http://lucene.apache.org/lucene-parent/lucene-highlighter: This is the highlighter for apache lucene java (The Apache Software Foundation) 
/*
 * 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.
 */
package org.apache.lucene.search.highlight;

import java.io.IOException;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.Objects;

import org.apache.lucene.analysis.Analyzer;
import org.apache.lucene.analysis.TokenStream;
import org.apache.lucene.analysis.tokenattributes.CharTermAttribute;
import org.apache.lucene.analysis.tokenattributes.OffsetAttribute;
import org.apache.lucene.util.PriorityQueue;


Marks up highlighted terms found in the best sections of text, using configurable Fragmenter, Scorer, Formatter, Encoder and tokenizers. This is Lucene's original Highlighter; there are others. 
/**
 * Marks up highlighted terms found in the best sections of
 * text, using configurable {@link Fragmenter}, {@link Scorer}, {@link Formatter},
 * {@link Encoder} and tokenizers.
 *
 * This is Lucene's original Highlighter; there are others.
 */
public class Highlighter
{
  public static final int DEFAULT_MAX_CHARS_TO_ANALYZE = 50*1024;

  private Formatter formatter;
  private Encoder encoder;
  private Scorer fragmentScorer;
  private int maxDocCharsToAnalyze = DEFAULT_MAX_CHARS_TO_ANALYZE;
  private Fragmenter textFragmenter = new SimpleFragmenter();

  public Highlighter(Scorer fragmentScorer)
  {
    this(new SimpleHTMLFormatter(),fragmentScorer);
  }

  public Highlighter(Formatter formatter, Scorer fragmentScorer)
  {
    this(formatter,new DefaultEncoder(),fragmentScorer);
  }

  public Highlighter(Formatter formatter, Encoder encoder, Scorer fragmentScorer)
  {
    ensureArgumentNotNull(formatter, "'formatter' must not be null");
    ensureArgumentNotNull(encoder, "'encoder' must not be null");
    ensureArgumentNotNull(fragmentScorer, "'fragmentScorer' must not be null");

    this.formatter = formatter;
    this.encoder = encoder;
    this.fragmentScorer = fragmentScorer;
  }

  
Highlights chosen terms in a text, extracting the most relevant section. This is a convenience method that calls getBestFragment(TokenStream, String) 
Params:
  • analyzer –   the analyzer that will be used to split text
into chunks
  • text – text to highlight terms in
  • fieldName – Name of field used to influence analyzer's tokenization policy
Throws:
Returns:highlighted text fragment or null if no terms found
/**
   * Highlights chosen terms in a text, extracting the most relevant section.
   * This is a convenience method that calls
   * {@link #getBestFragment(TokenStream, String)}
   *
   * @param analyzer   the analyzer that will be used to split <code>text</code>
   * into chunks
   * @param text text to highlight terms in
   * @param fieldName Name of field used to influence analyzer's tokenization policy
   *
   * @return highlighted text fragment or null if no terms found
   * @throws InvalidTokenOffsetsException thrown if any token's endOffset exceeds the provided text's length
   */
  public final String getBestFragment(Analyzer analyzer, String fieldName,String text)
    throws IOException, InvalidTokenOffsetsException
  {
    TokenStream tokenStream = analyzer.tokenStream(fieldName, text);
    return getBestFragment(tokenStream, text);
  }

  
Highlights chosen terms in a text, extracting the most relevant section.
The document text is analysed in chunks to record hit statistics
across the document. After accumulating stats, the fragment with the highest score
is returned

