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Posted to dev@lucene.apache.org by "Jack Krupansky (JIRA)" <ji...@apache.org> on 2013/04/24 19:09:15 UTC
[jira] [Commented] (LUCENE-4955) NGramTokenFilter increments
positions for each gram
[ https://issues.apache.org/jira/browse/LUCENE-4955?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13640657#comment-13640657 ]
Jack Krupansky commented on LUCENE-4955:
----------------------------------------
I think that ngram filter and edge-ngram filter are rather different cases.
With edge-ngram it is abundantly clear that all of the edge ngrams "stack up" at the same position (at least for "front" edge ngrams!). But embedded ngrams seem more like a stretching out of the token, from one token to a sequence of tokens. Actually, it is k overlayed sequences, where k = maxGramSize minus minGramSize plus 1.
I think the solution should be to have a "mode" which indicates whether the "intent" is merely variations (sub-tokens) for the token at the same position vs. a stretching the token into a sequence of tokens. Maybe call it "expansionMode": "stack" vs. "sequence".
But even for the latter, I would definitely recommend that each of the k sequences should restart the position at the original token position.
> NGramTokenFilter increments positions for each gram
> ---------------------------------------------------
>
> Key: LUCENE-4955
> URL: https://issues.apache.org/jira/browse/LUCENE-4955
> Project: Lucene - Core
> Issue Type: Bug
> Components: modules/analysis
> Affects Versions: 4.3
> Reporter: Simon Willnauer
> Fix For: 5.0, 4.4
>
> Attachments: highlighter-test.patch, LUCENE-4955.patch
>
>
> NGramTokenFilter increments positions for each gram rather for the actual token which can lead to rather funny problems especially with highlighting. if this filter should be used for highlighting is a different story but today this seems to be a common practice in many situations to highlight sub-term matches.
> I have a test for highlighting that uses ngram failing with a StringIOOB since tokens are sorted by position which causes offsets to be mixed up due to ngram token filter.
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