function [sys,x0,str,ts] = SSTS__tether_MMET_III_discrete_force_sfunction(t,x,u,flag,NN)

% /*M-FILE Function SSTS__tether_MMET_III_discrete_force_sfunction MMM SSTSLAB */
% /*==================================================================================================
%  Simple Space Tether Simulation Laboratory Toolbox for Matlab 7.x
%
%  Copyright 2007 The SxLAB Family - Yi Chen - leo.chen.yi@gmail.com
% ====================================================================================================
%File description:
%  this sfunction is to calculate the discrete force for each point masses
%        from 1~N ( MMET-III axial model with discrete point masses )
%
%Input:
%
%   t -- time                                         [Not modify]
%   x -- state of s funciton                          [Not modify]
%   u -- inputs vector , where                        [by user]
%               u(1) -- N, number of point masses ( 20 in default)
%               u(2) -- ro, tether density - kg/(m^3)
%               u(3) -- sectional_area,tether cross-section - m^2
%               u(4) -- length, tether length - m
%               u(5) -- mass of payload - kg
%               u(6) -- velocity_psi, angular velocity - rad/s
%   flag -- flat status                               [No modify]
%   option -- 1 ,'+'
%             2 ,'-'
%Output:
%   sys -- outputs vector        [by user]
%   x0  -- sfunction std output  [Not modify]
%   str -- sfunction std output  [Not modify]
%   ts  -- sfunction std output  [Not modify]
%
% Appendix comments:
%  follow the example of timestwo.m, provide by matlab
%
% Usage:
%
%
%===================================================================================================
%  See Also:         SSTS__tether_MMET_III_discrete_force_sfunction
%                    SSTS__tether_MMET_II_discrete_number_sfunction_II
%                    SSTS__tether_MMET_II_discrete_number_sfunction_III
%
%===================================================================================================
%
%===================================================================================================
%Revision -
%Date        Name    Description of Change email                 Location
%26-Jan-2007 Chen Yi Initial               chenyi2005@gmail.com  Glasgow
%HISTORY$
%==================================================================================================*/

% SSTS__tether_MMET_III_discrete_force_sfunction Begin

% The following outlines the general structure of an S-function.
switch flag,

    case 0,
        [sys,x0,str,ts] = mdlInitializeSizes( NN );

    case 3
        sys = mdlOutputs(t,x,u);

        %%%%%%%%%%%%%%%%%%%
        % Unhandled flags %
        %%%%%%%%%%%%%%%%%%%
        % There are no termination tasks (flag=9) to be handled.
        % Also, there are no continuous or discrete states,
        % so flags 1,2, and 4 are not used, so return an emptyu
        % matrix
    case { 1, 2, 4, 9 }
        sys=[];

    otherwise
        error(['Unhandled flag = ',num2str(flag)]);

end

% SSTS__tether_MMET_III_discrete_force_sfunction End


%=============================================================================
% mdlInitializeSizes
% Return the sizes, initial conditions, and sample times for the S-function.
%=============================================================================
%
function [sys,x0,str,ts]=mdlInitializeSizes( NN )

% %load data from mat file
% load('SSTS__tether_MMET_III_Point_Mass_Number.mat');
% %set variables according to the order in SSTS__tether_MMET_III.mdl
% % mat file always have simulation time as the first line
% % save('SSTS__tether_MMET_III_Point_Mass_Number.mat', 'N_number');
% NN = N_number(2,1);
% %with 2 end point
%Point Masses Number
% NN = 20;

sizes = simsizes;
sizes.NumContStates  = 0;
sizes.NumDiscStates  = 0;  % x
sizes.NumOutputs     = NN/2;  % sys
sizes.NumInputs      = 6;  % u
sizes.DirFeedthrough = 1;  %  if 0, not direct feedthrough, we need have flag = 2
%  if 1, has direct feedthrough, we dont need
%  flag = 2 ,
sizes.NumSampleTimes = 1;   % at least one sample time is needed

sys = simsizes(sizes);

str = [];     % str is always an empty matrix
x0  = []; % initialize the initial conditions
ts  = [-1 0]; % inherited sample time ,initialize the array of sample times
% end mdlInitializeSizes


%=============================================================================
% mdlOutputs
% Return the output vector for the S-function
%=============================================================================
%
function sys = mdlOutputs(t,x,u)
% [sys] =sum_point_mass(u,option);
[ sys ]= SSTS__tether_MMET_III_discrete_number_force(u(1),u(2),u(3),u(4),u(5),u(6));
%               u(1) -- N, number of point masses ( 20 in default)
%               u(2) -- ro, tether density - kg/(m^3)
%               u(3) -- sectional_area,tether cross-section - m^2
%               u(4) -- length, tether length - m
%               u(5) -- mass of payload - kg
%               u(6) -- velocity_psi, angular velocity - rad/s


% end mdlOutputs