code: def PerfectSquare_metric(bpmn_graph):
      
  
      all_types_count = Counter([node[1]['type'] for node in bpmn_graph.get_nodes() if node[1]['type']])
      sorted_counts = [count for _, count in all_types_count.most_common()]
  
      potential_perfect_square = min(len(sorted_counts), int(sqrt(sum(sorted_counts))))
  
      for i in range(potential_perfect_square, 0, -1):
          if sum(sorted_counts[:potential_perfect_square]) >= potential_perfect_square * potential_perfect_square:
              return potential_perfect_square
          else:
              potential_perfect_square -= 1
  
      return 0
docs: Returns the value of the Perfect Square metric
      ("Given a set of element types ranked
      in decreasing order of the number of their instances,
      the PSM is the (unique) largest number
      such that the top p types occur(together)
      at least p2 times.")
      for the BPMNDiagramGraph instance.
  
      :param bpmn_graph: an instance of BpmnDiagramGraph representing BPMN model.