# Package org.apache.lucene.collation

Unicode collation support.

See: Description

Class Summary
CollationAttributeFactory: Converts each token into its `CollationKey`, and then encodes the bytes as an index term.
CollationDocValuesField: Indexes collation keys as a single-valued `SortedDocValuesField`.
CollationKeyAnalyzer: Configures `KeywordTokenizer` with `CollationAttributeFactory`.

## Package org.apache.lucene.collation Description

Unicode collation support.

`Collation` converts each token into its binary `CollationKey` using the provided `Collator`, allowing it to be stored as an index term.

## Use Cases

- Efficient sorting of terms in languages that use non-Unicode character orderings. (Lucene Sort using a Locale can be very slow.)
- Efficient range queries over fields that contain terms in languages that use non-Unicode character orderings. (Range queries using a Locale can be very slow.)
- Effective Locale-specific normalization (case differences, diacritics, etc.). (`LowerCaseFilter` and `ASCIIFoldingFilter` provide these services in a generic way that doesn't take into account locale-specific needs.)

## Example Usages

### Farsi Range Queries

       // "fa" Locale is not supported by Sun JDK 1.4 or 1.5
       Collator collator = Collator.getInstance(new Locale("ar"));
       CollationKeyAnalyzer analyzer = new CollationKeyAnalyzer(collator);
       Path dirPath = Files.createTempDirectory("tempIndex");
       Directory dir = FSDirectory.open(dirPath);
       IndexWriter writer = new IndexWriter(dir, new IndexWriterConfig(analyzer));
       Document doc = new Document();
       doc.add(new TextField("content", "ساب", Field.Store.YES));
       writer.addDocument(doc);
       writer.close();
       IndexReader ir = DirectoryReader.open(dir);
       IndexSearcher is = new IndexSearcher(ir);

       QueryParser aqp = new QueryParser("content", analyzer);
       aqp.setAnalyzeRangeTerms(true);

       // Unicode order would include U+0633 in [ U+062F - U+0698 ], but Farsi
       // orders the U+0698 character before the U+0633 character, so the single
       // indexed Term above should NOT be returned by a ConstantScoreRangeQuery
       // with a Farsi Collator (or an Arabic one for the case when Farsi is not
       // supported).
       ScoreDoc[] result
         = is.search(aqp.parse("[ د TO ژ ]"), null, 1000).scoreDocs;
       assertEquals("The index Term should not be included.", 0, result.length);

### Danish Sorting

       Analyzer analyzer
         = new CollationKeyAnalyzer(Collator.getInstance(new Locale("da", "dk")));
       Path dirPath = Files.createTempDirectory("tempIndex");
       Directory dir = FSDirectory.open(dirPath);
       IndexWriter writer = new IndexWriter(dir, new IndexWriterConfig(analyzer));
       String[] tracer = new String[] { "A", "B", "C", "D", "E" };
       String[] data = new String[] { "HAT", "HUT", "HÅT", "HØT", "HOT" };
       String[] sortedTracerOrder = new String[] { "A", "E", "B", "D", "C" };
       for (int i = 0 ; i < data.length ; ++i) {
         Document doc = new Document();
         doc.add(new StoredField("tracer", tracer[i]));
         doc.add(new TextField("contents", data[i], Field.Store.NO));
         writer.addDocument(doc);
       }
       writer.close();
       IndexReader ir = DirectoryReader.open(dir);
       IndexSearcher searcher = new IndexSearcher(ir);
       Sort sort = new Sort();
       sort.setSort(new SortField("contents", SortField.STRING));
       Query query = new MatchAllDocsQuery();
       ScoreDoc[] result = searcher.search(query, null, 1000, sort).scoreDocs;
       for (int i = 0 ; i < result.length ; ++i) {
         Document doc = searcher.doc(result[i].doc);
         assertEquals(sortedTracerOrder[i], doc.getValues("tracer")[0]);
       }

### Turkish Case Normalization

       Collator collator = Collator.getInstance(new Locale("tr", "TR"));
       collator.setStrength(Collator.PRIMARY);
       Analyzer analyzer = new CollationKeyAnalyzer(collator);
       Path dirPath = Files.createTempDirectory("tempIndex");
       Directory dir = FSDirectory.open(dirPath);
       IndexWriter writer = new IndexWriter(dir, new IndexWriterConfig(analyzer));
       Document doc = new Document();
       doc.add(new TextField("contents", "DIGY", Field.Store.NO));
       writer.addDocument(doc);
       writer.close();
       IndexReader ir = DirectoryReader.open(dir);
       IndexSearcher is = new IndexSearcher(ir);
       QueryParser parser = new QueryParser("contents", analyzer);
       Query query = parser.parse("dıgy");   // U+0131: dotless i
       ScoreDoc[] result = is.search(query, null, 1000).scoreDocs;
       assertEquals("The index Term should be included.", 1, result.length);

## Caveats and Comparisons

WARNING: Make sure you use exactly the same `Collator` at index and query time -- `CollationKey`s are only comparable when produced by the same `Collator`. Since `RuleBasedCollator`s are not independently versioned, it is unsafe to search against stored `CollationKey`s unless the following are exactly the same (best practice is to store this information with the index and check that they remain the same at query time):

1. JVM vendor
2. JVM version, including patch version
3. The language (and country and variant, if specified) of the Locale used when constructing the collator via `Collator.getInstance(java.util.Locale)`.
4. The collation strength used - see `Collator.setStrength(int)`

`ICUCollationKeyAnalyzer`, available in the icu analysis module, uses ICU4J's `Collator`, which makes its version available, thus allowing collation to be versioned independently from the JVM. `ICUCollationKeyAnalyzer` is also significantly faster and generates significantly shorter keys than `CollationKeyAnalyzer`. See http://site.icu-project.org/charts/collation-icu4j-sun for key generation timing and key length comparisons between ICU4J and `java.text.Collator` over several languages.

`CollationKey`s generated by `java.text.Collator`s are not compatible with those those generated by ICU Collators. Specifically, if you use `CollationKeyAnalyzer` to generate index terms, do not use `ICUCollationKeyAnalyzer` on the query side, or vice versa.