code: package edu.jhu.thrax.hadoop.paraphrasing;
  
  import java.io.IOException;
  import java.util.ArrayList;
  import java.util.List;
  
  import org.apache.hadoop.conf.Configuration;
  import org.apache.hadoop.mapreduce.Reducer;
  
  import edu.jhu.thrax.hadoop.datatypes.FeatureMap;
  import edu.jhu.thrax.hadoop.datatypes.RuleWritable;
  import edu.jhu.thrax.hadoop.features.annotation.AnnotationFeature;
  import edu.jhu.thrax.hadoop.features.annotation.AnnotationFeatureFactory;
  import edu.jhu.thrax.hadoop.features.pivot.PivotedAnnotationFeature;
  import edu.jhu.thrax.hadoop.features.pivot.PivotedFeature;
  import edu.jhu.thrax.hadoop.features.pivot.PivotedFeatureFactory;
  import edu.jhu.thrax.util.BackwardsCompatibility;
  import edu.jhu.thrax.util.FormatUtils;
  import edu.jhu.thrax.util.Vocabulary;
  
  public class AggregationCombiner
      extends Reducer<RuleWritable, FeatureMap, RuleWritable, FeatureMap> {
  
    private List<PivotedFeature> pivotedFeatures;
  
    protected void setup(Context context) throws IOException, InterruptedException {
      Configuration conf = context.getConfiguration();
      String vocabulary_path = conf.getRaw("thrax.work-dir") + "vocabulary/part-*";
      Vocabulary.initialize(conf, vocabulary_path);
  
      pivotedFeatures = new ArrayList<PivotedFeature>();
      List<AnnotationFeature> annotationFeatures = new ArrayList<AnnotationFeature>();
  
      String features = BackwardsCompatibility.equivalent(conf.get("thrax.features", ""));
      for (String f_name : FormatUtils.P_COMMA_OR_SPACE.split(features)) {
        PivotedFeature pf = PivotedFeatureFactory.get(f_name);
        if (pf != null) {
          pivotedFeatures.add(pf);
        } else {
          AnnotationFeature af = AnnotationFeatureFactory.get(f_name);
          if (af != null) {
            annotationFeatures.add(af);
          }
        }
      }
      if (!annotationFeatures.isEmpty()) pivotedFeatures.add(new PivotedAnnotationFeature());
    }
  
    protected void reduce(RuleWritable key, Iterable<FeatureMap> values, Context context)
        throws IOException, InterruptedException {
      FeatureMap merged = new FeatureMap();
  
      for (PivotedFeature feature : pivotedFeatures)
        feature.initializeAggregation();
      for (FeatureMap feature_map : values) {
        for (PivotedFeature feature : pivotedFeatures) {
          try {
            feature.aggregate(feature_map);
          } catch (Exception e) {
            throw new RuntimeException(key.toString() + " on " + feature.getName() + ": "
                + e.getMessage());
          }
        }
      }
      for (PivotedFeature feature : pivotedFeatures)
        merged.put(feature.getName(), feature.finalizeAggregation());
      context.write(key, merged);
    }
  }
  