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StempelFilter.cs
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StempelFilter.cs
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using Lucene.Net.Analysis.TokenAttributes;
using System.Text;
namespace Lucene.Net.Analysis.Stempel
{
/*
* 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 stemming algorithm.
/// <para>
/// Note: the input to the stemming filter must already be in lower case, so you
/// will need to use <see cref="Analysis.Core.LowerCaseFilter"/> or <see cref="Analysis.Core.LowerCaseTokenizer"/> farther down the
/// <see cref="Tokenizer"/> chain in order for this to work properly!
/// </para>
/// </summary>
public sealed class StempelFilter : TokenFilter
{
private readonly ICharTermAttribute termAtt;
private readonly IKeywordAttribute keywordAtt;
private readonly StempelStemmer stemmer;
private readonly int minLength;
/// <summary>
/// Minimum length of input words to be processed. Shorter words are returned
/// unchanged.
/// </summary>
public static readonly int DEFAULT_MIN_LENGTH = 3;
/// <summary>
/// Create filter using the supplied stemming table.
/// </summary>
/// <param name="in">input token stream</param>
/// <param name="stemmer">stemmer</param>
public StempelFilter(TokenStream @in, StempelStemmer stemmer)
: this(@in, stemmer, DEFAULT_MIN_LENGTH)
{
}
/// <summary>
/// Create filter using the supplied stemming table.
/// </summary>
/// <param name="in">input token stream</param>
/// <param name="stemmer">stemmer</param>
/// <param name="minLength">For performance reasons words shorter than minLength
/// characters are not processed, but simply returned.</param>
public StempelFilter(TokenStream @in, StempelStemmer stemmer, int minLength)
: base(@in)
{
this.stemmer = stemmer;
this.minLength = minLength;
this.termAtt = AddAttribute<ICharTermAttribute>();
this.keywordAtt = AddAttribute<IKeywordAttribute>();
}
/// <summary>
/// Returns the next input <see cref="Token"/>, after being stemmed
/// </summary>
public override bool IncrementToken()
{
if (m_input.IncrementToken())
{
if (!keywordAtt.IsKeyword && termAtt.Length > minLength)
{
StringBuilder sb = stemmer.Stem(termAtt.ToString());
if (sb != null) // if we can't stem it, return unchanged
termAtt.SetEmpty().Append(sb);
}
return true;
}
else
{
return false;
}
}
}
}