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https://issues.apache.org/jira/browse/LUCENE-3297?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13080611#comment-13080611
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Dawid Weiss commented on LUCENE-3297:
-------------------------------------
Ok, I've found the issue I was looking for. Mike, this should be solved by
changing the logical organization of the fst. This would also solve the issue
of storing an "empty" automaton and its output (which is currently hardcoded in
an unpleasant way). The solution is to introduce an epsilon-labeled arc
(epsilon meaning an out-of-alphabet symbol, possibly a flag). The "root" of the
fst would be an eps-labeled transition to the actual root node. Then, an empty
automaton is basically an eps-terminal transition and prefix sharing you talk
about in this issue is whatever can be pushed onto this eps-arc. The automaton
remains deterministic, it's just a small change of logic (and addition of the
special eps symbol).
> FST doesn't fully share common prefix across all outputs
> --------------------------------------------------------
>
> Key: LUCENE-3297
> URL: https://issues.apache.org/jira/browse/LUCENE-3297
> Project: Lucene - Java
> Issue Type: Improvement
> Components: core/FSTs
> Reporter: Michael McCandless
> Priority: Minor
>
> FST will try to share prefixes of outputs when possible, however in the [I
> think unusual in practice] case where all outputs share a common prefix, FST
> really ought to store this just once, on the root arc, but instead it's only
> able to push back to the N root arcs. It's sort of an off-by-one on how far
> back the pushing goes...
> One [synthetic] example where this makes a big difference is the new
> Test2BPostings test, when it uses MemoryCodec, because this test has 26 terms
> (letters of alphabet) and each term has exactly the same long (~85 MB) all 1s
> byte[] as the postings. If we fixed this issue, then the resulting FST would
> only be ~85 MB but now instead it needs to be ~85 * 26 MB.
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