## Copyright (C) 1996, 1999, 2000, 2002, 2003, 2004, 2005, 2006, 2007
##               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 3 of the License, or (at
## your option) any later version.
##
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## 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, see
## <http://www.gnu.org/licenses/>.

## -*- 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)
    print_usage ();
  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;
        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