Params:
  • tokenStream –   a stream of tokens identified in the text parameter, including offset information.
This is typically produced by an analyzer re-parsing a document's
text. Some work may be done on retrieving TokenStreams more efficiently
by adding support for storing original text position data in the Lucene
index but this support is not currently available (as of Lucene 1.4 rc2).
  • text – text to highlight terms in
Throws:
Returns:highlighted text fragment or null if no terms found
/**
   * Highlights chosen terms in a text, extracting the most relevant section.
   * The document text is analysed in chunks to record hit statistics
   * across the document. After accumulating stats, the fragment with the highest score
   * is returned
   *
   * @param tokenStream   a stream of tokens identified in the text parameter, including offset information.
   * This is typically produced by an analyzer re-parsing a document's
   * text. Some work may be done on retrieving TokenStreams more efficiently
   * by adding support for storing original text position data in the Lucene
   * index but this support is not currently available (as of Lucene 1.4 rc2).
   * @param text text to highlight terms in
   *
   * @return highlighted text fragment or null if no terms found
   * @throws InvalidTokenOffsetsException thrown if any token's endOffset exceeds the provided text's length
   */
  public final String getBestFragment(TokenStream tokenStream, String text)
    throws IOException, InvalidTokenOffsetsException
  {
    String[] results = getBestFragments(tokenStream,text, 1);
    if (results.length > 0)
    {
      return results[0];
    }
    return null;
  }

  
Highlights chosen terms in a text, extracting the most relevant sections. This is a convenience method that calls getBestFragments(TokenStream, String, int) 
Params:
  • analyzer –   the analyzer that will be used to split text
into chunks
  • fieldName –     the name of the field being highlighted (used by analyzer)
  • text –          text to highlight terms in
  • maxNumFragments –  the maximum number of fragments.
Throws:
Returns:highlighted text fragments (between 0 and maxNumFragments number of fragments)
/**
   * Highlights chosen terms in a text, extracting the most relevant sections.
   * This is a convenience method that calls
   * {@link #getBestFragments(TokenStream, String, int)}
   *
   * @param analyzer   the analyzer that will be used to split <code>text</code>
   * into chunks
   * @param fieldName     the name of the field being highlighted (used by analyzer)
   * @param text          text to highlight terms in
   * @param maxNumFragments  the maximum number of fragments.
   *
   * @return highlighted text fragments (between 0 and maxNumFragments number of fragments)
   * @throws InvalidTokenOffsetsException thrown if any token's endOffset exceeds the provided text's length
   */
  public final String[] getBestFragments(
    Analyzer analyzer,
    String fieldName,
    String text,
    int maxNumFragments)
    throws IOException, InvalidTokenOffsetsException
  {
    TokenStream tokenStream = analyzer.tokenStream(fieldName, text);
    return getBestFragments(tokenStream, text, maxNumFragments);
  }

  
Highlights chosen terms in a text, extracting the most relevant sections.
The document text is analysed in chunks to record hit statistics
across the document. After accumulating stats, the fragments with the highest scores
are returned as an array of strings in order of score (contiguous fragments are merged into
one in their original order to improve readability)

Params:
  • text –          text to highlight terms in
  • maxNumFragments –  the maximum number of fragments.
Throws:
Returns:highlighted text fragments (between 0 and maxNumFragments number of fragments)
/**
   * Highlights chosen terms in a text, extracting the most relevant sections.
   * The document text is analysed in chunks to record hit statistics
   * across the document. After accumulating stats, the fragments with the highest scores
   * are returned as an array of strings in order of score (contiguous fragments are merged into
   * one in their original order to improve readability)
   *
   * @param text          text to highlight terms in
   * @param maxNumFragments  the maximum number of fragments.
   *
   * @return highlighted text fragments (between 0 and maxNumFragments number of fragments)
   * @throws InvalidTokenOffsetsException thrown if any token's endOffset exceeds the provided text's length
   */
  public final String[] getBestFragments(
    TokenStream tokenStream,
    String text,
    int maxNumFragments)
    throws IOException, InvalidTokenOffsetsException
  {
    maxNumFragments = Math.max(1, maxNumFragments); //sanity check

    TextFragment[] frag =getBestTextFragments(tokenStream,text, true,maxNumFragments);

    //Get text
    ArrayList<String> fragTexts = new ArrayList<>();
    for (int i = 0; i < frag.length; i++)
    {
      if ((frag[i] != null) && (frag[i].getScore() > 0))
      {
        fragTexts.add(frag[i].toString());
      }
    }
    return fragTexts.toArray(new String[0]);
  }


  
Low level api to get the most relevant (formatted) sections of the document.
This method has been made public to allow visibility of score information held in TextFragment objects.
Thanks to Jason Calabrese for help in redefining the interface.

