src/Lucene.Net.Core/Analysis/StopAnalyzer.cs

/* 
 * 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 System.Collections.Generic;
using Version = Lucene.Net.Util.Version;

namespace Lucene.Net.Analysis
{
    
    /// <summary> Filters <see cref="LetterTokenizer" /> with <see cref="LowerCaseFilter" /> and
    /// <see cref="StopFilter" />.
    /// 
    /// <a name="version"/>
    /// <p/>
    /// You must specify the required <see cref="Version" /> compatibility when creating
    /// StopAnalyzer:
    /// <list type="bullet">
    /// <item>As of 2.9, position increments are preserved</item>
    /// </list>
    /// </summary>
    
    public sealed class StopAnalyzer:Analyzer
    {
        private readonly ISet<string> stopWords;
        private readonly bool enablePositionIncrements;

        /// <summary>An unmodifiable set containing some common English words that are not usually useful
        /// for searching.
        /// </summary>
        public static ISet<string> ENGLISH_STOP_WORDS_SET;
        
        /// <summary> Builds an analyzer which removes words in ENGLISH_STOP_WORDS.</summary>
        public StopAnalyzer(Version matchVersion)
        {
            stopWords = ENGLISH_STOP_WORDS_SET;
            enablePositionIncrements = StopFilter.GetEnablePositionIncrementsVersionDefault(matchVersion);
        }

        /// <summary>Builds an analyzer with the stop words from the given set.</summary>
        public StopAnalyzer(Version matchVersion, ISet<string> stopWords)
        {
            this.stopWords = stopWords;
            enablePositionIncrements = StopFilter.GetEnablePositionIncrementsVersionDefault(matchVersion);
        }
        
        /// <summary> Builds an analyzer with the stop words from the given file.
        /// 
        /// </summary>
        /// <seealso cref="WordlistLoader.GetWordSet(System.IO.FileInfo)">
        /// </seealso>
        /// <param name="matchVersion">See <a href="#version">above</a>
        /// </param>
        /// <param name="stopwordsFile">File to load stop words from
        /// </param>
        public StopAnalyzer(Version matchVersion, System.IO.FileInfo stopwordsFile)
        {
            stopWords = WordlistLoader.GetWordSet(stopwordsFile);
            enablePositionIncrements = StopFilter.GetEnablePositionIncrementsVersionDefault(matchVersion);
        }

        /// <summary>Builds an analyzer with the stop words from the given reader. </summary>
        /// <seealso cref="WordlistLoader.GetWordSet(System.IO.TextReader)">
        /// </seealso>
        /// <param name="matchVersion">See <a href="#Version">above</a>
        /// </param>
        /// <param name="stopwords">Reader to load stop words from
        /// </param>
        public StopAnalyzer(Version matchVersion, System.IO.TextReader stopwords)
        {
            stopWords = WordlistLoader.GetWordSet(stopwords);
            enablePositionIncrements = StopFilter.GetEnablePositionIncrementsVersionDefault(matchVersion);
        }

        /// <summary>Filters LowerCaseTokenizer with StopFilter. </summary>
        public override TokenStream TokenStream(System.String fieldName, System.IO.TextReader reader)
        {
            return new StopFilter(enablePositionIncrements, new LowerCaseTokenizer(reader), stopWords);
        }
        
        /// <summary>Filters LowerCaseTokenizer with StopFilter. </summary>
        private class SavedStreams
        {
            public SavedStreams(StopAnalyzer enclosingInstance)
            {
                InitBlock(enclosingInstance);
            }
            private void  InitBlock(StopAnalyzer enclosingInstance)
            {
                this.enclosingInstance = enclosingInstance;
            }
            private StopAnalyzer enclosingInstance;
            public StopAnalyzer Enclosing_Instance
            {
                get
                {
                    return enclosingInstance;
                }
                
            }
            internal Tokenizer source;
            internal TokenStream result;
        }
        
        public override TokenStream ReusableTokenStream(System.String fieldName, System.IO.TextReader reader)
        {
            var streams = (SavedStreams) PreviousTokenStream;
            if (streams == null)
            {
                streams = new SavedStreams(this) {source = new LowerCaseTokenizer(reader)};
                streams.result = new StopFilter(enablePositionIncrements, streams.source, stopWords);
                PreviousTokenStream = streams;
            }
            else
                streams.source.Reset(reader);
            return streams.result;
        }
        static StopAnalyzer()
        {
            {
                var stopWords = new System.String[]{"a", "an", "and", "are", "as", "at", "be", "but", "by", "for", "if", "in", "into", "is", "it", "no", "not", "of", "on", "or", "such", "that", "the", "their", "then", "there", "these", "they", "this", "to", "was", "will", "with"};
                var stopSet = new CharArraySet(stopWords.Length, false);
                stopSet.AddAll(stopWords);
                ENGLISH_STOP_WORDS_SET = CharArraySet.UnmodifiableSet(stopSet);
            }
        }
    }
}