GreekLowerCaseFilter.cs

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/*
 *
 * 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.
 *
*/

using Lucene.Net.Analysis.Tokenattributes;

namespace Lucene.Net.Analysis.El
{
    /*
     * Normalizes token text to lower case, removes some Greek diacritics,
     * and standardizes final sigma to sigma. 
     *
     */
    public sealed class GreekLowerCaseFilter : TokenFilter
    {
        private ITermAttribute termAtt;

        public GreekLowerCaseFilter(TokenStream _in)
            : base(_in)
        {
            termAtt = AddAttribute<ITermAttribute>();
        }

        public override bool IncrementToken()
        {
            if (input.IncrementToken())
            {
                char[] chArray = termAtt.TermBuffer();
                int chLen = termAtt.TermLength();
                // TODO: iterate codepoints to support supp. characters
                for (int i = 0; i < chLen; i++)
                {
                    chArray[i] = (char)lowerCase(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 char.ToLower((char)codepoint);
            }
        }
    }
}