function u = fish2d(f)
% Poisson solver in 2D based on matlab fft
% square geometry, homogeneous Dirichlet boundary conditions
% requires uniform mesh (2^k-1)^2 interior nodes 
%
% C. T. Kelley, January 6, 1994
%
% This code comes with no guarantee or warranty of any kind.
%
% Solves - u_xx - u_yy = f
%
% Requires sintv.m and isintv.m
%
% function u = fishsd(f)
%
%
global fish_lal fish_ual ua;
n=length(f);
m=sqrt(n);
hh=.5/(m+1);
h2=(m+1)*(m+1);
e=ones(m,1);
%
% Don't compute eigenvalues, factor tridiagonal matrices
% or allocate space for the 2-D representation of the solution
% more than once. Note the use of global variables to store
% fish_ual, fish_lal, and ua.
%
if exist('fish_ual')==1
    [f1, f2]=size(fish_ual);
    m1=sqrt(f1);
else
    m1=m;
end
if (exist('fish_ual') == 0) | (m1 ~= m)
%
% allocate room for ua, the 2D representation of the solution
%
    ua=zeros(m,m);
%
% set up diagonal of system using eigenvalues of 1D operator
%
    x=1:m; x=pi*hh*x'; lam=sin(x); lam=h2*(2*ones(m,1) + 4*lam.*lam);
    en=h2*ones(n,1); fn=en; gn=en; dx=zeros(n,1);
    for i=1:m
        fn(i*m)=0;
        gn((i-1)*m+1)=0;
        dx((i-1)*m+1:i*m)=lam(i)*ones(m,1);
    end
    al=spdiags([-fn, dx, -gn], [-1,0,1], n,n);
%
% factor the tridiagonal matrix for solution in the x direction
%
    [fish_lal,fish_ual]=lu(al);
end
%
% Map f into a rectangular array.
% Take sine transform in y direction.
%
ua(:)=f;
ua=sintv(ua).';
%
% Solve in the x direction
%
ua(:)=fish_ual\(fish_lal\ua(:));
%
% Take inverse transform.
%
ua=isintv((ua.'));
%
% Map uz into a linear array.
%
u=ua(:);