%--------------------------
 % @Author: Jingqiao Hu
 % @Date: 2020-10-29 15:04:18
 % @LastEditors: Please set LastEditors
 % @LastEditTime: 2020-11-28 21:49:16
%--------------------------
function f = elastic_force(F, dF, nodeNum, edofMat, lx, u, l)

    f = zeros(nodeNum*2, 1);
    jac = lx^2/4;
    eleNum = size(edofMat,1);
    
    for gp = 1:4
        dFe = dF{gp};                      % 4 * 8 * m
        Fe  = F{gp, 1};                    % 4 * m
        P2(1, :, :)  = PK1_fast(u, l, Fe); % 4 * m
        
        fe = squeeze(mtimesx(P2, dFe)) * jac; % 8*m

        for ele = 1:eleNum   
            edof = edofMat(ele, :);
            f(edof) = f(edof) - fe(:, ele); % NOTE: MINUS!
        end        
    end 
end

function PP = PK1(u,l,F)
    % Neo hookean
    JJ = log(det(F));
    Finv = inv(F)';
    PP = u*(F-Finv) + l*JJ*Finv;

    % Linear
    % I = eye(3,3);
    % PP = u*(F + F'-2*I) + l * (trace(F)-3) * I;
end