docs: private float calculateClusterQuality (ArrayList<Instance> docs,
  			HashSparseVector centroid) {
  		float quality = 0.0f;
  		HashSparseVector c = centroid;
  
  		for (int i = 0; i < docs.size(); ++i) {
  			Instance doc = docs.get(i);
  
  			quality += c.distanceEuclidean((HashSparseVector) doc.getData());
  		}
  
  		return quality;
  	}