function mg5ptdemo(p)
%MG5PTDEMO  graphical demonstration of multigrid convergence
%
% demonstration of multigrid for 5-point Laplacian: plots
% successive iterates of a multigrid v-cycle; uses mesh-size
% h=1/2^p, with default value for p.
% Douglas N. Arnold, 2009-10-15

if nargin == 0
  p=4;
end
niters = 10;

% supply f
%f = @(x,y) -2*pi^2*sin(pi*x).*sin(pi*y);
f = @(x,y) -190*x.^2.*(1-y)+26.4*exp((x-.25).^2+(y-.25).^2);

% set mesh size
n = 2^p-1;
h = 1/(n+1);
x = linspace(0,1,n+2);   % x grid points including boundaries
y = linspace(0,1,n+2);   % y grid points including boundaries

[X,Y] = meshgrid(x,y);      % 2d arrays of x,y values
X = X';                     % transpose so that X(i,j),Y(i,j) are
Y = Y';                     % coordinates of (i,j) grid point, i.e., ((i-1)h,(j-1)h)

Iint = 2:n+1;                 % indices of interior points in x
Jint = 2:n+1;                 % indices of interior points in y
Xint = X(Iint,Jint);        % interior points
Yint = Y(Iint,Jint);

fh = f(Xint,Yint);          % fh is a mesh function on interior grid points

%%%%%%  Direct solve %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% set uh on the boundary to given values (interior values will be ignored)
uh = zeros(n+2,n+2);

% form 5-point Laplacian matrix (times h^2)
I = speye(n);
e = ones(n,1);
A = spdiags([-e,4*e,-e],[-1,0,1],n,n);
J = spdiags([-e,-e],[-1,1],n,n);
Lh = kron(I,A) + kron(J,I);

fh = f(Xint,Yint);          % fh is a mesh function on interior grid points
% multiply right hand side by h^2 and adjust for boundary condition
gh = h^2*fh;
gh(:,1) = gh(:,1) - uh(Iint,1);
gh(:,n) = gh(:,n) - uh(Iint,n+2);
gh(1,:) = gh(1,:) - uh(1,Jint);
gh(n,:) = gh(n,:) - uh(n+2,Jint);

% convert the right hand side from a grid function into a vector
F = reshape(gh,n*n,1);

% Solve the linear system for the grid function uh on the interior, written
% as vector
U = Lh\F;  

% convert from vector into grid function use this for uh at the interior
% grid points (uh is already given at the boundary grid points)
uh(Iint,Jint) = reshape(U,n,n);
% plot results:
figure(1)
clf
uhplot=surf(X,Y,uh);
set(uhplot,'facecolor','yellow')
if p > 5
  set(uhplot,'edgecolor','none')
end
set(gca,'zlim',[-1,1])
l=light('position',[-5,5,10]);
lighting gouraud
%%%%%%  End direct solve %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

%%%%%% multigrid v-cycle %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% set up initial iterate (zero grid function)
u = zeros(size(fh));
% set up initial iterate (random grid function)
%u = 2*rand(size(fh))-1;
U = [zeros(1,n+2);zeros(n,1),u,zeros(n,1);zeros(1,n+2)];

figure(2)
clf
uplot=surf(X,Y,U);
set(uplot,'facecolor','yellow')
if p > 5
  set(uplot,'edgecolor','none')
end
set(gca,'zlim',[-1,1])
hold on
l=light('position',[-5,5,10]);
lighting gouraud
% print max norm error
fprintf('iteration    0.   max norm error: %9.5f\n',max(max(abs(uh-U))))

% iterate, updating plot each time
for niter = 1:niters
  pause
  u = mg5pt(fh,u);
  clf
  U = [zeros(1,n+2);zeros(n,1),u,zeros(n,1);zeros(1,n+2)];
  uplot=surf(X,Y,U);
  set(uplot,'facecolor','yellow')
  if p > 5
    set(uplot,'edgecolor','none')
  end
  set(gca,'zlim',[-1,1])
  l=light('position',[-5,5,10]);
  lighting gouraud
  % print max norm error
  fprintf('iteration %4d.   max norm error: %9.5f\n',niter,max(max(abs(uh-U))))
end