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From "Dawid Weiss (JIRA)" <>
Subject [jira] [Updated] (LUCENE-8084) FSTs can be very space-inefficient on array-expanded nodes
Date Thu, 07 Dec 2017 11:27:03 GMT


Dawid Weiss updated LUCENE-8084:
    Attachment: capture-4.png

> FSTs can be very space-inefficient on array-expanded nodes
> ----------------------------------------------------------
>                 Key: LUCENE-8084
>                 URL:
>             Project: Lucene - Core
>          Issue Type: Improvement
>            Reporter: Dawid Weiss
>            Priority: Minor
>         Attachments: capture-4.png
> We have FSTs which operate on a larger alphabet (keys in int) space and emit character
sequence outputs. I noticed that certain nodes get expanded into fixed-size arrays to accelerate
lookups (binary search). This has a potential problem, however, when the outputs emit larger
blobs of data (say, one of the outputs is very long, all the others are small). Then the fixed-size
array is very much overallocated, as evident on the attached picture.
> I wonder if it'd be better to encode the array as fixed-size, but without the outputs
and use a local fixed-size pointer into a "value pool" somewhere next to the node's arcs.
Theoretically this "value pool" could even be shared by all of automaton's nodes (saved once
at the end or flushed periodically). 
> Just a thought.

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