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From Eva Popenda <Eva.Pope...@abas.de>
Subject Problem with NGramAnalyzer, PhraseQuery and Highlighter
Date Mon, 18 Apr 2016 15:27:33 GMT
Hi,

I have a problem when using the Highlighter with N-GramAnalyzer and PhraseQuery:
Searching for a substring with length = N (4 in my case) yields the following exception:

java.lang.IllegalArgumentException: Less than 2 subSpans.size():1
at org.apache.lucene.search.spans.ConjunctionSpans.<init>(ConjunctionSpans.java:40)
at org.apache.lucene.search.spans.NearSpansOrdered.<init>(NearSpansOrdered.java:56)
at org.apache.lucene.search.spans.SpanNearQuery$SpanNearWeight.getSpans(SpanNearQuery.java:232)
at org.apache.lucene.search.highlight.WeightedSpanTermExtractor.extractWeightedSpanTerms(WeightedSpanTermExtractor.java:292)
at org.apache.lucene.search.highlight.WeightedSpanTermExtractor.extract(WeightedSpanTermExtractor.java:137)
at org.apache.lucene.search.highlight.WeightedSpanTermExtractor.getWeightedSpanTerms(WeightedSpanTermExtractor.java:506)
at org.apache.lucene.search.highlight.QueryScorer.initExtractor(QueryScorer.java:219)
at org.apache.lucene.search.highlight.QueryScorer.init(QueryScorer.java:187)
at org.apache.lucene.search.highlight.Highlighter.getBestTextFragments(Highlighter.java:196)

Below is a JUnit-Test reproducing this behavior. In case of searching for a string with more
than N characters or using NGramPhraseQuery this problem doesn't occur.
Why is it that more than 1 subSpans are required?

public class HighlighterTest {

    @Rule
    public final ExpectedException exception = ExpectedException.none();

    @Test
    public void testHighlighterWithPhraseQueryThrowsException() throws IOException, InvalidTokenOffsetsException
{

        final Analyzer analyzer = new NGramAnalyzer(4);
        final String fieldName = "substring";

        final List<BytesRef> list = new ArrayList<>();
        list.add(new BytesRef("uchu"));
        final PhraseQuery query = new PhraseQuery(fieldName, list.toArray(new BytesRef[list.size()]));

        final QueryScorer fragmentScorer = new QueryScorer(query, fieldName);
        final SimpleHTMLFormatter formatter = new SimpleHTMLFormatter("<b>", "</b>");

        exception.expect(IllegalArgumentException.class);
        exception.expectMessage("Less than 2 subSpans.size():1");

        final Highlighter highlighter = new Highlighter(formatter,TextEncoder.NONE.getEncoder(),
fragmentScorer);
        highlighter.setTextFragmenter(new SimpleFragmenter(100));
        final String fragment = highlighter.getBestFragment(analyzer, fieldName, "Buchung");

        assertEquals("B<b>uchu</b>ng",fragment);

    }

public final class NGramAnalyzer extends Analyzer {

    private final int minNGram;

    public NGramAnalyzer(final int minNGram) {
        super();
        this.minNGram = minNGram;
    }

    @Override
    protected TokenStreamComponents createComponents(final String fieldName) {
        final Tokenizer source = new NGramTokenizer(minNGram, minNGram);
        return new TokenStreamComponents(source);
    }

}

}

Thanks and cheers,
Eva

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