src/Lucene.Net.Analysis.Common/Analysis/En/PorterStemFilter.cs

using Lucene.Net.Analysis.TokenAttributes;

namespace Lucene.Net.Analysis.En
{
    /*
     * 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>
    /// Transforms the token stream as per the Porter stemming algorithm.
    /// <para/>
    ///    Note: the input to the stemming filter must already be in lower case,
    ///    so you will need to use LowerCaseFilter or LowerCaseTokenizer farther
    ///    down the Tokenizer chain in order for this to work properly!
    ///    <para/>
    ///    To use this filter with other analyzers, you'll want to write an
    ///    Analyzer class that sets up the TokenStream chain as you want it.
    ///    To use this with LowerCaseTokenizer, for example, you'd write an
    ///    analyzer like this:
    ///    <para/>
    ///    <code>
    ///    class MyAnalyzer : Analyzer {
    ///      protected override TokenStreamComponents CreateComponents(string fieldName, TextReader reader) {
    ///        Tokenizer source = new LowerCaseTokenizer(version, reader);
    ///        return new TokenStreamComponents(source, new PorterStemFilter(source));
    ///      }
    ///    }
    ///    </code>
    ///    <para>
    ///    Note: This filter is aware of the <see cref="KeywordAttribute"/>. To prevent
    ///    certain terms from being passed to the stemmer
    ///    <see cref="KeywordAttribute.IsKeyword"/> should be set to <code>true</code>
    ///    in a previous <see cref="TokenStream"/>.
    /// 
    ///    Note: For including the original term as well as the stemmed version, see
    ///   <see cref="Miscellaneous.KeywordRepeatFilterFactory"/>
    /// </para>
    /// </summary>
    public sealed class PorterStemFilter : TokenFilter
    {
        private readonly PorterStemmer stemmer = new PorterStemmer();
        private readonly ICharTermAttribute termAtt;
        private readonly IKeywordAttribute keywordAttr;

        public PorterStemFilter(TokenStream @in) : base(@in)
        {
            termAtt = AddAttribute<ICharTermAttribute>();
            keywordAttr = AddAttribute<IKeywordAttribute>();
        }

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

            if ((!keywordAttr.IsKeyword) && stemmer.Stem(termAtt.Buffer, 0, termAtt.Length))
            {
                termAtt.CopyBuffer(stemmer.ResultBuffer, 0, stemmer.ResultLength);
            }
            return true;
        }
    }
}