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https://issues.apache.org/jira/browse/LUCENE-8920?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16965650#comment-16965650
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Bruno Roustant commented on LUCENE-8920:
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Hum, I was confused by the special case of END_LABEL. Yes, maybe we can rely on 
the bitTable of the current arc. So we need the label range, the bit table and 
the first label. I'm going to try to remove the labels.

> Reduce size of FSTs due to use of direct-addressing encoding 
> -------------------------------------------------------------
>
>                 Key: LUCENE-8920
>                 URL: https://issues.apache.org/jira/browse/LUCENE-8920
>             Project: Lucene - Core
>          Issue Type: Improvement
>            Reporter: Michael Sokolov
>            Priority: Minor
>         Attachments: TestTermsDictRamBytesUsed.java
>
>          Time Spent: 3h 20m
>  Remaining Estimate: 0h
>
> Some data can lead to worst-case ~4x RAM usage due to this optimization. 
> Several ideas were suggested to combat this on the mailing list:
> bq. I think we can improve thesituation here by tracking, per-FST instance, 
> the size increase we're seeing while building (or perhaps do a preliminary 
> pass before building) in order to decide whether to apply the encoding. 
> bq. we could also make the encoding a bit more efficient. For instance I 
> noticed that arc metadata is pretty large in some cases (in the 10-20 bytes) 
> which make gaps very costly. Associating each label with a dense id and 
> having an intermediate lookup, ie. lookup label -> id and then id->arc offset 
> instead of doing label->arc directly could save a lot of space in some cases? 
> Also it seems that we are repeating the label in the arc metadata when 
> array-with-gaps is used, even though it shouldn't be necessary since the 
> label is implicit from the address?



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