`CollationKeyFilter` converts each token into its binary `CollationKey` using the provided `Collator`, and then encode the `CollationKey` as a String using {@link org.apache.lucene.util.IndexableBinaryStringTools}, to allow 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.). ({@link org.apache.lucene.analysis.LowerCaseFilter} and {@link org.apache.lucene.analysis.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);
      RAMDirectory ramDir = new RAMDirectory();
      IndexWriter writer = new IndexWriter
        (ramDir, analyzer, true, IndexWriter.MaxFieldLength.LIMITED);
      Document doc = new Document();
      doc.add(new Field("content", "\u0633\u0627\u0628",
                        Field.Store.YES, Field.Index.ANALYZED));
      writer.addDocument(doc);
      writer.close();
      IndexSearcher is = new IndexSearcher(ramDir, true);

      // The AnalyzingQueryParser in Lucene's contrib allows terms in range queries
      // to be passed through an analyzer - Lucene's standard QueryParser does not
      // allow this.
      AnalyzingQueryParser aqp = new AnalyzingQueryParser("content", analyzer);
      aqp.setLowercaseExpandedTerms(false);

      // 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("[ \u062F TO \u0698 ]"), 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")));
      RAMDirectory indexStore = new RAMDirectory();
      IndexWriter writer = new IndexWriter
        (indexStore, analyzer, true, IndexWriter.MaxFieldLength.LIMITED);
      String[] tracer = new String[] { "A", "B", "C", "D", "E" };
      String[] data = new String[] { "HAT", "HUT", "H\u00C5T", "H\u00D8T", "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 Field("tracer", tracer[i], Field.Store.YES, Field.Index.NO));
        doc.add(new Field("contents", data[i], Field.Store.NO, Field.Index.ANALYZED));
        writer.addDocument(doc);
      }
      writer.close();
      Searcher searcher = new IndexSearcher(indexStore, true);
      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);
      RAMDirectory ramDir = new RAMDirectory();
      IndexWriter writer = new IndexWriter
        (ramDir, analyzer, true, IndexWriter.MaxFieldLength.LIMITED);
      Document doc = new Document();
      doc.add(new Field("contents", "DIGY", Field.Store.NO, Field.Index.ANALYZED));
      writer.addDocument(doc);
      writer.close();
      IndexSearcher is = new IndexSearcher(ramDir, true);
      QueryParser parser = new QueryParser("contents", analyzer);
      Query query = parser.parse("d\u0131gy");   // 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 {@link java.text.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 {@link java.text.Collator#getInstance(java.util.Locale)}.
4. The collation strength used - see {@link java.text.Collator#setStrength(int)}

`ICUCollationKeyFilter`, available in the icu package in Lucene's contrib area, uses ICU4J's `Collator`, which makes its version available, thus allowing collation to be versioned independently from the JVM. `ICUCollationKeyFilter` is also significantly faster and generates significantly shorter keys than `CollationKeyFilter`. 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 `CollationKeyFilter` to generate index terms, do not use `ICUCollationKeyFilter` on the query side, or vice versa.