function [] = FESolveX2D()
% MATLAB based 2-D XFEM Solver
% J. Grogan (2012)
clear all
% Define Mesh
NumX=3;
NumY=1;
delX=1.;
delY=1.;
for j=1:NumY+1
   for i=1:NumX+1
       index=i+(NumX+1)*(j-1);
       Node(index,1)=single((i-1.))*delX;
       Node(index,2)=single((j-1.))*delY;
   end
end
numNodes=(NumX+1)*(NumY+1);
for j=1:NumY
    for i=1:NumX
        index=i+NumX*(j-1);
        Element(index,1)=i+(NumX+1)*(j-1);
        Element(index,2)=i+(NumX+1)*(j-1)+1;
        Element(index,3)=i+(NumX+1)*(j)+1;
        Element(index,4)=i+(NumX+1)*(j);
    end
end
numElem=(NumX)*(NumY);
% dofs per node
ndof=2;
% Define Section Properties
rho=1.;
% initial interface position
dpos=0.25;
% Initial temperatures
Tnew=zeros(numNodes*2,1);
Bound=zeros(numNodes*2,1);
stored(1)=dpos;
for e=1:numElem
    for n=1:4
        crdn=Node(Element(e,n),1);
        if crdn<=dpos
            Tnew(2*Element(e,n)-1)=1.;
            Bound(2*Element(e,n)-1)=1.;
        end
    end
end
% Define Time Step
dtime=0.1;
tsteps=1;
time=0.;
% penalty term
beta=0.;
% Loop through time steps
for ts=1:tsteps
    % Get interface velocity
     d(1)=dpos+delX;
     d(2)=dpos+3*delX/4;
     d(3)=dpos+delX/4;
     d(4)=dpos;
     for e=1:numElem
         crdn1=Node(Element(e,1),1);
         crdn2=Node(Element(e,2),1);
        for j=1:4
            if d(j)>=crdn1 & d(j)<crdn2                
                   elen=abs(crdn2-crdn1);
                   ajacob=elen/2.;
                   point=(d(j)-crdn1)/ajacob-1.;
                   theta(1)=abs(crdn1-dpos)*sign(crdn1-dpos);
                   theta(2)=abs(crdn2-dpos)*sign(crdn2-dpos); 
                   tmp1a=Tnew(Element(e,1)*2-1);
                   tmp1b=Tnew(Element(e,1)*2);
                   tmp2a=Tnew(Element(e,2)*2-1);
                   tmp2b=Tnew(Element(e,1)*2);
                   xi=point;
                   gm(1)=(1.-xi)/2.;
                   gm(3)=(1.+xi)/2.;
                   term=theta(1)*gm(1)+theta(2)*gm(3);
                   gm(2)=gm(1)*(abs(term)-abs(theta(1)));
                   gm(4)=gm(3)*(abs(term)-abs(theta(2)));               
                   t(j)=gm(1)*tmp1a+gm(2)*tmp1b+gm(3)*tmp2a+gm(4)*tmp2b;
             end
         end
     end 
    vel=(0.5/delX)*(2*t(1)+t(2)-t(3)-2*t(4));
%    vel=0.1;
    % Update interface position
    dpos=dpos+vel*dtime;
    stored(ts+1)=dpos;
    K=zeros(numNodes*ndof,numNodes*ndof);
    M=zeros(numNodes*ndof,numNodes*ndof);
    pforce=zeros(numNodes*ndof,1);
    % Loop Through Elements
    for e=1:numElem
        Ke=zeros(4*ndof);
        Me=zeros(4*ndof);
        for icrd=1:4;
            crdnx(icrd)=Node(Element(e,icrd),1);
            crdny(icrd)=Node(Element(e,icrd),2);
            theta(icrd)=abs(crdnx(icrd)-dpos)*sign(crdnx(icrd)-dpos);
        end   
        if sign(theta(1))~=sign(theta(2))
