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From "Hiroaki Kawai (JIRA)" <>
Subject [jira] Commented: (LUCENE-1224) NGramTokenFilter creates bad TokenStream
Date Thu, 15 May 2008 09:29:55 GMT


Hiroaki Kawai commented on LUCENE-1224:

Q: Why it is necessary to index
A: Because it was necessary to show how the query is performed. 
That is the point I wanted to address. 

Q: testNGrams don't test the issue
A: Exactlly. it don't test the issue.
I modified the test because it failed with my patch, that
Token.toString() prints additional incrementPosition information.
I read the existing test program, and found that current test
program depends on Token.toString() method.
I thought we'd better test it without Token.toString().
Current test program tests that the Token have NO positionIncrement.

testIndexAndQuery is the very test that address the issue.
Please don't drop it. Think the case, we want to search a word 
that contain "abcd" with 2-gram index.
The test does searching "abcd" with 2,3-gram.

We have the 2gram of abcde; 'ab', 'bc', 'cd', 'de'.
Reffering the current lucene implementation, the position gap 
of 'ab' and 'bc' must be 1.

> NGramTokenFilter creates bad TokenStream
> ----------------------------------------
>                 Key: LUCENE-1224
>                 URL:
>             Project: Lucene - Java
>          Issue Type: Bug
>          Components: contrib/*
>            Reporter: Hiroaki Kawai
>            Assignee: Grant Ingersoll
>            Priority: Critical
>         Attachments: LUCENE-1224.patch, NGramTokenFilter.patch, NGramTokenFilter.patch
> With current trunk NGramTokenFilter(min=2,max=4) , I index "abcdef" string into an index,
but I can't query it with "abc". If I query with "ab", I can get a hit result.
> The reason is that the NGramTokenFilter generates badly ordered TokenStream. Query is
based on the Token order in the TokenStream, that how stemming or phrase should be anlayzed
is based on the order (Token.positionIncrement).
> With current filter, query string "abc" is tokenized to : ab bc abc 
> meaning "query a string that has ab bc abc in this order".
> Expected filter will generate : ab abc(positionIncrement=0) bc
> meaning "query a string that has (ab|abc) bc in this order"
> I'd like to submit a patch for this issue. :-)

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