code: @Override
      public int numChunksSoFar(String id) {
          if (!chunks.containsKey(id))
              return 0;
          return chunks.get(id).size();
      }
docs: Returns the number of chunks
  accumulated for a given id so far
  
  @param id the id to get the
  number of chunks for
  @return the number of chunks accumulated
  for a given id so far