function dydt = reactor_ode(t,y,gas,vdot,area,heatflux)
% REACTOR ODE system for a generic zero-dimensional reactor.
%
%    Function REACTOR evaluates the system of ordinary differential
%    equations for a zero-dimensional reactor with arbitrary heat
%    transfer and  volume change.
%
% Solution vector components:
%    y(1)   Total internal energy U
%    y(2)   Volume V
%    y(3)   Mass of species 1
%    ....
%    y(2+nsp) Mass of last species
%

[m,n] = size(y);
dydt = zeros(m,n);

for j = 1:n
    this_y = y(:,j);
    int_energy = this_y(1);
    vol = this_y(2);
    masses = this_y(3:end);

    % evaluate the total mass, and the specific internal energy and volume.
    total_mass = sum(masses);
    u_mass = int_energy/total_mass;
    v_mass = vol/total_mass;

    % set the state of the gas by specifying (u,v,{Y_k})
    setMassFractions(gas,masses);
    setState_UV(gas, [u_mass v_mass]);
    p = pressure(gas);

    % volume equation
    vdt = feval(vdot, t, vol, gas);

    % energy equation
    a = feval(area, t, vol);
    q = feval(heatflux, t, gas);
    udt = -p * vdt + a * q;

    % species equations
    ydt = total_mass * ydot(gas);

    % set up column vector for dydt
    dydt(:,j) = [udt
        vdt
        ydt ];
end