%% zerocount
%% return the number of "interpolated" zeros between samples of a periodic function

function z = zerocount(X)
eps = 10^(-10);
X = X(:);                  % put in column vector form
N = length(X);
Y = [X(2:N);X(1)] ;
XY = X.*Y;                         % sign changes where this is negative or zero 
z1 = length(find(XY < -eps));      % a zero between samples
z2 = length(find(abs(XY) <= eps)); % an approximate zero at a sample, the zero is double counted.
z = z1+z2/2;