code: def regular_to_sparse(self):
          
  
          return mapping_util.regular_to_sparse_from_sparse_mappings(
              regular_to_unmasked_sparse=self.regular_to_unmasked_sparse,
              unmasked_sparse_to_sparse=self.unmasked_sparse_to_sparse).astype('int')
docs: The 1D index mappings between the regular-grid and masked sparse-grid.