Throws:
  • IOException – If there is a low-level I/O error
  • InvalidTokenOffsetsException – thrown if any token's endOffset exceeds the provided text's length
/**
   * Low level api to get the most relevant (formatted) sections of the document.
   * This method has been made public to allow visibility of score information held in TextFragment objects.
   * Thanks to Jason Calabrese for help in redefining the interface.
   * @throws IOException If there is a low-level I/O error
   * @throws InvalidTokenOffsetsException thrown if any token's endOffset exceeds the provided text's length
   */
  public final TextFragment[] getBestTextFragments(
    TokenStream tokenStream,
    String text,
    boolean mergeContiguousFragments,
    int maxNumFragments)
    throws IOException, InvalidTokenOffsetsException
  {
    ArrayList<TextFragment> docFrags = new ArrayList<>();
    StringBuilder newText=new StringBuilder();

    CharTermAttribute termAtt = tokenStream.addAttribute(CharTermAttribute.class);
    OffsetAttribute offsetAtt = tokenStream.addAttribute(OffsetAttribute.class);
    TextFragment currentFrag =  new TextFragment(newText,newText.length(), docFrags.size());

    if (fragmentScorer instanceof QueryScorer) {
      ((QueryScorer) fragmentScorer).setMaxDocCharsToAnalyze(maxDocCharsToAnalyze);
    }

    TokenStream newStream = fragmentScorer.init(tokenStream);
    if(newStream != null) {
      tokenStream = newStream;
    }
    fragmentScorer.startFragment(currentFrag);
    docFrags.add(currentFrag);

    FragmentQueue fragQueue = new FragmentQueue(maxNumFragments);

    try
    {

      String tokenText;
      int startOffset;
      int endOffset;
      int lastEndOffset = 0;
      textFragmenter.start(text, tokenStream);

      TokenGroup tokenGroup=new TokenGroup(tokenStream);

      tokenStream.reset();
      for (boolean next = tokenStream.incrementToken(); next && (offsetAtt.startOffset()< maxDocCharsToAnalyze);
            next = tokenStream.incrementToken())
      {
        if(  (offsetAtt.endOffset()>text.length())
          ||
          (offsetAtt.startOffset()>text.length())
          )
        {
          throw new InvalidTokenOffsetsException("Token "+ termAtt.toString()
              +" exceeds length of provided text sized "+text.length());
        }
        if((tokenGroup.getNumTokens() >0)&&(tokenGroup.isDistinct()))
        {
          //the current token is distinct from previous tokens -
          // markup the cached token group info
          startOffset = tokenGroup.getStartOffset();
          endOffset = tokenGroup.getEndOffset();
          tokenText = text.substring(startOffset, endOffset);
          String markedUpText=formatter.highlightTerm(encoder.encodeText(tokenText), tokenGroup);
          //store any whitespace etc from between this and last group
          if (startOffset > lastEndOffset)
            newText.append(encoder.encodeText(text.substring(lastEndOffset, startOffset)));
          newText.append(markedUpText);
          lastEndOffset=Math.max(endOffset, lastEndOffset);
          tokenGroup.clear();

          //check if current token marks the start of a new fragment
          if(textFragmenter.isNewFragment())
          {
            currentFrag.setScore(fragmentScorer.getFragmentScore());
            //record stats for a new fragment
            currentFrag.textEndPos = newText.length();
            currentFrag =new TextFragment(newText, newText.length(), docFrags.size());
            fragmentScorer.startFragment(currentFrag);
            docFrags.add(currentFrag);
          }
        }

        tokenGroup.addToken(fragmentScorer.getTokenScore());

//        if(lastEndOffset>maxDocBytesToAnalyze)
//        {
//          break;
//        }
      }
      currentFrag.setScore(fragmentScorer.getFragmentScore());

      if(tokenGroup.getNumTokens() >0)
      {
        //flush the accumulated text (same code as in above loop)
        startOffset = tokenGroup.getStartOffset();
        endOffset = tokenGroup.getEndOffset();
        tokenText = text.substring(startOffset, endOffset);
        String markedUpText=formatter.highlightTerm(encoder.encodeText(tokenText), tokenGroup);
        //store any whitespace etc from between this and last group
        if (startOffset > lastEndOffset)
          newText.append(encoder.encodeText(text.substring(lastEndOffset, startOffset)));
        newText.append(markedUpText);
        lastEndOffset=Math.max(lastEndOffset,endOffset);
      }

