## Copyright (C) 1996, 1999 Auburn University.  All rights reserved.
##
## This file is part of Octave.
##
## Octave 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 2, or (at your option) any
## later version.
##
## Octave 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 Octave; see the file COPYING.  If not, write to the Free
## Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
## 02110-1301 USA.

## -*- texinfo -*-
## @deftypefn {Function File} {}  sysadd (@var{gsys}, @var{hsys})
## returns @var{sys} = @var{gsys} + @var{hsys}.
## @itemize @bullet
## @item Exits with
## an error if @var{gsys} and @var{hsys} are not compatibly dimensioned.
## @item Prints a warning message is system states have identical names;
## duplicate names are given a suffix to make them unique.
## @item @var{sys} input/output names are taken from @var{gsys}.
## @end itemize
## @example
## @group
##           ________
##      ----|  gsys  |---
## u   |    ----------  +|
## -----                (_)----> y
##     |     ________   +|
##      ----|  hsys  |---
##           --------
## @end group
## @end example
## @end deftypefn

## Author: John Ingram <ingraje@eng.auburn.edu>
## Created: July 1996
## Updated for variable number of arguments July 1999 A. S. Hodel

function sys = sysadd (varargin)

  if(nargin < 1)
    usage("sysadd: sys = sysysadd(Gsys{,Hsys, ...})");
  endif

  ## collect all arguments
  arglist = {};
  for kk=1:nargin
    arglist{kk} = varargin{kk};
    if(!isstruct(arglist{kk}))
      error("sysadd: argument %d is not a data structure",kk);
    endif
  endfor

  ## check system dimensions
  [n,nz,mg,pg,Gyd] = sysdimensions(arglist{1});
  for kk=2:nargin
    [n,nz,mh,ph,Hyd] = sysdimensions(arglist{kk});
    if(mg != mh)
      error("arg 1 has %d inputs; arg %d has vs %d inputs",mg,kk,mh);
    elseif(pg != ph)
      error("arg 1 has %d outputs; arg %d has vs %d outputs",pg,kk,ph);
    elseif(norm(Gyd - Hyd))
      warning("cannot add a discrete output to a continuous output");
      error("Output type mismatch: arguments 1 and %d\n",kk);
    endif
  endfor

  ## perform the add
  if (nargin == 2)
    Gsys = arglist{1};
    Hsys = arglist{2};

    # check if adding scalar transfer functions with identical denoms
    [Gn, Gnz, Gm, Gp] = sysdimensions(Gsys);
    [Hn, Hnz, Hm, Hp] = sysdimensions(Hsys);
    if ( Gm ==1 & Gp == 1 & Hm == 1 & Hp == 1 & Gn == Hn & Gnz == Hnz )
      # dimensions are compatible, check if can add
      [Gnum,Gden,GT,Gin,Gout] = sys2tf(Gsys);
      [Hnum,Hden,HT,Hin,Hout] = sys2tf(Hsys);
      if (length(Hden) == length(Gden) )
        if( (Hden == Gden) & (HT == GT) )
          sys = tf(Gnum+Hnum,Gden,GT,Gin,Gout);

          return;   # return prematurely since the add is done.
        endif
      endif
    endif

    ## make sure in ss form
    Gsys = sysupdate(Gsys,"ss");
    Hsys = sysupdate(Hsys,"ss");
    Gin = sysgetsignals(Gsys,"in");
    Gout = sysgetsignals(Gsys,"out");
    Hin = sysgetsignals(Hsys,"in");
    Hout = sysgetsignals(Hsys,"out");

    ## change signal names to avoid warning messages from sysgroup
    Gsys = syssetsignals(Gsys,"in",__sysdefioname__(length(Gin),"Gin_u"));
    Gsys = syssetsignals(Gsys,"out",__sysdefioname__(length(Gout),"Gout_u"));
    Hsys = syssetsignals(Hsys,"in",__sysdefioname__(length(Hin),"Hin_u"));
    Hsys = syssetsignals(Hsys,"out",__sysdefioname__(length(Hout),"Hout_u"));

    sys = sysgroup(Gsys,Hsys);

    eyin = eye(mg);
    eyout = eye(pg);

    sys = sysscale(sys,[eyout, eyout],[eyin;eyin],Gout,Gin);

  else
    ## multiple systems (or a single system); combine together one by one
    sys = arglist{1};
    for kk=2:length(arglist)
      sys = sysadd(sys,arglist{kk});
    endfor
  endif

endfunction