% 
% advance phi and eta one time step by RK3. Space discretisation use one of following :
%    1 : Godunov
%    2 : LLF
%    3 : LF
%    4 : NORMAL
%    5 : NLF
% 
%***********************************************************************
%  This code is written by Chiu-Yen Kao
%  Copyright is reserved  !!

function [phi,u,v] = advance(phi,f,u,v,runtime,dt)
tol = 1e-7;
phi_method = 1;
global nxs nys nxe nye

phi = enforceBC(phi);

rhs1 = getRHS(phi,u,v);        

for i = nxs : nxe
   for j = nys : nye
      phi(i,j) = phi(i,j) + dt*rhs1(i,j);
   end
end
phi = enforceBC(phi);
[u,v] = pp_to_uv(phi);