## 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} =} sysmult (@var{Asys}, @var{Bsys})
## Compute @math{sys = Asys*Bsys} (series connection):
## @example
## @group
## u   ----------     ----------
## --->|  Bsys  |---->|  Asys  |--->
##     ----------     ----------
## @end group
## @end example
## A warning occurs if there is direct feed-through from an input 
## or a continuous state of @var{Bsys}, through a discrete output 
## of @var{Bsys}, to a continuous state or output in @var{Asys}
## (system data structure does not recognize discrete inputs).
## @end deftypefn

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

function sys = sysmult (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(ph != mg)
      error ("arg %d has %d outputs; arg %d has %d inputs", kk, ph, kk-1, mg);
    endif
    [n, nz, mg, pg, Gyd] = sysdimensions (arglist{kk});   # for next iteration
  endfor

  ## perform the multiply
  if (nargin == 2)
    Asys = arglist{1};
    Bsys = arglist{2};

    [An, Anz, Am, Ap] = sysdimensions (Asys);
    [Bn, Bnz, Bm, Bp] = sysdimensions (Bsys);

    [Aa, Ab, Ac, Ad, Atsam, An, Anz, ...
     Astname, Ainname, Aoutname, Ayd] = sys2ss(Asys);

    [Ba, Bb, Bc, Bd, Btsam, Bn, Bnz, ...
     Bstname, Binname, Boutname, Byd] = sys2ss(Bsys);

    if (Byd)
      ## check direct feed-through of inputs through discrete outputs
      alist = find (Byd);
      if (An)
        bd = Ab(1:An) * Bd(alist,:);
        if (norm (bd, 1))
          warning ("sysmult: inputs -> Bsys discrete outputs -> continuous states of Asys");
        endif
      endif
      ## check direct feed-through of continuous state through discrete outputs
      if (Bn)
        bc = Ab(1:An) * Bc(alist,1:(Bn));
        if (norm (bc, 1))
          warning ("sysmult: Bsys states -> Bsys discrete outputs -> continuous states of Asys");
        endif
      endif
    endif

    ## change signal names to avoid spurious warnings from sysgroup
    Asys = syssetsignals (Asys, "in",
			  __sysdefioname__ (Am, "A_sysmult_tmp_name"));

    Bsys = syssetsignals (Bsys, "out",
			  __sysdefioname__ (Bp, "B_sysmult_tmp_name"));

    sys = sysgroup (Asys, Bsys);

    ## connect outputs of B to inputs of A
    sys = sysconnect (sys, Ap+(1:Bp), 1:Am);

    ## now keep only  outputs of A and inputs of B
    sys = sysprune (sys, 1:Ap, Am+(1:Bm));

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

endfunction