Sebastian Lutze created LUCENE-4133:
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             Summary: FastVectorHighlighter: A weighted approach for ordered 
fragments
                 Key: LUCENE-4133
                 URL: https://issues.apache.org/jira/browse/LUCENE-4133
             Project: Lucene - Java
          Issue Type: Improvement
          Components: modules/highlighter
    Affects Versions: 4.0, 5.0
            Reporter: Sebastian Lutze
            Priority: Minor
             Fix For: 4.0
         Attachments: LUCENE-4133.patch

The FastVectorHighlighter currently disregards IDF-weights for matching terms 
within generated fragments. In the worst case, a fragment, which contains high 
number of very common words, is scored higher, than a fragment that contains 
*all* of the terms which have been used in the original query.

This patch provides ordered fragments with IDF-weighted terms:

*For each distinct matching term per fragment:* 
_weight = weight + IDF * boost_

*For each fragment:* 
_weight = weight * numTerms * 1 / sqrt( numTerms )_

|weight| total weight of fragment 
|IDF| inverse document frequency for each distinct matching term
|boost| query boost as provided, for example _term^2_
|numTerms| total number of matching terms per fragment 


*Method:*

{code:java}
  public void add( int startOffset, int endOffset, List<WeightedPhraseInfo> 
phraseInfoList ) {
    
    float totalBoost = 0;
    
    List<SubInfo> subInfos = new ArrayList<SubInfo>();
    HashSet<String> distinctTerms = new HashSet<String>();
    
    int length = 0;

    for( WeightedPhraseInfo phraseInfo : phraseInfoList ){
      subInfos.add( new SubInfo( phraseInfo.getText(), 
phraseInfo.getTermsOffsets(), phraseInfo.getSeqnum() ) );
      for ( TermInfo ti :  phraseInfo.getTermsInfos()) {
        if ( distinctTerms.add( ti.getText() ) )
          totalBoost += ti.getWeight() * phraseInfo.getBoost();
        length++;
      }
    }
    totalBoost *= length * ( 1 / Math.sqrt( length ) );
    
    getFragInfos().add( new WeightedFragInfo( startOffset, endOffset, subInfos, 
totalBoost ) );
  }
{code}

The ranking-formula should be the same, or at least similar, to that one used 
in QueryTermScorer.

*This patch contains:*

* a changed class-member in FieldPhraseList (termInfos to termsInfos)
* a changed local variable in SimpleFieldFragList (score to totalBoost)
* adds a missing @override in SimpleFragListBuilder
* class WeightedFieldFragList, a implementation of FieldFragList
* class WeightedFragListBuilder, a implementation of BaseFragListBuilder
* class WeightedFragListBuilderTest, a simple test-case 
* updated docs for FVH 

Last part (see also LUCENE-4091, LUCENE-4107, LUCENE-4113) of LUCENE-3440. 


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