src/Lucene.Net.Analysis.Common/Analysis/Miscellaneous/ScandinavianFoldingFilter.cs

using Lucene.Net.Analysis.TokenAttributes;
using Lucene.Net.Analysis.Util;

namespace Lucene.Net.Analysis.Miscellaneous
{
    /*
     * 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>
    /// This filter folds Scandinavian characters åÅäæÄÆ->a and öÖøØ->o.
    /// It also discriminate against use of double vowels aa, ae, ao, oe and oo, leaving just the first one.
    /// <para/>
    /// It's is a semantically more destructive solution than <see cref="ScandinavianNormalizationFilter"/> but
    /// can in addition help with matching raksmorgas as räksmörgås.
    /// <para/>
    /// blåbærsyltetøj == blåbärsyltetöj == blaabaarsyltetoej == blabarsyltetoj
    /// räksmörgås == ræksmørgås == ræksmörgaos == raeksmoergaas == raksmorgas
    /// <para/>
    /// Background:
    /// Swedish åäö are in fact the same letters as Norwegian and Danish åæø and thus interchangeable
    /// when used between these languages. They are however folded differently when people type
    /// them on a keyboard lacking these characters.
    /// <para/>
    /// In that situation almost all Swedish people use a, a, o instead of å, ä, ö.
    /// <para/>
    /// Norwegians and Danes on the other hand usually type aa, ae and oe instead of å, æ and ø.
    /// Some do however use a, a, o, oo, ao and sometimes permutations of everything above.
    /// <para/>
    /// This filter solves that mismatch problem, but might also cause new.
    /// </summary>
    /// <seealso cref="ScandinavianNormalizationFilter"/>
    public sealed class ScandinavianFoldingFilter : TokenFilter
    {
        public ScandinavianFoldingFilter(TokenStream input)
            : base(input)
        {
            charTermAttribute = AddAttribute<ICharTermAttribute>();
        }

        private readonly ICharTermAttribute charTermAttribute;

        private const char AA = '\u00C5'; // Å
        private const char aa = '\u00E5'; // å
        private const char AE = '\u00C6'; // Æ
        private const char ae = '\u00E6'; // æ
        private const char AE_se = '\u00C4'; // Ä
        private const char ae_se = '\u00E4'; // ä
        private const char OE = '\u00D8'; // Ø
        private const char oe = '\u00F8'; // ø
        private const char OE_se = '\u00D6'; // Ö
        private const char oe_se = '\u00F6'; //ö


        public override bool IncrementToken()
        {
            if (!m_input.IncrementToken())
            {
                return false;
            }

            char[] buffer = charTermAttribute.Buffer;
            int length = charTermAttribute.Length;


            int i;
            for (i = 0; i < length; i++)
            {

                if (buffer[i] == aa || buffer[i] == ae_se || buffer[i] == ae)
                {

                    buffer[i] = 'a';

                }
                else if (buffer[i] == AA || buffer[i] == AE_se || buffer[i] == AE)
                {

                    buffer[i] = 'A';

                }
                else if (buffer[i] == oe || buffer[i] == oe_se)
                {

                    buffer[i] = 'o';

                }
                else if (buffer[i] == OE || buffer[i] == OE_se)
                {

                    buffer[i] = 'O';

                }
                else if (length - 1 > i)
                {

                    if ((buffer[i] == 'a' || buffer[i] == 'A') && (buffer[i + 1] == 'a' || buffer[i + 1] == 'A' || buffer[i + 1] == 'e' || buffer[i + 1] == 'E' || buffer[i + 1] == 'o' || buffer[i + 1] == 'O'))
                    {

                        length = StemmerUtil.Delete(buffer, i + 1, length);

                    }
                    else if ((buffer[i] == 'o' || buffer[i] == 'O') && (buffer[i + 1] == 'e' || buffer[i + 1] == 'E' || buffer[i + 1] == 'o' || buffer[i + 1] == 'O'))
                    {

                        length = StemmerUtil.Delete(buffer, i + 1, length);

                    }
                }
            }

            charTermAttribute.Length = length;

            return true;
        }
    }
}