code: public Long getDelta(String name, long count) {
          Long previous = lookup.put(name, count);
          return calculateDelta(name, previous, count);
      }
docs: Calculates the delta.  If this is a new value that has not been seen before, zero will be assumed to be the
  initial value. Updates the internal map with the supplied count.
  
  @param name  the name of the counter
  @param count the counter value
  @return the delta