## Copyright (C) 1996, 1999, 2000, 2002, 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.
##
## 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, see
## <http://www.gnu.org/licenses/>.

## -*- texinfo -*-
## @deftypefn {Function File} {@var{sys} =} syssub (@var{Gsys}, @var{Hsys})
## Return @math{sys = Gsys - Hsys}.
##
## @strong{Method}
##
## @var{Gsys} and @var{Hsys} are connected in parallel.
## The input vector is connected to both systems; the outputs are
## subtracted.  Returned system names are those of @var{Gsys}.
## @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 numbers of input arguments by July 1999 A. S. Hodel

function sys = syssub (varargin)

  if (nargin < 1)
    print_usage ();
  endif

  ## collect all arguments
  arglist = {};
  for kk = 1:nargin
    arglist{kk} = varargin{kk};
    if (! isstruct (arglist{kk}))
      error ("syssub: 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", kk);
    endif
  endfor

  ## perform the subtract
  if (nargin == 2)
    Gsys = arglist{1};
    Hsys = arglist{2};
    if (strcmp (sysgettype (Gsys), "tf") || strcmp (sysgettype (Hsys), "tf"))
      ## see if subtracting  transfer functions with identical denominators
      [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
        ## if not, we go on and do the usual thing...
      endif
    endif

    ## make sure in ss form
    Gsys = sysupdate (Gsys, "ss");
    Hsys = sysupdate (Hsys, "ss");

    ## 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 = syssub (sys, arglist{kk});
    endfor
  endif

endfunction