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https://issues.apache.org/jira/browse/LUCENE-8231?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16419121#comment-16419121
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Jim Ferenczi commented on LUCENE-8231:
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I tried this approach and generated a new FST with the remap chars. The size of
the FST after conversion is 4MB + 1MB for the separated Hanja FST which is
roughly the same size as the FST with the hangul syllab and the Hanja together
(5.4MB). I also ran the HantecRel indexation and it tooks approximatively 235s
to build (I tried multiple times and the times were pretty consistent) with
root caching for the 255 first arcs. That's surprising because it's slower than
the FST with hangul syllab and root caching (200s) so I wonder if this feature
is worth the complexity ? I checked the size of the root caching for the 11,171
syllabs for Hangul and it takes approximatively 250k so that's not bad
considering that this version is faster.
I'll try the compression for compounds now.
> Nori, a Korean analyzer based on mecab-ko-dic
> ---------------------------------------------
>
> Key: LUCENE-8231
> URL: https://issues.apache.org/jira/browse/LUCENE-8231
> Project: Lucene - Core
> Issue Type: New Feature
> Reporter: Jim Ferenczi
> Priority: Major
> Attachments: LUCENE-8231.patch, LUCENE-8231.patch
>
>
> There is a dictionary similar to IPADIC but for Korean called mecab-ko-dic:
> It is available under an Apache license here:
> https://bitbucket.org/eunjeon/mecab-ko-dic
> This dictionary was built with MeCab, it defines a format for the features
> adapted for the Korean language.
> Since the Kuromoji tokenizer uses the same format for the morphological
> analysis (left cost + right cost + word cost) I tried to adapt the module to
> handle Korean with the mecab-ko-dic. I've started with a POC that copies the
> Kuromoji module and adapts it for the mecab-ko-dic.
> I used the same classes to build and read the dictionary but I had to make
> some modifications to handle the differences with the IPADIC and Japanese.
> The resulting binary dictionary takes 28MB on disk, it's bigger than the
> IPADIC but mainly because the source is bigger and there are a lot of
> compound and inflect terms that define a group of terms and the segmentation
> that can be applied.
> I attached the patch that contains this new Korean module called -godori-
> nori. It is an adaptation of the Kuromoji module so currently
> the two modules don't share any code. I wanted to validate the approach first
> and check the relevancy of the results. I don't speak Korean so I used the
> relevancy
> tests that was added for another Korean tokenizer
> (https://issues.apache.org/jira/browse/LUCENE-4956) and tested the output
> against mecab-ko which is the official fork of mecab to use the mecab-ko-dic.
> I had to simplify the JapaneseTokenizer, my version removes the nBest output
> and the decomposition of too long tokens. I also
> modified the handling of whitespaces since they are important in Korean.
> Whitespaces that appear before a term are attached to that term and this
> information is used to compute a penalty based on the Part of Speech of the
> token. The penalty cost is a feature added to mecab-ko to handle
> morphemes that should not appear after a morpheme and is described in the
> mecab-ko page:
> https://bitbucket.org/eunjeon/mecab-ko
> Ignoring whitespaces is also more inlined with the official MeCab library
> which attach the whitespaces to the term that follows.
> I also added a decompounder filter that expand the compounds and inflects
> defined in the dictionary and a part of speech filter similar to the Japanese
> that removes the morpheme that are not useful for relevance (suffix, prefix,
> interjection, ...). These filters don't play well with the tokenizer if it
> can
> output multiple paths (nBest output for instance) so for simplicity I removed
> this ability and the Korean tokenizer only outputs the best path.
> I compared the result with mecab-ko to confirm that the analyzer is working
> and ran the relevancy test that is defined in HantecRel.java included
> in the patch (written by Robert for another Korean analyzer). Here are the
> results:
> ||Analyzer||Index Time||Index Size||MAP(CLASSIC)||MAP(BM25)||MAP(GL2)||
> |Standard|35s|131MB|.007|.1044|.1053|
> |CJK|36s|164MB|.1418|.1924|.1916|
> |Korean|212s|90MB|.1628|.2094|.2078|
> I find the results very promising so I plan to continue to work on this
> project. I started to extract the part of the code that could be shared with
> the
> Kuromoji module but I wanted to share the status and this POC first to
> confirm that this approach is viable. The advantages of using the same model
> than
> the Japanese analyzer are multiple: we don't have a Korean analyzer at the
> moment ;), the resulting dictionary is small compared to other libraries that
> use the mecab-ko-dic (the FST takes only 5.4MB) and the Tokenizer prunes the
> lattice on the fly to select the best path efficiently.
> The dictionary can be built directly from the godori module with the
> following command:
> ant regenerate (you need to create the resource directory (mkdir
> lucene/analysis/godori/src/resources/org/apache/lucene/analysis/ko/dict)
> first since the dictionary is not included in the patch).
> I've also added some minimal tests in the module to play with the analysis.
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