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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