% The function var_acap.m computes the atrial elastance and derivative 
% of the elastance for a desired time step.
%
% Function arguments:
%	Cs - atrial end-systolic compliance (ml/mmHg)
%	Cd - atrial diastolic compliance (ml/mmHg)
%	Tp - previous cardiac cycle length (s)
%	sumdeltaT -  time surpassed in current cardiac cycle (s)
%
% Function outputs:
%	E - atrial elastance at the desired time step (mmHg/ml)
%	dE - derivative of E at the desired time step (mmHg/ml-s)
%

function [E,dE] = var_acap(Cs,Cd,Tp,sumdeltaT)

% Making the atrium a passive element, if Cd is less 
% than Cs.
if (Cd <= Cs)

	E = 1/Cd;
	dE = 0;

else

	% Assigning variables.
	Ts = 0.15*sqrt(Tp);
	Tir = Ts/2;

	% Calculating values during isovolumic contraction phase.
	mbrealscalar(sumdeltaT < Ts);
	if (sumdeltaT < Ts)

		E = (((1/Cs)-(1/Cd))/2) * (1-cos((pi/Ts)*sumdeltaT)) + (1/Cd);
		dE = (pi/Ts)*(((1/Cs)-(1/Cd))/2) * sin((pi/Ts)*sumdeltaT);

	% Setting values during diastolic filling phase.
	mbrealscalar(sumdeltaT >= Ts+Tir);
	elseif (sumdeltaT >= Ts+Tir)

		E = 1/Cd;
		dE = 0;

	% Calculating values during isovolumic relaxation phase.
	else

		E = (((1/Cs)-(1/Cd))/2) * (1+cos((pi/Tir)*(sumdeltaT-Ts))) + (1/Cd);
		dE = -(pi/Tir)*(((1/Cs)-(1/Cd))/2) * sin((pi/Tir)*(sumdeltaT-Ts));

	end

end