            % enriched element
            enr=8;
            % get interface position on element
            elen=abs(crdnx(2)-crdnx(1));
            frac=abs(dpos-crdnx(1))/elen;
            len1=2.*frac;
            len2=2.*(1.-frac);           
            % devide element for sub integration
            mid1=-1+len1/2.;
            mid2=1-len2/2.;
            gx(1)=mid1-(len1/2.)/sqrt(3.);
            gx(2)=mid1+(len1/2.)/sqrt(3.);
            gx(3)=mid1+(len1/2.)/sqrt(3.);
            gx(4)=mid1-(len1/2.)/sqrt(3.);
            gx(5)=mid2-(len2/2.)/sqrt(3.);
            gx(6)=mid2+(len2/2.)/sqrt(3.);
            gx(7)=mid2+(len2/2.)/sqrt(3.);
            gx(8)=mid2-(len2/2.)/sqrt(3.); 
            gpos=1/sqrt(3.);
            hx(1)=-gpos;
            hx(2)=-gpos;
            hx(3)=+gpos;
            hx(4)=+gpos;
            hx(5)=-gpos;
            hx(6)=-gpos;
            hx(7)=+gpos;
            hx(8)=+gpos;  
            for iw=1:4
                w(iw)=frac/2.;
                w(iw+4)=(1.-frac)/2.;
            end      
        else
            % regular element - fix extra dofs 
            enr=4;
            gpos=1/sqrt(3.);
            gx(1)=-gpos;
            gx(2)=gpos;
            gx(3)=gpos;
            gx(4)=-gpos;
            hx(1)=-gpos;
            hx(2)=-gpos;
            hx(3)=gpos;
            hx(4)=gpos;
            for iw=1:4
                w(iw)=1.;
            end             
        end          
        % Loop Through Int Points
        for i=1:enr;
            g=gx(i);
            h=hx(i);                     
            phi(1)=0.25*(1.-g)*(1.-h);
            phi(3)=0.25*(1.+g)*(1.-h);
            phi(5)=0.25*(1.+g)*(1.+h);
            phi(7)=0.25*(1.-g)*(1.+h);  
            iLS=theta(1)*phi(1)+theta(2)*phi(3)+theta(3)*phi(5)+theta(4)*phi(7);
            if iLS<0.
                cond=0.;
                spec=0.01;
            else
                cond=1.;
                spec=1.;
            end
            for iter=1:4
                phi(2*iter)=phi(2*iter-1)*(abs(iLS)-abs(theta(iter)));
            end
			phig(1)=0.25*-(1.-h);
			phig(3)=0.25*(1.-h);
			phig(5)=0.25*(1.+h);
			phig(7)=0.25*-(1.+h);
			phih(1)=0.25*-(1.-g);
			phih(3)=0.25*-(1.+g);
			phih(5)=0.25*(1.+g);
			phih(7)=0.25*(1.-g);	
            diLSg=sign(iLS)*(phig(1)*theta(1)+phig(3)*theta(2)+phig(5)*theta(3)+phig(7)*theta(4));
            diLSh=sign(iLS)*(phih(1)*theta(1)+phih(3)*theta(2)+phih(5)*theta(3)+phih(7)*theta(4));
            for iter=1:4                      
                phig(2*iter)=phig(2*iter-1)*(abs(iLS)-abs(theta(iter)))+phi(2*iter-1)*diLSg;
                phih(2*iter)=phih(2*iter-1)*(abs(iLS)-abs(theta(iter)))+phi(2*iter-1)*diLSh;
            end           
			rjac=zeros(2,2);
			for iter=1:4
				rjac(1,1)=rjac(1,1)+phig(2*iter-1)*crdnx(iter);
				rjac(1,2)=rjac(1,2)+phig(2*iter-1)*crdny(iter);
