> Let's say I have a document that
> contains the following text:
>
> "Graph Algorithms is one of the most important topics in
> computer science"
>
> And a query "!Graph Algorithms" then the term Graph in the
> query should have
> a double weight because the offset of Graph is 0 (and it is
> even) - we apply
> this doubling of weight only if a '!' operator precedes the
> term and if its
> offset from the document is even.
I modified the TokenOffsetPayloadTokenFilter and created
TermPositionPayloadTokenFilter.
Index time you can use WhitespaceTokenizer + TermPositionPayloadTokenFilter to
assign payload values of 2.0f to the tokens that have an even term position.
Modifying the QueryParser to change the meaning of ! operator is very
troublesome.
If you can convert your query "!Graph Algorithms" to "Graph|2.0 Algorithms" you
can use DelimitedPayloadTokenFilter to set payload of marked term.
Additionally you need to everride QueryParser to return PayloadTermQuery
and scorePayload method of DefaultSimilarity.
By doing so payloads will be included in score calculation.
public class PayloadAnalyzer extends Analyzer {
public TokenStream tokenStream(String fieldName, Reader reader) {
TokenStream result = new WhitespaceTokenizer(reader);
result = new DelimitedPayloadTokenFilter(result, '|', new
FloatEncoder());
return result;
}
public static void main(String[] args) throws ParseException {
QueryParser qp = new QueryParser(Version.LUCENE_29, "f", new
PayloadAnalyzer());
System.out.println(qp.parse("Graph|2.0 Algorithms").toString());
}
}
public class TermPositionPayloadTokenFilter extends TokenFilter {
protected PayloadAttribute payAtt;
protected PositionIncrementAttribute posIncrAtt;
private static final Payload evenPayload = new
Payload(PayloadHelper.encodeFloat(2.0f));
private int termPosition = 0;
public TermPositionPayloadTokenFilter(TokenStream input) {
super(input);
payAtt = (PayloadAttribute) addAttribute(PayloadAttribute.class);
posIncrAtt = (PositionIncrementAttribute)
addAttribute(PositionIncrementAttribute.class);
}
public final boolean incrementToken() throws IOException {
if (input.incrementToken()) {
if ((termPosition % 2) == 0)
payAtt.setPayload(evenPayload);
termPosition += posIncrAtt.getPositionIncrement();
return true;
} else {
return false;
}
}
public static void main(String[] args) throws IOException {
String test = "Graph Algorithms is one of the most important topics in
computer science";
TokenStream tokenStream = new TermPositionPayloadTokenFilter(new
WhitespaceTokenizer(new StringReader(test)));
TermAttribute termAtt = (TermAttribute)
tokenStream.getAttribute(TermAttribute.class);
PayloadAttribute payloadAtt = (PayloadAttribute)
tokenStream.getAttribute(PayloadAttribute.class);
while (tokenStream.incrementToken()) {
System.out.print(termAtt.term());
Payload payload = payloadAtt.getPayload();
if (payload != null)
System.out.println(" Payload = " +
PayloadHelper.decodeFloat(payload.toByteArray()));
else
System.out.println(" Payload is null.");
}
}
}
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