src/Lucene.Net.Analysis.Common/Analysis/Cz/CzechStemmer.cs

using Lucene.Net.Analysis.Util;

namespace Lucene.Net.Analysis.Cz
{
    /*
     * 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.
     */

    /// <summary>
    /// Light Stemmer for Czech.
    /// <para>
    /// Implements the algorithm described in:  
    /// <c>
    /// Indexing and stemming approaches for the Czech language
    /// </c>
    /// http://portal.acm.org/citation.cfm?id=1598600
    /// </para>
    /// </summary>
    public class CzechStemmer
    {
        /// <summary>
        /// Stem an input buffer of Czech text.
        /// <para><b>NOTE</b>: Input is expected to be in lowercase, 
        /// but with diacritical marks</para>
        /// </summary>
        /// <param name="s"> input buffer </param>
        /// <param name="len"> length of input buffer </param>
        /// <returns> length of input buffer after normalization</returns>
        public virtual int Stem(char[] s, int len)
        {
            len = RemoveCase(s, len);
            len = RemovePossessives(s, len);
            if (len > 0)
            {
                len = Normalize(s, len);
            }
            return len;
        }

        private int RemoveCase(char[] s, int len)
        {
            if (len > 7 && StemmerUtil.EndsWith(s, len, "atech"))
            {
                return len - 5;
            }

            if (len > 6 && (StemmerUtil.EndsWith(s, len, "ětem") || StemmerUtil.EndsWith(s, len, "etem") || StemmerUtil.EndsWith(s, len, "atům")))
            {
                return len - 4;
            }

            if (len > 5 && (StemmerUtil.EndsWith(s, len, "ech") || StemmerUtil.EndsWith(s, len, "ich") || StemmerUtil.EndsWith(s, len, "ích") || StemmerUtil.EndsWith(s, len, "ého") || StemmerUtil.EndsWith(s, len, "ěmi") || StemmerUtil.EndsWith(s, len, "emi") || StemmerUtil.EndsWith(s, len, "ému") || StemmerUtil.EndsWith(s, len, "ěte") || StemmerUtil.EndsWith(s, len, "ete") || StemmerUtil.EndsWith(s, len, "ěti") || StemmerUtil.EndsWith(s, len, "eti") || StemmerUtil.EndsWith(s, len, "ího") || StemmerUtil.EndsWith(s, len, "iho") || StemmerUtil.EndsWith(s, len, "ími") || StemmerUtil.EndsWith(s, len, "ímu") || StemmerUtil.EndsWith(s, len, "imu") || StemmerUtil.EndsWith(s, len, "ách") || StemmerUtil.EndsWith(s, len, "ata") || StemmerUtil.EndsWith(s, len, "aty") || StemmerUtil.EndsWith(s, len, "ých") || StemmerUtil.EndsWith(s, len, "ama") || StemmerUtil.EndsWith(s, len, "ami") || StemmerUtil.EndsWith(s, len, "ové") || StemmerUtil.EndsWith(s, len, "ovi") || StemmerUtil.EndsWith(s, len, "ými")))
            {
                return len - 3;
            }

            if (len > 4 && (StemmerUtil.EndsWith(s, len, "em") || StemmerUtil.EndsWith(s, len, "es") || StemmerUtil.EndsWith(s, len, "ém") || StemmerUtil.EndsWith(s, len, "ím") || StemmerUtil.EndsWith(s, len, "ům") || StemmerUtil.EndsWith(s, len, "at") || StemmerUtil.EndsWith(s, len, "ám") || StemmerUtil.EndsWith(s, len, "os") || StemmerUtil.EndsWith(s, len, "us") || StemmerUtil.EndsWith(s, len, "ým") || StemmerUtil.EndsWith(s, len, "mi") || StemmerUtil.EndsWith(s, len, "ou")))
            {
                return len - 2;
            }

            if (len > 3)
            {
                switch (s[len - 1])
                {
                    case 'a':
                    case 'e':
                    case 'i':
                    case 'o':
                    case 'u':
                    case 'ů':
                    case 'y':
                    case 'á':
                    case 'é':
                    case 'í':
                    case 'ý':
                    case 'ě':
                        return len - 1;
                }
            }

            return len;
        }

        private int RemovePossessives(char[] s, int len)
        {
            if (len > 5 && (StemmerUtil.EndsWith(s, len, "ov") || StemmerUtil.EndsWith(s, len, "in") || StemmerUtil.EndsWith(s, len, "ův")))
            {
                return len - 2;
            }

            return len;
        }

        private int Normalize(char[] s, int len)
        {
            if (StemmerUtil.EndsWith(s, len, "čt")) // čt -> ck
            {
                s[len - 2] = 'c';
                s[len - 1] = 'k';
                return len;
            }

            if (StemmerUtil.EndsWith(s, len, "št")) // št -> sk
            {
                s[len - 2] = 's';
                s[len - 1] = 'k';
                return len;
            }

            switch (s[len - 1])
            {
                case 'c': // [cč] -> k
                case 'č':
                    s[len - 1] = 'k';
                    return len;
                case 'z': // [zž] -> h
                case 'ž':
                    s[len - 1] = 'h';
                    return len;
            }

            if (len > 1 && s[len - 2] == 'e')
            {
                s[len - 2] = s[len - 1]; // e* > *
                return len - 1;
            }

            if (len > 2 && s[len - 2] == 'ů')
            {
                s[len - 2] = 'o'; // *ů* -> *o*
                return len;
            }

            return len;
        }
    }
}