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https://issues.apache.org/jira/browse/LUCENE-3297?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13080878#comment-13080878
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Michael McCandless commented on LUCENE-3297:
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We sort of have this today, in the root arc (FST.getFirstArc), but it "avoids"
eps by not setting the arc's label at all, ie you're only allowed/expected to
use this arc's target state, output/nextFinalOutput. This arc is how the
consumer of the FST API "gets started" in accessing the FST.
Adding eps label would make me nervous :) For our FST impl (limited because we
only support determinized FSTs) we'd never see an eps transition anywhere else
right? Ie, it'd only be for the root arc.
So the FST already today can represent a globally shared output prefix; the
challenge in fixing this issue is to fix the Builder impl to be able to push
the output all the way back onto this root arc; the phase where we push outputs
as far back as possible doesn't push far enough...
> 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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