[jira] [Created] (LUCENE-8084) FSTs can be very space-inefficient on array-expanded nodes

["Dawid Weiss (JIRA)" <ji...@apache.org>]

Dawid Weiss created LUCENE-8084:
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             Summary: FSTs can be very space-inefficient on array-expanded nodes
                 Key: LUCENE-8084
                 URL: https://issues.apache.org/jira/browse/LUCENE-8084
             Project: Lucene - Core
          Issue Type: Improvement
            Reporter: Dawid Weiss
            Priority: Minor


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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