def split_users(unique_uid, n_heldout_users=10000):   
   """Split train / validation / test users.
      Select 10K users as heldout users, 10K users as validation users, and the rest of the users for training
   Args:
       unique_uid (Array): [randomly permutated User Index]
       n_heldout_users (int)
   """
   n_users = len(unique_uid)
   tr_users = unique_uid[:(n_users - n_heldout_users * 2)]
   vd_users = unique_uid[(n_users - n_heldout_users * 2): (n_users - n_heldout_users)]
   te_users = unique_uid[(n_users - n_heldout_users):]
   return tr_users, vd_users, te_users