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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.search.section;

import java.io.IOException;

import org.apache.lucene.index.AtomicReader;
import org.apache.lucene.index.Term;
import org.apache.lucene.util.BytesRef;

/**
 * TermNode for SectionSearchQueryPlan
 *
 */
public class TermNode extends AbstractTerminalNode {
  protected int _positionInPhrase;

  public TermNode(Term term, AtomicReader reader) throws IOException {
    this(term, 0, reader);
  }

  public TermNode(Term term, int positionInPhrase, AtomicReader reader) throws IOException {
    super(term, reader);
    _positionInPhrase = positionInPhrase; // relative position in a phrase
  }

  @Override
  public int fetchSec(int targetSec) throws IOException {
    if (_posLeft > 0) {
      while (true) {
        _curPos = _dp.nextPosition();
        _posLeft--;

        if (readSecId() >= targetSec) return _curSec;

        if (_posLeft <= 0) break;
      }
    }
    _curSec = SectionSearchQueryPlan.NO_MORE_SECTIONS;
    return _curSec;
  }

  @Override
  protected int fetchPos() throws IOException {
    if (_posLeft > 0) {
      _curPos = _dp.nextPosition();
      _posLeft--;
      return _curPos;
    }
    _curPos = SectionSearchQueryPlan.NO_MORE_POSITIONS;
    return _curPos;
  }

  public int readSecId() throws IOException {
    BytesRef payload = _dp.getPayload();
    if (payload != null) {
      _curSec = intDecoders[payload.length].decode(payload.bytes);
    } else {
      _curSec = -1;
    }
    return _curSec;
  }

  private abstract static class IntDecoder {
    abstract int decode(byte[] d);
  }

  private static final IntDecoder[] intDecoders = { new IntDecoder() {
    @Override
    int decode(byte[] d) {
      return 0;
    }
  }, new IntDecoder() {
    @Override
    int decode(byte[] d) {
      return (d[0] & 0xff);
    }
  }, new IntDecoder() {
    @Override
    int decode(byte[] d) {
      return (d[0] & 0xff) | ((d[1] & 0xff) << 8);
    }
  }, new IntDecoder() {
    @Override
    int decode(byte[] d) {
      return (d[0] & 0xff) | ((d[1] & 0xff) << 8) | ((d[2] & 0xff) << 16);
    }
  }, new IntDecoder() {
    @Override
    int decode(byte[] d) {
      return (d[0] & 0xff) | ((d[1] & 0xff) << 8) | ((d[2] & 0xff) << 16) | ((d[3] & 0xff) << 24);
    }
  } };
}