isFiniteSLOW or something?
Does the new random test case tend not to hit the super-slow cases...?
> SpecialOperations.isFinite can have TERRIBLE TERRIBLE runtime in certain
> situations
> -----------------------------------------------------------------------------------
>
> Key: LUCENE-2970
> URL: https://issues.apache.org/jira/browse/LUCENE-2970
> Project: Lucene - Java
> Issue Type: Bug
> Affects Versions: 4.0
> Reporter: Robert Muir
> Assignee: Robert Muir
> Fix For: 4.0
>
> Attachments: LUCENE-2970.patch
>
>
> in an application of mine, i experienced some very slow query times with
> finite automata (all the DFAs are acyclic)
> It turned out, the slowdown is some terrible runtime in
> SpecialOperations.isFinite <-- this is used to determine if the DFA is
> acyclic or not.
> (in this case I am talking about even up to minutes of cpu).
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