				rjac(2,1)=rjac(2,1)+phih(2*iter-1)*crdnx(iter);
				rjac(2,2)=rjac(2,2)+phih(2*iter-1)*crdny(iter);
            end
			djac=rjac(1,1)*rjac(2,2)-rjac(1,2)*rjac(2,1);
			rjaci(1,1)= rjac(2,2)/djac;
			rjaci(2,2)= rjac(1,1)/djac;
			rjaci(1,2)=-rjac(1,2)/djac;
			rjaci(2,1)=-rjac(2,1)/djac ; 
            for iter=1:8              
                phix(iter)=rjaci(1,1)*phig(iter)+rjaci(1,2)*phih(iter);
                phiy(iter)=rjaci(2,1)*phig(iter)+rjaci(2,2)*phih(iter);
            end	           
            we=w(i)*djac;
            Ke=Ke+we*cond*(phix'*phix+phiy'*phiy);
            Me=Me+(we*rho*spec*phi'*phi)/dtime;
        end  
        % Add penalty term and get temp gradient on interface
        if enr==8;
            xi=2.*frac-1;
            yi=-1.;                     
            gm(1)=0.25*(1.-xi)*(1.-yi);
            gm(3)=0.25*(1.+xi)*(1.-yi);
            gm(5)=0.25*(1.+xi)*(1.+yi);
            gm(7)=0.25*(1.-xi)*(1.+yi);                      
            term=theta(1)*gm(1)+theta(2)*gm(3);
            gm(2)=gm(1)*(abs(iLS)-abs(theta(1)));
            gm(4)=gm(3)*(abs(iLS)-abs(theta(2)));
            gm(6)=gm(5)*(abs(iLS)-abs(theta(3)));
            gm(8)=gm(7)*(abs(iLS)-abs(theta(4)));
            pen=beta*(gm'*gm);
            pfL=beta*1.*gm';  
            Ke=Ke+pen;
        else
            pen=zeros(8);
            pfL=zeros(8,1);
        end
        % Assemble Global Matrices
        gnum(1)=2*Element(e,1)-1;
        gnum(2)=2*Element(e,2)-1;
        gnum(3)=2*Element(e,3)-1;
        gnum(4)=2*Element(e,4)-1;      
        for i=1:4;
            for j=1:4;
                K(gnum(j),gnum(i))=K(gnum(j),gnum(i))+Ke(2*j-1,2*i-1);
                K(gnum(j)+1,gnum(i)+1)=K(gnum(j)+1,gnum(i)+1)+Ke(2*j,2*i);
                M(gnum(j),gnum(i))=M(gnum(j),gnum(i))+Me(2*j-1,2*i-1);
                M(gnum(j)+1,gnum(i)+1)=M(gnum(j)+1,gnum(i)+1)+Me(2*j,2*i);                
            end
        end
        for i=1:4;
            pforce(gnum(i))=pforce(gnum(i))+pfL(2*i-1);
            pforce(gnum(i)+1)=pforce(gnum(i)+1)+pfL(2*i);
        end
    end
    %Remove inactive DOFs(Reduce Matrices)
    RHS=M*Tnew; 
    A=K+M;
    Sub=A*Bound; 
    iindex=0;   
    for i=1:ndof*numNodes;  
        if Bound(i)==0.;
            iindex=iindex+1;
            RHSred(iindex)=RHS(i)-Sub(i)+pforce(i);
            jindex=0;
            for j=1:ndof*numNodes; 
               if Bound(j)==0.; 
                    jindex=jindex+1;
                    Ared(iindex,jindex)=A(i,j);   
               end
            end
        end
    end 
    %Solve
    StiffI=Ared^-1;
    Tnewr=(Ared^-1)*RHSred';
    iindex=0;   
    for i=1:ndof*numNodes;
        if Bound(i)==0.;
            iindex=iindex+1;
            Tnew(i)=Tnewr(iindex);
        end       
    end
    Tnew
end
stored'