@Override
 public void reduce(
     GenericKey key,
     Iterator<GenericValue> values,
     OutputCollector<GenericKey, GenericValue> output,
     Reporter reporter)
     throws IOException {
   if (key.getSecondary() < Preprocesser.MINIMUM_ID) { // vector
     output.collect(key, values.next());
     if (values.hasNext()) assert false : "Vectors should not get grouped by combiner: " + key;
   } else { // addend
     reporter.progress();
     int counter = 0;
     float sim = 0;
     HalfPair hp = null;
     while (values.hasNext()) {
       hp = (HalfPair) values.next().get();
       sim += hp.getSimilarity();
       if (counter++ % REPORTER_INTERVAL == 0) reporter.progress();
     }
     if (hp != null) {
       payload.set(hp.getID(), sim);
       outValue.set(payload);
       output.collect(key, outValue);
     } else {
       assert false : "There is nothing to combine!";
     }
   }
 }
    @Override
    public void reduce(
        GenericKey key,
        Iterator<GenericValue> values,
        OutputCollector<VectorPair, FloatWritable> output,
        Reporter reporter)
        throws IOException {

      int vectorID = key.getPrimary();
      assert (key.getSecondary() == -1);
      // the vector is the first value
      VectorComponentArrayWritable vector = (VectorComponentArrayWritable) values.next().get();
      // half pairs are sorted such that all equal pairs are consecutive
      if (values.hasNext()) {
        reporter.incrCounter(APS.COMBINED, 1);
        HalfPair hp1 = (HalfPair) values.next().get();
        float similarity = hp1.getSimilarity();
        HalfPair hp2;
        int counter = 0;
        while (values.hasNext()) {
          reporter.incrCounter(APS.COMBINED, 1);
          if (counter++ % REPORTER_INTERVAL == 0) reporter.progress();
          hp2 = (HalfPair) values.next().get();
          if (hp1.equals(hp2)) {
            similarity += hp2.getSimilarity();
          } else {
            // output
            outputHelper(hp1, vectorID, vector, similarity, output, reporter);
            // start new stripe
            hp1 = hp2;
            similarity = hp1.getSimilarity();
          }
        }
        // output the last one
        outputHelper(hp1, vectorID, vector, similarity, output, reporter);
      }
    }