Compute Hilbert indices. 

    Parameters
    ----------
    xint: (N, d) int numpy.ndarray

    Returns
    -------
    h: (N,) int numpy.ndarray
        Hilbert indices

def hilbert_array(xint):
    """Compute Hilbert indices. 

    Parameters
    ----------
    xint: (N, d) int numpy.ndarray

    Returns
    -------
    h: (N,) int numpy.ndarray
        Hilbert indices 
    """
    N, d = xint.shape
    h = np.zeros(N, int64)
    for n in range(N):
        h[n] = Hilbert_to_int(xint[n, :])
    return h