def pcca(P, m):
    """
    PCCA+ spectral clustering method with optimized memberships [1]_

    Clusters the first m eigenvectors of a transition matrix in order to cluster the states.
    This function does not assume that the transition matrix is fully connected. Disconnected sets
    will automatically define the first metastable states, with perfect membership assignments.

    Parameters
    ----------
    P : ndarray (n,n)
        Transition matrix.

    m : int
        Number of clusters to group to.

    Returns
    -------
    chi by default, or (chi,rot) if return_rot = True

    chi : ndarray (n x m)
        A matrix containing the probability or membership of each state to be assigned to each cluster.
        The rows sum to 1.

    References
    ----------
    [1] S. Roeblitz and M. Weber, Fuzzy spectral clustering by PCCA+:
        application to Markov state models and data classification.
        Adv Data Anal Classif 7, 147-179 (2013).
    [2] F. Noe, multiset PCCA and HMMs, in preparation.

    """
    # imports
    from msmtools.estimation import connected_sets
    from msmtools.analysis import eigenvalues, is_transition_matrix, hitting_probability

    # validate input
    n = np.shape(P)[0]
    if (m > n):
        raise ValueError("Number of metastable states m = " + str(m)+
                         " exceeds number of states of transition matrix n = " + str(n))
    if not is_transition_matrix(P):
        raise ValueError("Input matrix is not a transition matrix.")

    # prepare output
    chi = np.zeros((n, m))

    # test connectivity
    components = connected_sets(P)
    # print "all labels ",labels
    n_components = len(components)  # (n_components, labels) = connected_components(P, connection='strong')
    # print 'n_components'

    # store components as closed (with positive equilibrium distribution)
    # or as transition states (with vanishing equilibrium distribution)
    closed_components = []
    transition_states = []
    for i in range(n_components):
        component = components[i]  # np.argwhere(labels==i).flatten()
        rest = list(set(range(n)) - set(component))
        # is component closed?
        if (np.sum(P[component, :][:, rest]) == 0):
            closed_components.append(component)
        else:
            transition_states.append(component)
    n_closed_components = len(closed_components)
    closed_states = np.concatenate(closed_components)
    if len(transition_states) == 0:
        transition_states = np.array([], dtype=int)
    else:
        transition_states = np.concatenate(transition_states)

    # check if we have enough clusters to support the disconnected sets
    if (m < len(closed_components)):
        raise ValueError("Number of metastable states m = " + str(m) + " is too small. Transition matrix has " +
                         str(len(closed_components)) + " disconnected components")

    # We collect eigenvalues in order to decide which
    closed_components_Psub = []
    closed_components_ev = []
    closed_components_enum = []
    for i in range(n_closed_components):
        component = closed_components[i]
        # print "component ",i," ",component
        # compute eigenvalues in submatrix
        Psub = P[component, :][:, component]
        closed_components_Psub.append(Psub)
        closed_components_ev.append(eigenvalues(Psub))
        closed_components_enum.append(i * np.ones((component.size), dtype=int))

    # flatten
    closed_components_ev_flat = np.hstack(closed_components_ev)
    closed_components_enum_flat = np.hstack(closed_components_enum)
    # which components should be clustered?
    component_indexes = closed_components_enum_flat[np.argsort(closed_components_ev_flat)][0:m]
    # cluster each component
    ipcca = 0
    for i in range(n_closed_components):
        component = closed_components[i]
        # how many PCCA states in this component?
        m_by_component = np.shape(np.argwhere(component_indexes == i))[0]

        # if 1, then the result is trivial
        if (m_by_component == 1):
            chi[component, ipcca] = 1.0
            ipcca += 1
        elif (m_by_component > 1):
            #print "submatrix: ",closed_components_Psub[i]
            chi[component, ipcca:ipcca + m_by_component] = _pcca_connected(closed_components_Psub[i], m_by_component)
            ipcca += m_by_component
        else:
            raise RuntimeError("Component " + str(i) + " spuriously has " + str(m_by_component) + " pcca sets")

    # finally assign all transition states
    # print "chi\n", chi
    # print "transition states: ",transition_states
    # print "closed states: ", closed_states
    if (transition_states.size > 0):
        # make all closed states absorbing, so we can see which closed state we hit first
        Pabs = P.copy()
        Pabs[closed_states, :] = 0.0
        Pabs[closed_states, closed_states] = 1.0
        for i in range(closed_states.size):
            # hitting probability to each closed state
            h = hitting_probability(Pabs, closed_states[i])
            for j in range(transition_states.size):
                # transition states belong to closed states with the hitting probability, and inherit their chi
                chi[transition_states[j]] += h[transition_states[j]] * chi[closed_states[i]]

    # check if we have m metastable sets. If less than m, we must raise
    nmeta = np.count_nonzero(chi.sum(axis=0))
    assert m <= nmeta, str(m) + " metastable states requested, but transition matrix only has " + str(nmeta) \
                       + ". Consider using a prior or request less metastable states. "

    # print "chi\n", chi
    return chi