      //Test what remains of the original text beyond the point where we stopped analyzing
      if (
//          if there is text beyond the last token considered..
          (lastEndOffset < text.length())
          &&
//          and that text is not too large...
          (text.length()<= maxDocCharsToAnalyze)
        )
      {
        //append it to the last fragment
        newText.append(encoder.encodeText(text.substring(lastEndOffset)));
      }

      currentFrag.textEndPos = newText.length();

      //sort the most relevant sections of the text
      for (Iterator<TextFragment> i = docFrags.iterator(); i.hasNext();)
      {
        currentFrag = i.next();

        //If you are running with a version of Lucene before 11th Sept 03
        // you do not have PriorityQueue.insert() - so uncomment the code below
        /*
                  if (currentFrag.getScore() >= minScore)
                  {
                    fragQueue.put(currentFrag);
                    if (fragQueue.size() > maxNumFragments)
                    { // if hit queue overfull
                      fragQueue.pop(); // remove lowest in hit queue
                      minScore = ((TextFragment) fragQueue.top()).getScore(); // reset minScore
                    }


                  }
        */
        //The above code caused a problem as a result of Christoph Goller's 11th Sept 03
        //fix to PriorityQueue. The correct method to use here is the new "insert" method
        // USE ABOVE CODE IF THIS DOES NOT COMPILE!
        fragQueue.insertWithOverflow(currentFrag);
      }

      //return the most relevant fragments
      TextFragment frag[] = new TextFragment[fragQueue.size()];
      for (int i = frag.length - 1; i >= 0; i--)
      {
        frag[i] = fragQueue.pop();
      }

      //merge any contiguous fragments to improve readability
      if(mergeContiguousFragments)
      {
        mergeContiguousFragments(frag);
        ArrayList<TextFragment> fragTexts = new ArrayList<>();
        for (int i = 0; i < frag.length; i++)
        {
          if ((frag[i] != null) && (frag[i].getScore() > 0))
          {
            fragTexts.add(frag[i]);
          }
        }
        frag= fragTexts.toArray(new TextFragment[0]);
      }

      return frag;

    }
    finally
    {
      if (tokenStream != null)
      {
        try
        {
          tokenStream.end();
          tokenStream.close();
        }
        catch (Exception e)
        {
        }
      }
    }
  }


  
Improves readability of a score-sorted list of TextFragments by merging any fragments
that were contiguous in the original text into one larger fragment with the correct order.
This will leave a "null" in the array entry for the lesser scored fragment.

Params:
  • frag – An array of document fragments in descending score
/** Improves readability of a score-sorted list of TextFragments by merging any fragments
   * that were contiguous in the original text into one larger fragment with the correct order.
   * This will leave a "null" in the array entry for the lesser scored fragment.
   *
   * @param frag An array of document fragments in descending score
   */
  private void mergeContiguousFragments(TextFragment[] frag)
  {
    boolean mergingStillBeingDone;
    if (frag.length > 1)
      do
      {
        mergingStillBeingDone = false; //initialise loop control flag
        //for each fragment, scan other frags looking for contiguous blocks
        for (int i = 0; i < frag.length; i++)
        {
          if (frag[i] == null)
          {
            continue;
          }
          //merge any contiguous blocks
          for (int x = 0; x < frag.length; x++)
          {
            if (frag[x] == null)
            {
              continue;
            }
            if (frag[i] == null)
            {
              break;
            }
            TextFragment frag1 = null;
            TextFragment frag2 = null;
            int frag1Num = 0;
            int frag2Num = 0;
            int bestScoringFragNum;
            int worstScoringFragNum;
            //if blocks are contiguous....
            if (frag[i].follows(frag[x]))
            {
              frag1 = frag[x];
              frag1Num = x;
              frag2 = frag[i];
              frag2Num = i;
            }
            else
              if (frag[x].follows(frag[i]))
              {
                frag1 = frag[i];
                frag1Num = i;
                frag2 = frag[x];
                frag2Num = x;
              }
            //merging required..
            if (frag1 != null)
            {
              if (frag1.getScore() > frag2.getScore())
              {
                bestScoringFragNum = frag1Num;
                worstScoringFragNum = frag2Num;
              }
              else
              {
                bestScoringFragNum = frag2Num;
                worstScoringFragNum = frag1Num;
              }
              frag1.merge(frag2);
              frag[worstScoringFragNum] = null;
              mergingStillBeingDone = true;
              frag[bestScoringFragNum] = frag1;
            }
          }
        }
      }
      while (mergingStillBeingDone);
  }


