## 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, 59 Temple Place, Suite 330, Boston, MA 02111 USA. 

## -*- 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 of Bsys or a continuous state of Bsys through a discrete 
## output of Bsys to a continuous state or output in Asys (system data structure
## does not recognize discrete inputs).
## @end deftypefn

function sys = sysmult(...)

  ## Written by John Ingram July 1996
  ## updated for variable number of arguments by A. S. Hodel July 1999

  save_val = implicit_str_to_num_ok;	# save for later
  implicit_str_to_num_ok = 1;

  if(nargin < 1)
    usage("sysmult: sys = sysmult(Asys{,Bsys,...})");
  endif

  ## collect all arguments
  arglist = list();
  va_start();
  for kk=1:nargin
    arglist(kk) = va_arg();
    if(!is_struct(nth(arglist,kk)))
      error("sysadd: argument %d is not a data structure",kk);
    endif
  endfor

  ## check system dimensions
  [n,nz,mg,pg,Gyd] = sysdimensions(nth(arglist,1));
  for kk=2:nargin
    [n,nz,mh,ph,Hyd] = sysdimensions(nth(arglist,kk));
    if(mh != pg)
      error("arg %d has %d outputs; arg %d has vs %d inputs",kk,ph,kk-1,mg);
    endif
    [n,nz,mg,pg,Gyd] = sysdimensions(nth(arglist,kk));   # for next iteration
  endfor

  ## perform the multiply
  if(nargin == 2)
    Asys = nth(arglist,1);   Bsys = nth(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 -> continous 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 = nth(arglist,1);
    for kk=2:length(arglist)
      sys = sysmult(sys,nth(arglist,kk));
    endfor
  endif
  implicit_str_to_num_ok = save_val;	# restore value  
endfunction