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https://issues.apache.org/jira/browse/LUCENE-1343?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12858891#action_12858891
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Robert Muir commented on LUCENE-1343:
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By the way, I have been running this with the ASCIIFoldingFilter tests and
ensuring its a superset (e.g. we have at least all their mappings).
But there are some bugs in ASCIIFoldingFilter that should be fixed:
For example, U+1E9B (LATIN SMALL LETTER LONG S WITH DOT ABOVE)
But in unicode. this is canonically equivalent to U+017F (LONG S) U+0307
(COMBINING DOT ABOVE)
AsciiFoldingFilter folds U+1E9B (LONG S WITH DOT) to an F
but it folds U+017F (LONG S) to an S
Unicode defines this character as a compatibility equivalent to S anyway, but
its worse that ASCIIFoldingFilter is canonically inconsistent with itself.
> A replacement for AsciiFoldingFilter that does a more thorough job of
> removing diacritical marks or non-spacing modifiers.
> --------------------------------------------------------------------------------------------------------------------------
>
> Key: LUCENE-1343
> URL: https://issues.apache.org/jira/browse/LUCENE-1343
> Project: Lucene - Java
> Issue Type: Improvement
> Components: Analysis
> Affects Versions: 3.1
> Reporter: Robert Haschart
> Assignee: Robert Muir
> Priority: Minor
> Fix For: 3.1
>
> Attachments: LUCENE-1343.patch, normalizer.jar, UnicodeCharUtil.java,
> UnicodeNormalizationFilter.java, UnicodeNormalizationFilterFactory.java,
> utr30.nrm
>
>
> The ISOLatin1AccentFilter takes Unicode characters that have diacritical
> marks and replaces them with a version of that character with the diacritical
> mark removed. For example é becomes e. However another equally valid way of
> representing an accented character in Unicode is to have the unaccented
> character followed by a non-spacing modifier character (like this: é )
> The ISOLatin1AccentFilter doesn't handle the accents in decomposed unicode
> characters at all. Additionally there are some instances where a word will
> contain what looks like an accented character, that is actually considered to
> be a separate unaccented character such as Ł but which to make searching
> easier you want to fold onto the latin1 lookalike version L .
> The UnicodeNormalizationFilter can filter out accents and diacritical marks
> whether they occur as composed characters or decomposed characters, it can
> also handle cases where as described above characters that look like they
> have diacritics (but don't) are to be folded onto the letter that they look
> like ( Ł -> L )
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