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Bobo is a Faceted Search implementation written purely in Java, an extension of Apache Lucene
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package com.browseengine.bobo.query;

import java.io.IOException;
import java.util.ArrayList;
import java.util.Map;

import org.apache.lucene.index.AtomicReader;
import org.apache.lucene.search.Explanation;
import org.apache.lucene.search.Scorer;

import com.browseengine.bobo.api.BoboSegmentReader;
import com.browseengine.bobo.facets.FacetHandler;
import com.browseengine.bobo.query.scoring.BoboDocScorer;
import com.browseengine.bobo.query.scoring.FacetScoreable;
import com.browseengine.bobo.query.scoring.FacetTermScoringFunctionFactory;
import com.browseengine.bobo.query.scoring.MultiplicativeFacetTermScoringFunctionFactory;

public class FacetBasedBoostScorerBuilder implements ScorerBuilder {
  protected final Map> _boostMaps;
  protected final FacetTermScoringFunctionFactory _scoringFunctionFactory;

  public FacetBasedBoostScorerBuilder(Map> boostMaps) {
    this(boostMaps, new MultiplicativeFacetTermScoringFunctionFactory());
  }

  protected FacetBasedBoostScorerBuilder(Map> boostMaps,
      FacetTermScoringFunctionFactory scoringFunctionFactory) {
    _boostMaps = boostMaps;
    _scoringFunctionFactory = scoringFunctionFactory;
  }

  @Override
  public Scorer createScorer(Scorer innerScorer, AtomicReader reader, boolean scoreDocsInOrder,
      boolean topScorer) throws IOException {
    if (!(reader instanceof BoboSegmentReader)) throw new IllegalArgumentException(
        "IndexReader is not BoboSegmentReader");

    return new FacetBasedBoostingScorer((BoboSegmentReader) reader, innerScorer);
  }

  @Override
  public Explanation explain(AtomicReader indexReader, int docid, Explanation innerExplaination)
      throws IOException {
    if (!(indexReader instanceof BoboSegmentReader)) throw new IllegalArgumentException(
        "IndexReader is not BoboSegmentReader");
    BoboSegmentReader reader = (BoboSegmentReader) indexReader;

    Explanation exp = new Explanation();
    exp.setDescription("FacetBasedBoost");

    float boost = 1.0f;
    for (Map.Entry> boostEntry : _boostMaps.entrySet()) {
      String facetName = boostEntry.getKey();
      FacetHandler handler = reader.getFacetHandler(facetName);
      if (!(handler instanceof FacetScoreable)) throw new IllegalArgumentException(facetName
          + " does not implement FacetScoreable");

      FacetScoreable facetScoreable = (FacetScoreable) handler;
      BoboDocScorer scorer = facetScoreable.getDocScorer(reader, _scoringFunctionFactory,
        boostEntry.getValue());
      float facetBoost = scorer.score(docid);

      Explanation facetExp = new Explanation();
      facetExp.setDescription(facetName);
      facetExp.setValue(facetBoost);
      facetExp.addDetail(scorer.explain(docid));
      boost *= facetBoost;
      exp.addDetail(facetExp);
    }
    exp.setValue(boost);
    exp.addDetail(innerExplaination);
    return exp;
  }

  private class FacetBasedBoostingScorer extends Scorer {
    private final Scorer _innerScorer;
    private final BoboDocScorer[] _facetScorers;

    private int _docid;

    public FacetBasedBoostingScorer(BoboSegmentReader reader, Scorer innerScorer) {
      super(innerScorer.getWeight());
      _innerScorer = innerScorer;

      ArrayList list = new ArrayList();

      for (Map.Entry> boostEntry : _boostMaps.entrySet()) {
        String facetName = boostEntry.getKey();
        FacetHandler handler = reader.getFacetHandler(facetName);
        if (!(handler instanceof FacetScoreable)) throw new IllegalArgumentException(facetName
            + " does not implement FacetScoreable");
        FacetScoreable facetScoreable = (FacetScoreable) handler;
        BoboDocScorer scorer = facetScoreable.getDocScorer(reader, _scoringFunctionFactory,
          boostEntry.getValue());
        if (scorer != null) list.add(scorer);
      }
      _facetScorers = list.toArray(new BoboDocScorer[list.size()]);
      _docid = -1;
    }

    @Override
    public float score() throws IOException {
      float score = _innerScorer.score();
      for (BoboDocScorer facetScorer : _facetScorers) {
        float fscore = facetScorer.score(_docid);
        if (fscore > 0.0) {
          score *= fscore;
        }
      }
      return score;
    }

    @Override
    public int docID() {
      return _docid;
    }

    @Override
    public int nextDoc() throws IOException {
      return (_docid = _innerScorer.nextDoc());
    }

    @Override
    public int advance(int target) throws IOException {
      return (_docid = _innerScorer.advance(target));
    }

    @Override
    public int freq() throws IOException {
      // TODO Auto-generated method stub
      return 0;
    }

    @Override
    public long cost() {
      // TODO Auto-generated method stub
      return 0;
    }
  }
}