another highlighter

I've written this highlighter for my project to support bi-gram token stream (general token stream (e.g. WhitespaceTokenizer) also supported. see test code in patch). The idea was inherited from my previous project with my colleague and LUCENE-644. This approach needs highlight fields to be TermVector.WITH_POSITIONS_OFFSETS, but is fast and can support N-grams. This depends on LUCENE-1448 to get refined term offsets.

usage:

TopDocs docs = searcher.search( query, 10 );
Highlighter h = new Highlighter();
FieldQuery fq = h.getFieldQuery( query );
for( ScoreDoc scoreDoc : docs.scoreDocs ){
  // fieldName="content", fragCharSize=100, numFragments=3
  String[] fragments = h.getBestFragments( fq, reader, scoreDoc.doc, "content", 100, 3 );
  if( fragments != null ){
    for( String fragment : fragments )
      System.out.println( fragment );
  }
}

features:

• fast for large docs
• supports not only whitespace-based token stream, but also "fixed size" N-gram (e.g. (2,2), not (1,3)) (can solve LUCENE-1489)
• supports PhraseQuery, phrase-unit highlighting with slops

  q="w1 w2"
  <b>w1 w2</b>
  ---------------
  q="w1 w2"~1
  <b>w1</b> w3 <b>w2</b> w3 <b>w1 w2</b>
  

• highlight fields need to be TermVector.WITH_POSITIONS_OFFSETS
• easy to apply patch due to independent package (contrib/highlighter2)
• uses Java 1.5
• looks query boost to score fragments (currently doesn't see idf, but it should be possible)
• pluggable FragListBuilder
• pluggable FragmentsBuilder

to do:

• term positions can be unnecessary when phraseHighlight==false
• collects performance numbers