code: def _construct_sparse_features(self, x):
          
          I, J, K = x.shape
          new_array_height = (x != 0).sum(axis=2).max()
          index_array = -np.ones((I, J, new_array_height), dtype='int64')
          value_array = -np.ones((I, J, new_array_height), dtype='float64')
          populate_sparse_features(x, index_array, value_array, I, J, K)
          return index_array, value_array
docs: Helper to construct a sparse representation of the features.