function Q_x = commGSP_signal_modularity(A, signal)
% Returns the scalar measure whose positive value reflects how much
% a graph signal is modular (more modular than anti-modular).
% A negative value reflects more anti-modular features than modular.
% The measure is the quadratic form of modularity matrix.
% INPUT:
%   A - (symmetric) matrix to decompose
%   signal - vector (length equal to size of A) representing graph signal
%   of size number_of_nodes x 1, or a set of signals organized in a matrix
%   of size number_of_nodes x number_of_signals
% OUTPUT
%   Q_x - scalar metric of signal's modularity structure, or a vector with
%   every entry corresponding to one of the signals is signal is a matrix

Q = commGSP_build_matrix(A, 'modularity');
Q_x = diag(signal'*Q*signal);

end