lucene/analysis/common/src/java/org/apache/lucene/analysis/el/GreekLowerCaseFilter.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.el;

import java.io.IOException;

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

/**
 * Normalizes token text to lower case, removes some Greek diacritics,
 * and standardizes final sigma to sigma. 
 */
public final class GreekLowerCaseFilter extends TokenFilter {
  private final CharTermAttribute termAtt = addAttribute(CharTermAttribute.class);

  /**
   * Create a GreekLowerCaseFilter that normalizes Greek token text.
   * 
   * @param in TokenStream to filter
   */
  public GreekLowerCaseFilter(TokenStream in) {
    super(in);
  }
  
  @Override
  public boolean incrementToken() throws IOException {
    if (input.incrementToken()) {
      char[] chArray = termAtt.buffer();
      int chLen = termAtt.length();
      for (int i = 0; i < chLen;) {
        i += Character.toChars(
            lowerCase(Character.codePointAt(chArray, i, chLen)), chArray, i);
       }
      return true;
    } else {
      return false;
    }
  }
  
  private int lowerCase(int codepoint) {
    switch(codepoint) {
      /* There are two lowercase forms of sigma:
       *   U+03C2: small final sigma (end of word)
       *   U+03C3: small sigma (otherwise)
       *   
       * Standardize both to U+03C3
       */
      case '\u03C2': /* small final sigma */
        return '\u03C3'; /* small sigma */
        
      /* Some greek characters contain diacritics.
       * This filter removes these, converting to the lowercase base form.
       */
        
      case '\u0386': /* capital alpha with tonos */
      case '\u03AC': /* small alpha with tonos */
        return '\u03B1'; /* small alpha */
        
      case '\u0388': /* capital epsilon with tonos */
      case '\u03AD': /* small epsilon with tonos */
        return '\u03B5'; /* small epsilon */
        
      case '\u0389': /* capital eta with tonos */
      case '\u03AE': /* small eta with tonos */
        return '\u03B7'; /* small eta */
        
      case '\u038A': /* capital iota with tonos */
      case '\u03AA': /* capital iota with dialytika */
      case '\u03AF': /* small iota with tonos */
      case '\u03CA': /* small iota with dialytika */
      case '\u0390': /* small iota with dialytika and tonos */
        return '\u03B9'; /* small iota */
        
      case '\u038E': /* capital upsilon with tonos */
      case '\u03AB': /* capital upsilon with dialytika */
      case '\u03CD': /* small upsilon with tonos */
      case '\u03CB': /* small upsilon with dialytika */
      case '\u03B0': /* small upsilon with dialytika and tonos */
        return '\u03C5'; /* small upsilon */
        
      case '\u038C': /* capital omicron with tonos */
      case '\u03CC': /* small omicron with tonos */
        return '\u03BF'; /* small omicron */
        
      case '\u038F': /* capital omega with tonos */
      case '\u03CE': /* small omega with tonos */
        return '\u03C9'; /* small omega */
        
      /* The previous implementation did the conversion below.
       * Only implemented for backwards compatibility with old indexes.
       */
        
      case '\u03A2': /* reserved */
        return '\u03C2'; /* small final sigma */
        
      default:
        return Character.toLowerCase(codepoint);
    }
  }
}