%MILSTRONG  Test strong convergence of Milstein: vectorized
%
% Solves   dX = r*X*(K-X) dt + beta*X dW,  X(0) = Xzero,
%        where r = 2, K= 1, beta = 1 and Xzero = 0.5.
%
% Discretized Brownian path over [0,1] has dt = 2^(-11).
% Milstein uses timesteps 128*dt, 64*dt, 32*dt, 16*dt (also dt for reference).
%
% Examines strong convergence at T=1:  E | X_L - X(T) |.
% Code is vectorized: all paths computed simultaneously.

rand('state',100)
r = 2; K = 1; beta = 0.25; Xzero = 0.5;   % problem parameters
T = 1; N = 2^(11); dt = T/N;              %             
M = 500;                                  % number of paths sampled
R = [1; 16; 32; 64; 128];                 % Milstein stepsizes are R*dt

dW = sqrt(dt)*randn(M,N);                 % Brownian increments
Xmil = zeros(M,5);                        % preallocate array
for p = 1:5                               
     Dt = R(p)*dt; L = N/R(p);            % L timesteps of size Dt = R dt
     Xtemp = Xzero*ones(M,1);
     for j = 1:L
          Winc = sum(dW(:,R(p)*(j-1)+1:R(p)*j),2);
          Xtemp = Xtemp + Dt*r*Xtemp.*(K-Xtemp) + beta*Xtemp.*Winc ...
              + 0.5*beta^2*Xtemp.*(Winc.^2 - Dt);
     end
     Xmil(:,p) = Xtemp;  % store Milstein solution at t =1
end

Xref = Xmil(:,1);                             % Reference solution
Xerr = abs(Xmil(:,2:5) - repmat(Xref,1,4));   % Error in each path
mean(Xerr);                                   % Mean pathwise erorrs
Dtvals = dt*R(2:5);                           % Milstein timesteps used

subplot(224)                                  % lower RH picture
loglog(Dtvals,mean(Xerr),'b*-'), hold on
loglog(Dtvals,Dtvals,'r--'), hold off         % reference slope of 1
axis([1e-3 1e-1 1e-4 1])
xlabel('\Delta t')
ylabel('Sample average of | X(T) - X_L |')
title('milstrong.m','FontSize',10)

%%%% Least squares fit of error = C * Dt^q %%%%
A = [ones(4,1), log(Dtvals)]; rhs = log(mean(Xerr)');
sol = A\rhs; q = sol(2)
resid = norm(A*sol - rhs)
