[jira] [Created] (LUCENE-4682) Reduce wasted bytes in FST due to array arcs

["Michael McCandless (JIRA)" <ji...@apache.org>]

Michael McCandless created LUCENE-4682:
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             Summary: Reduce wasted bytes in FST due to array arcs
                 Key: LUCENE-4682
                 URL: https://issues.apache.org/jira/browse/LUCENE-4682
             Project: Lucene - Core
          Issue Type: Improvement
          Components: core/FSTs
            Reporter: Michael McCandless
            Priority: Minor


When a node is close to the root, or it has many outgoing arcs, the FST writes the arcs as an array (each arc gets N bytes), so we can e.g. bin search on lookup.

The problem is N is set to the max(numBytesPerArc), so if you have an outlier arc e.g. with a big output, you can waste many bytes for all the other arcs that didn't need so many bytes.

I generated Kuromoji's FST and found it has 271187 wasted bytes vs total size 1535612 = ~18% wasted.

It would be nice to reduce this.

One thing we could do without packing is: in addNode, if we detect that number of wasted bytes is above some threshold, then don't do the expansion.

Another thing, if we are packing: we could record stats in the first pass about which nodes wasted the most, and then in the second pass (paack) we could set the threshold based on the top X% nodes that waste ...

Another idea is maybe to deref large outputs, so that the numBytesPerArc is more uniform ...

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