[jira] [Commented] (LUCENE-6595) CharFilter offsets correction is wonky

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

    [ https://issues.apache.org/jira/browse/LUCENE-6595?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14616411#comment-14616411 ] 

Michael McCandless commented on LUCENE-6595:
--------------------------------------------

[~caomanhdat] will you have time to fold in some of the feedback above, to minimize API changes?  Or I can try to, if you're too busy...

> CharFilter offsets correction is wonky
> --------------------------------------
>
>                 Key: LUCENE-6595
>                 URL: https://issues.apache.org/jira/browse/LUCENE-6595
>             Project: Lucene - Core
>          Issue Type: Bug
>            Reporter: Michael McCandless
>         Attachments: LUCENE-6595.patch, LUCENE-6595.patch
>
>
> Spinoff from this original Elasticsearch issue: https://github.com/elastic/elasticsearch/issues/11726
> If I make a MappingCharFilter with these mappings:
> {noformat}
>   ( -> 
>   ) -> 
> {noformat}
> i.e., just erase left and right paren, then tokenizing the string
> "(F31)" with e.g. WhitespaceTokenizer, produces a single token F31,
> with start offset 1 (good).
> But for its end offset, I would expect/want 4, but it produces 5
> today.
> This can be easily explained given how the mapping works: each time a
> mapping rule matches, we update the cumulative offset difference,
> conceptually as an array like this (it's encoded more compactly):
> {noformat}
>   Output offset: 0 1 2 3
>    Input offset: 1 2 3 5
> {noformat}
> When the tokenizer produces F31, it assigns it startOffset=0 and
> endOffset=3 based on the characters it sees (F, 3, 1).  It then asks
> the CharFilter to correct those offsets, mapping them backwards
> through the above arrays, which creates startOffset=1 (good) and
> endOffset=5 (bad).
> At first, to fix this, I thought this is an "off-by-1" and when
> correcting the endOffset we really should return
> 1+correct(outputEndOffset-1), which would return the correct value (4)
> here.
> But that's too naive, e.g. here's another example:
> {noformat}
>   cccc -> cc
> {noformat}
> If I then tokenize cccc, today we produce the correct offsets (0, 4)
> but if we do this "off-by-1" fix for endOffset, we would get the wrong
> endOffset (2).
> I'm not sure what to do here...



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