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https://issues.apache.org/jira/browse/LUCENE-9983?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17354237#comment-17354237
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Haoyu Zhai commented on LUCENE-9983:
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Oh I realized we're still gonna iterate on those frozen set
[here|https://github.com/apache/lucene/blob/main/lucene/core/src/java/org/apache/lucene/util/automaton/Operations.java#L705]
so maybe bitset is not a good choice? What about just iterate over the keys
and create a {{FronzenIntSet}} based on that? Since we're anyway gonna copy
those keys so it should only add a little more overhead comparing to the
current implementation, while getting the benefit of using a light weight, sort
free data structure?
> Stop sorting determinize powersets unnecessarily
> ------------------------------------------------
>
> Key: LUCENE-9983
> URL: https://issues.apache.org/jira/browse/LUCENE-9983
> Project: Lucene - Core
> Issue Type: Improvement
> Reporter: Michael McCandless
> Priority: Major
>
> Spinoff from LUCENE-9981.
> Today, our {{Operations.determinize}} implementation builds powersets of all
> subsets of NFA states that "belong" in the same determinized state, using
> [this algorithm|https://en.wikipedia.org/wiki/Powerset_construction].
> To hold each powerset, we use a malleable {{SortedIntSet}} and periodically
> freeze it to a {{FrozenIntSet}}, also sorted. We pay a high price to keep
> these growing maps of int key, int value sorted by key, e.g. upgrading to a
> {{TreeMap}} once the map is large enough (> 30 entries).
> But I think sorting is entirely unnecessary here! Really all we need is the
> ability to add/delete keys from the map, and hashCode / equals (by key only –
> ignoring value!), and to freeze the map (a small optimization that we could
> skip initially). We only use these maps to lookup in the (growing)
> determinized automaton whether this powerset has already been seen.
> Maybe we could simply poach the {{IntIntScatterMap}} implementation from
> [HPPC|https://github.com/carrotsearch/hppc]? And then change its
> {{hashCode}}/{{equals }}to only use keys (not values).
> This change should be a big speedup for the kinds of (admittedly adversarial)
> regexps we saw on LUCENE-9981.
>
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