function [AHess] = AHessian(T,R,Q,H,P,Y,DT,DYss,DOm,DH,DP,start,mf,kalman_tol,riccati_tol)
% function [AHess] = AHessian(T,R,Q,H,P,Y,DT,DYss,DOm,DH,DP,start,mf,kalman_tol,riccati_tol)
%
% computes the asymptotic hessian matrix of the log-likelihood function of
% a state space model (notation as in kalman_filter.m in DYNARE
% Thanks to  Nikolai Iskrev
%
% NOTE: the derivative matrices (DT,DR ...) are 3-dim. arrays with last
% dimension equal to the number of structural parameters

% Copyright (C) 2011 Dynare Team
%
% This file is part of Dynare.
%
% Dynare is free software: you can redistribute it and/or modify
% it under the terms of the GNU General Public License as published by
% the Free Software Foundation, either version 3 of the License, or
% (at your option) any later version.
%
% Dynare is distributed in the hope that it will be useful,
% but WITHOUT ANY WARRANTY; without even the implied warranty of
% MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
% GNU General Public License for more details.
%
% You should have received a copy of the GNU General Public License
% along with Dynare.  If not, see <http://www.gnu.org/licen


    k = size(DT,3);                                 % number of structural parameters
    smpl = size(Y,2);                               % Sample size.
    mm   = size(T,2);                               % Number of state variables.
    a    = zeros(mm,1);                             % State vector.
    Om   = R*Q*transpose(R);                        % Variance of R times the vector of structural innovations.
    t    = 0;                                       % Initialization of the time index.
    oldK = 0;
    notsteady   = 1;                                % Steady state flag.
    F_singular  = 1;

    AHess  = zeros(k,k);                             % Initialization of the Hessian
    Da    = zeros(mm,k);                             % State vector.
    Dv = zeros(length(mf),k);
    
%     for ii = 1:k
%         DOm = DR(:,:,ii)*Q*transpose(R) + R*DQ(:,:,ii)*transpose(R) + R*Q*transpose(DR(:,:,ii)); 
%     end
    
    while notsteady & t<smpl
        t  = t+1;
        v  = Y(:,t)-a(mf);
        F  = P(mf,mf) + H;
        if rcond(F) < kalman_tol
            if ~all(abs(F(:))<kalman_tol)
                return
            else
                a = T*a;
                P = T*P*transpose(T)+Om;
            end
        else
            F_singular = 0;
            iF     = inv(F);
            K      = P(:,mf)*iF;

            [DK,DF,DP1] = computeDKalman(T,DT,DOm,P,DP,DH,mf,iF,K);
            
            		for ii = 1:k
                        Dv(:,ii)   = -Da(mf,ii) - DYss(mf,ii);
                        Da(:,ii)   = DT(:,:,ii)*(a+K*v) + T*(Da(:,ii)+DK(:,:,ii)*v + K*Dv(:,ii));
                    end
                    vecDPmf = reshape(DP(mf,mf,:),[],k);
                    iPmf = inv(P(mf,mf));
                    if t>=start
                        AHess = AHess + Dv'*iPmf*Dv + .5*(vecDPmf' * kron(iPmf,iPmf) * vecDPmf);
                    end
            a      = T*(a+K*v);                   
            P      = T*(P-K*P(mf,:))*transpose(T)+Om;
            DP     = DP1;
        end
        notsteady = max(max(abs(K-oldK))) > riccati_tol;
        oldK = K;
    end

    if F_singular
        error('The variance of the forecast error remains singular until the end of the sample')
    end

    
    if t < smpl
        t0 = t+1;
        while t < smpl
            t = t+1;
            v = Y(:,t)-a(mf);
                  	for ii = 1:k
                        Dv(:,ii)   = -Da(mf,ii)-DYss(mf,ii);
                        Da(:,ii)   = DT(:,:,ii)*(a+K*v) + T*(Da(:,ii)+DK(:,:,ii)*v + K*Dv(:,ii));
                    end
             if t>=start
                AHess = AHess + Dv'*iPmf*Dv; 
             end   
            a = T*(a+K*v);
        end
        AHess = AHess + .5*(smpl+t0-1)*(vecDPmf' * kron(iPmf,iPmf) * vecDPmf);
%         for ii = 1:k;
%             for jj = 1:ii
%              H(ii,jj) = trace(iPmf*(.5*DP(mf,mf,ii)*iPmf*DP(mf,mf,jj) + Dv(:,ii)*Dv(:,jj)'));
%             end
%         end
    end    
    
AHess = -AHess;  
% end of main function    
    
function [DK,DF,DP1] = computeDKalman(T,DT,DOm,P,DP,DH,mf,iF,K)

            k      = size(DT,3);
            tmp    = P-K*P(mf,:);

for ii = 1:k
    DF(:,:,ii)  = DP(mf,mf,ii) + DH(:,:,ii); 
    DiF(:,:,ii) = -iF*DF(:,:,ii)*iF;
    DK(:,:,ii)  = DP(:,mf,ii)*iF + P(:,mf)*DiF(:,:,ii);
    Dtmp        = DP(:,:,ii) - DK(:,:,ii)*P(mf,:) - K*DP(mf,:,ii);
    DP1(:,:,ii) = DT(:,:,ii)*tmp*T' + T*Dtmp*T' + T*tmp*DT(:,:,ii)' + DOm(:,:,ii);
end

% end of computeDKalman