code: def save_fast_format(self, filename):
          
          items, _ = zip(*sorted(self.items.items(), key=lambda x: x[1]))
          items = {"items": items,
                   "unk_index": self.unk_index,
                   "name": self.name}
  
          json.dump(items, open("{}_items.json".format(filename), 'w'))
          np.save(open("{}_vectors.npy".format(filename), 'wb'), self.vectors)
docs: Save a reach instance in a fast format.
  
          The reach fast format stores the words and vectors of a Reach instance
          separately in a JSON and numpy format, respectively.
  
          Parameters
          ----------
          filename : str
              The prefix to add to the saved filename. Note that this is not the
              real filename under which these items are stored.
              The words and unk_index are stored under "{filename}_words.json",
              and the numpy matrix is saved under "{filename}_vectors.npy".