[LUCENE-8084] FSTs can be very space-inefficient on array-expanded nodes

• Type: Improvement
• Status: Open
• Priority: Minor
• Resolution: Unresolved
• Lucene Fields: New
• Assignee: Unassigned
• Reporter: Dawid Weiss
• Created: 07/Dec/17 11:24
• Updated: 28/Aug/22 15:22

Description

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.

Issue Links

• relates to: LUCENE-4682 Reduce wasted bytes in FST due to array arcs