content: package org.apache.lucene.collation;
  
  /**
   * Licensed to the Apache Software Foundation (ASF) under one or more
   * contributor license agreements.  See the NOTICE file distributed with
   * this work for additional information regarding copyright ownership.
   * The ASF licenses this file to You under the Apache License, Version 2.0
   * (the "License"); you may not use this file except in compliance with
   * the License.  You may obtain a copy of the License at
   *
   *     http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  
  
  import org.apache.lucene.analysis.TokenFilter;
  import org.apache.lucene.analysis.TokenStream;
  import org.apache.lucene.analysis.tokenattributes.CharTermAttribute;
  import org.apache.lucene.util.IndexableBinaryStringTools;
  
  import java.io.IOException;
  import java.text.Collator;
  
  
  /**
   * <p>
   *   Converts each token into its {@link java.text.CollationKey}, and then
   *   encodes the CollationKey with {@link IndexableBinaryStringTools}, to allow 
   *   it to be stored as an index term.
   * </p>
   * <p>
   *   <strong>WARNING:</strong> Make sure you use exactly the same Collator at
   *   index and query time -- CollationKeys 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
   *   CollationKeys 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):
   * </p>
   * <ol>
   *   <li>JVM vendor</li>
   *   <li>JVM version, including patch version</li>
   *   <li>
   *     The language (and country and variant, if specified) of the Locale
   *     used when constructing the collator via
   *     {@link Collator#getInstance(java.util.Locale)}.
   *   </li>
   *   <li>
   *     The collation strength used - see {@link Collator#setStrength(int)}
   *   </li>
   * </ol> 
   * <p>
   *   The <code>ICUCollationKeyFilter</code> in the icu package of 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
   *   <a href="http://site.icu-project.org/charts/collation-icu4j-sun"
   *   >http://site.icu-project.org/charts/collation-icu4j-sun</a> for key
   *   generation timing and key length comparisons between ICU4J and
   *   java.text.Collator over several languages.
   * </p>
   * <p>
   *   CollationKeys generated by java.text.Collators 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.
   * </p>
   */
  public final class CollationKeyFilter extends TokenFilter {
    private final Collator collator;
    private final CharTermAttribute termAtt = addAttribute(CharTermAttribute.class);
  
    /**
     * @param input Source token stream
     * @param collator CollationKey generator
     */
    public CollationKeyFilter(TokenStream input, Collator collator) {
      super(input);
      // clone in case JRE doesnt properly sync,
      // or to reduce contention in case they do
      this.collator = (Collator) collator.clone();
    }
  
    @Override
    public boolean incrementToken() throws IOException {
      if (input.incrementToken()) {
        byte[] collationKey = collator.getCollationKey(termAtt.toString()).toByteArray();
        int encodedLength = IndexableBinaryStringTools.getEncodedLength(
            collationKey, 0, collationKey.length);
        termAtt.resizeBuffer(encodedLength);
        termAtt.setLength(encodedLength);
        IndexableBinaryStringTools.encode(collationKey, 0, collationKey.length,
            termAtt.buffer(), 0, encodedLength);
        return true;
      } else {
        return false;
      }
    }
  }
  