def simplex_index(x, m, n):
    r"""
    Return the index of the point x in the lexicographic order of the
    integer points of the (m-1)-dimensional simplex :math:`\{x \mid x_0
    + \cdots + x_{m-1} = n\}`.

    Parameters
    ----------
    x : array_like(int, ndim=1)
        Integer point in the simplex, i.e., an array of m nonnegative
        itegers that sum to n.

    m : scalar(int)
        Dimension of each point. Must be a positive integer.

    n : scalar(int)
        Number which the coordinates of each point sum to. Must be a
        nonnegative integer.

    Returns
    -------
    idx : scalar(int)
        Index of x.

    """
    if m == 1:
        return 0

    decumsum = np.cumsum(x[-1:0:-1])[::-1]
    idx = num_compositions(m, n) - 1
    for i in range(m-1):
        if decumsum[i] == 0:
            break
        idx -= num_compositions(m-i, decumsum[i]-1)
    return idx