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https://issues.apache.org/jira/browse/LUCENE-5013?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13664652#comment-13664652
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Shawn Heisey commented on LUCENE-5013:
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Does it make sense to have this filter do the Scandinavian folding *before* the
ascii folding, rather than after? Would that cause fewer search misses and
false positives, or more? Would it make sense to leave the ASCII step out, and
let the user run it separately, either before or after according to the way
they want it to work?
One of the things I really like about the ICU filters is that they handle
international notions of uppercase and lowercase, so you're not dealing with
just ASCII characters. The example given on the wiki page is ß/SS, which
honestly means little to me with my uneducated (American) viewpoint. If this
filter can do something similar for the differences between Scandinavian
languages, that would really be useful.
> ScandinavianInterintelligableASCIIFoldingFilter
> -----------------------------------------------
>
> Key: LUCENE-5013
> URL: https://issues.apache.org/jira/browse/LUCENE-5013
> Project: Lucene - Core
> Issue Type: New Feature
> Components: modules/analysis
> Affects Versions: 4.3
> Reporter: Karl Wettin
> Priority: Trivial
> Attachments: LUCENE-5013.txt
>
>
> This filter is an augmentation of output from ASCIIFoldingFilter,
> it discriminate against double vowels aa, ae, ao, oe and oo, leaving just the
> first one.
> blåbærsyltetøj == blåbärsyltetöj == blaabaarsyltetoej == blabarsyltetoj
> räksmörgås == ræksmørgås == ræksmörgaos == raeksmoergaas == raksmorgas
> Caveats:
> Since this is a filtering on top of ASCIIFoldingFilter äöåøæ already has been
> folded down to aoaoae when handled by this filter it will cause effects such
> as:
> bøen -> boen -> bon
> åene -> aene -> ane
> I find this to be a trivial problem compared to not finding anything at all.
> Background:
> Swedish åäö is in fact the same letters as Norwegian and Danish åæø and thus
> interchangeable in when used between these languages. They are however folded
> differently when people type them on a keyboard lacking these characters and
> ASCIIFoldingFilter handle ä and æ differently.
> When a Swedish person is lacking umlauted characters on the keyboard they
> consistently type a, a, o instead of å, ä, ö. Foreigners also tend to use a,
> a, o.
> In Norway people tend to type aa, ae and oe instead of å, æ and ø. Some use
> a, a, o. I've also seen oo, ao, etc. And permutations. Not sure about Denmark
> but the pattern is probably the same.
> This filter solves that problem, but might also cause new.
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