lucene/analysis/common/src/java/org/apache/lucene/analysis/tr/TurkishLowerCaseFilter.java

/*
 * 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.
 */
package org.apache.lucene.analysis.tr;


import java.io.IOException;

import org.apache.lucene.analysis.TokenFilter;
import org.apache.lucene.analysis.TokenStream;
import org.apache.lucene.analysis.tokenattributes.CharTermAttribute;

/**
 * Normalizes Turkish token text to lower case.
 * <p>
 * Turkish and Azeri have unique casing behavior for some characters. This
 * filter applies Turkish lowercase rules. For more information, see <a
 * href="http://en.wikipedia.org/wiki/Turkish_dotted_and_dotless_I"
 * >http://en.wikipedia.org/wiki/Turkish_dotted_and_dotless_I</a>
 * </p>
 */
public final class TurkishLowerCaseFilter extends TokenFilter {
  private static final int LATIN_CAPITAL_LETTER_I = '\u0049';
  private static final int LATIN_SMALL_LETTER_I = '\u0069';
  private static final int LATIN_SMALL_LETTER_DOTLESS_I = '\u0131';
  private static final int COMBINING_DOT_ABOVE = '\u0307';
  private final CharTermAttribute termAtt = addAttribute(CharTermAttribute.class);
  
  /**
   * Create a new TurkishLowerCaseFilter, that normalizes Turkish token text 
   * to lower case.
   * 
   * @param in TokenStream to filter
   */
  public TurkishLowerCaseFilter(TokenStream in) {
    super(in);
  }
  
  @Override
  public final boolean incrementToken() throws IOException {
    boolean iOrAfter = false;
    
    if (input.incrementToken()) {
      final char[] buffer = termAtt.buffer();
      int length = termAtt.length();
      for (int i = 0; i < length;) {
        final int ch = Character.codePointAt(buffer, i, length);
    
        iOrAfter = (ch == LATIN_CAPITAL_LETTER_I || 
            (iOrAfter && Character.getType(ch) == Character.NON_SPACING_MARK));
        
        if (iOrAfter) { // all the special I turkish handling happens here.
          switch(ch) {
            // remove COMBINING_DOT_ABOVE to mimic composed lowercase
            case COMBINING_DOT_ABOVE:
              length = delete(buffer, i, length);
              continue;
            // i itself, it depends if it is followed by COMBINING_DOT_ABOVE
            // if it is, we will make it small i and later remove the dot
            case LATIN_CAPITAL_LETTER_I:
              if (isBeforeDot(buffer, i + 1, length)) {
                buffer[i] = LATIN_SMALL_LETTER_I;
              } else {
                buffer[i] = LATIN_SMALL_LETTER_DOTLESS_I;
                // below is an optimization. no COMBINING_DOT_ABOVE follows,
                // so don't waste time calculating Character.getType(), etc
                iOrAfter = false;
              }
              i++;
              continue;
          }
        }
        
        i += Character.toChars(Character.toLowerCase(ch), buffer, i);
      }
      
      termAtt.setLength(length);
      return true;
    } else
      return false;
  }
  
  
  /**
   * lookahead for a combining dot above.
   * other NSMs may be in between.
   */
  private boolean isBeforeDot(char s[], int pos, int len) {
    for (int i = pos; i < len;) {
      final int ch = Character.codePointAt(s, i, len);
      if (Character.getType(ch) != Character.NON_SPACING_MARK)
        return false;
      if (ch == COMBINING_DOT_ABOVE)
        return true;
      i += Character.charCount(ch);
    }
    
    return false;
  }
  
  /**
   * delete a character in-place.
   * rarely happens, only if COMBINING_DOT_ABOVE is found after an i
   */
  private int delete(char s[], int pos, int len) {
    if (pos < len) 
      System.arraycopy(s, pos + 1, s, pos, len - pos - 1);
    
    return len - 1;
  }
}