## Copyright (C) 1996, 1998, 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} {} sysgroup (@var{asys}, @var{bsys})
## Combines two systems into a single system
##
## @strong{Inputs}
## @var{asys}, @var{bsys}: system data structures
##
## @strong{Outputs}
## @math{sys = @r{block diag}(asys,bsys)}
## @example
## @group
##          __________________
##          |    ________    |
## u1 ----->|--> | asys |--->|----> y1
##          |    --------    |
##          |    ________    |
## u2 ----->|--> | bsys |--->|----> y2
##          |    --------    |
##          ------------------
##               Ksys
## @end group
## @end example
## The function also rearranges the internal state-space realization of @var{sys}
## so that the
## continuous states come first and the discrete states come last.
## If there are duplicate names, the second name has a unique suffix appended
## on to the end of the name.
## @end deftypefn

## Author: A. S. Hodel <a.s.hodel@eng.auburn.edu>
## Created: August 1995
## modified by John Ingram July 1996
## A. S. Hodel: modified for variable number of arguments 1999

function sys = sysgroup (varargin)

  save_emp = empty_list_elements_ok;
  empty_list_elements_ok = 1;

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

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

  if(nargin == 2)
    ## the usual case; group the two systems together
    Asys = nth(arglist,1);
    Bsys = nth(arglist,2);

    ## extract information from Asys, Bsys to consruct sys
    Asys = sysupdate(Asys,"ss");
    Bsys = sysupdate(Bsys,"ss");
    [n1,nz1,m1,p1] = sysdimensions(Asys);
    [n2,nz2,m2,p2] = sysdimensions(Bsys);
    [Aa,Ab,Ac,Ad,Atsam,An,Anz,Ast,Ain,Aout,Ayd] = sys2ss(Asys);
    [Ba,Bb,Bc,Bd,Btsam,Bn,Bnz,Bst,Bin,Bout,Byd] = sys2ss(Bsys);
    nA = An + Anz;
    nB = Bn + Bnz;

    if(p1*m1*p2*m2 == 0)
      error("sysgroup: argument lacks inputs and/or outputs");

    elseif((Atsam + Btsam > 0) & (Atsam * Btsam == 0) )
      warning("sysgroup: creating combination of continuous and discrete systems")

    elseif(Atsam != Btsam)
      error("sysgroup: Asys.tsam=%e, Bsys.tsam =%e", Atsam, Btsam);
    endif

    A = [Aa,zeros(nA,nB); zeros(nB,nA),Ba];
    B = [Ab,zeros(nA,m2); zeros(nB,m1),Bb];
    C = [Ac,zeros(p1,nB); zeros(p2,nA),Bc];
    D = [Ad,zeros(p1,m2); zeros(p2,m1),Bd];
    tsam = max(Atsam,Btsam);

    ## construct combined signal names; stnames must check for pure gain blocks
    if(isempty(Ast))
      stname = Bst;
    elseif(isempty(Bst))
      stname = Ast;
    else
      stname  = append(Ast, Bst);
    endif
    inname  = append(Ain, Bin);
    outname = append(Aout,Bout);

    ## Sort states into continous first, then discrete
    dstates = ones(1,(nA+nB));
    if(An)
      dstates(1:(An)) = zeros(1,An);
    endif
    if(Bn)
      dstates((nA+1):(nA+Bn)) = zeros(1,Bn);
    endif
    [tmp,pv] = sort(dstates);
    A = A(pv,pv);
    B = B(pv,:);
    C = C(:,pv);
    stname = stname(pv);

    ## check for duplicate signal names
    inname = __sysgroupn__ (inname, "input");
    stname = __sysgroupn__ (stname, "state");
    outname = __sysgroupn__ (outname, "output");

    ## mark discrete outputs
    outlist = find([Ayd, Byd]);

    ## build new system
    sys = ss2sys(A,B,C,D,tsam,An+Bn,Anz+Bnz,stname,inname,outname);

  else
    ## multiple systems (or a single system); combine together one by one
    sys = nth(arglist,1);
    for kk=2:length(arglist)
      printf("sysgroup: kk=%d\n",kk);
      sys = sysgroup(sys,nth(arglist,kk));
    endfor
  endif

  empty_list_elements_ok = save_emp;

endfunction