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https://issues.apache.org/jira/browse/LUCENE-4682?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Michael McCandless updated LUCENE-4682:
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Attachment: fstByteStats.txt
I was curious about the number of times each byte value occurs in an FST ...
because we could pick an uncommon one and use it as periodic marker (eg ever 5
arcs or something) to enable binary searching.
So I ran this on two FSTs ... first one is all Wikipedia terms and second one
is FreeDB suggester (has some non-ascii song titles...).
Not sure exactly what to conclude since a byte's frequency is FST dependent ...
> Reduce wasted bytes in FST due to array arcs
> --------------------------------------------
>
> Key: LUCENE-4682
> URL: https://issues.apache.org/jira/browse/LUCENE-4682
> Project: Lucene - Core
> Issue Type: Improvement
> Components: core/FSTs
> Reporter: Michael McCandless
> Priority: Minor
> Attachments: fstByteStats.txt, kuromoji.wasted.bytes.txt,
> LUCENE-4682.patch
>
>
> When a node is close to the root, or it has many outgoing arcs, the FST
> writes the arcs as an array (each arc gets N bytes), so we can e.g. bin
> search on lookup.
> The problem is N is set to the max(numBytesPerArc), so if you have an outlier
> arc e.g. with a big output, you can waste many bytes for all the other arcs
> that didn't need so many bytes.
> I generated Kuromoji's FST and found it has 271187 wasted bytes vs total size
> 1535612 = ~18% wasted.
> It would be nice to reduce this.
> One thing we could do without packing is: in addNode, if we detect that
> number of wasted bytes is above some threshold, then don't do the expansion.
> Another thing, if we are packing: we could record stats in the first pass
> about which nodes wasted the most, and then in the second pass (paack) we
> could set the threshold based on the top X% nodes that waste ...
> Another idea is maybe to deref large outputs, so that the numBytesPerArc is
> more uniform ...
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