  
Highlights terms in the  text , extracting the most relevant sections
and concatenating the chosen fragments with a separator (typically "...").
The document text is analysed in chunks to record hit statistics
across the document. After accumulating stats, the fragments with the highest scores
are returned in order as "separator" delimited strings.

Params:
  • text –        text to highlight terms in
  • maxNumFragments –  the maximum number of fragments.
  • separator –  the separator used to intersperse the document fragments (typically "...")
Throws:
Returns:highlighted text
/**
   * Highlights terms in the  text , extracting the most relevant sections
   * and concatenating the chosen fragments with a separator (typically "...").
   * The document text is analysed in chunks to record hit statistics
   * across the document. After accumulating stats, the fragments with the highest scores
   * are returned in order as "separator" delimited strings.
   *
   * @param text        text to highlight terms in
   * @param maxNumFragments  the maximum number of fragments.
   * @param separator  the separator used to intersperse the document fragments (typically "...")
   *
   * @return highlighted text
   * @throws InvalidTokenOffsetsException thrown if any token's endOffset exceeds the provided text's length
   */
  public final String getBestFragments(
    TokenStream tokenStream,
    String text,
    int maxNumFragments,
    String separator)
    throws IOException, InvalidTokenOffsetsException
  {
    String sections[] =  getBestFragments(tokenStream,text, maxNumFragments);
    StringBuilder result = new StringBuilder();
    for (int i = 0; i < sections.length; i++)
    {
      if (i > 0)
      {
        result.append(separator);
      }
      result.append(sections[i]);
    }
    return result.toString();
  }

  public int getMaxDocCharsToAnalyze() {
    return maxDocCharsToAnalyze;
  }

  public void setMaxDocCharsToAnalyze(int maxDocCharsToAnalyze) {
    this.maxDocCharsToAnalyze = maxDocCharsToAnalyze;
  }

  public Fragmenter getTextFragmenter()
  {
    return textFragmenter;
  }

  public void setTextFragmenter(Fragmenter fragmenter)
  {
    textFragmenter = Objects.requireNonNull(fragmenter);
  }

  
Returns:Object used to score each text fragment
/**
   * @return Object used to score each text fragment
   */
  public Scorer getFragmentScorer()
  {
    return fragmentScorer;
  }

  public void setFragmentScorer(Scorer scorer)
  {
    fragmentScorer = Objects.requireNonNull(scorer);
  }

  public Encoder getEncoder() {
    return encoder;
  }

  public void setEncoder(Encoder encoder) {
    this.encoder = Objects.requireNonNull(encoder);
  }

  
Throws an IllegalArgumentException with the provided message if 'argument' is null.

Params:
  • argument – the argument to be null-checked
  • message –  the message of the exception thrown if argument == null
/**
   * Throws an IllegalArgumentException with the provided message if 'argument' is null.
   *
   * @param argument the argument to be null-checked
   * @param message  the message of the exception thrown if argument == null
   */
  private static void ensureArgumentNotNull(Object argument, String message) {
    if (argument == null) {
      throw new IllegalArgumentException(message);
    }
  }

  static class FragmentQueue extends PriorityQueue<TextFragment>
  {
    FragmentQueue(int size)
    {
      super(size);
    }

    @Override
    public final boolean lessThan(TextFragment fragA, TextFragment fragB)
    {
      if (fragA.getScore() == fragB.getScore())
        return fragA.fragNum > fragB.fragNum;
      else
        return fragA.getScore() < fragB.getScore();
    }
  }
}