## Copyright (C) 1996 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{csys}, @var{Acd}, @var{Ccd}] = } syscont (@var{sys})
## Extract the purely continuous subsystem of an input system.
## 
## @strong{Inputs}
## @var{sys} is a system data structure
## 
## @strong{Outputs}
## @table @var
## @item csys
##  is the purely continuous input/output connections of @var{sys}
## @item Acd
## @itemx Ccd
##  connections from discrete states to continuous states,
##                discrete states to continuous outputs, respectively.
## 
##  returns @var{csys} empty if no continuous/continous path exists
## @end table
## 
## @end deftypefn
 
## Author: John Ingram <ingraje@eng.auburn.edu>
## Created: August 1996

function [csys, Acd, Ccd] = syscont (sys)

  save_empty = empty_list_elements_ok;
  empty_list_elements_ok = 1;

  if (nargin != 1)
    usage("[csys,Acd,Ccd,Dcd] = syscont(sys)");
  elseif (!is_struct(sys))
    error("sys must be in system data structure form");
  endif

  sys = sysupdate(sys,"ss");
  [n_tot,st_c,st_d,y_c,y_d] = syscont_disc(sys);	# get ranges

  ## assume there's nothing there; build partitions as appropriate
  Acc = Acd = Bcc = Ccc = Ccd = Dcc = [];

  if(isempty(st_c) & isempty(y_c))
    error("syscont: expecting continous states and/or continous outputs");
  elseif (isempty(st_c))
    warning("syscont: no continuous states");
  elseif(isempty(y_c))
    warning("syscont: no continuous outputs");
  endif

  [sys_a, sys_b, sys_c, sys_d ] = sys2ss(sys);
  [sys_stname, sys_inname, sys_outname] = sysgetsignals(sys);
  [sys_n, sys_nz, sys_m, sys_p] = sysdimensions(sys);
  if(!isempty(st_c))
    Acc = sys_a(st_c,st_c);
    stname = sys_stname(st_c);
    Bcc = sys_b(st_c,:);
    Ccc = sys_c(y_c,st_c);
    Acd = sys_a(st_c,st_d);
  else
    stname=[];
  endif
  outname = sys_outname(y_c);
  Dcc = sys_d(y_c,:);
  Ccd = sys_c(y_c,st_d);
  inname = sys_inname;
  
  csys = ss2sys(Acc,Bcc,Ccc,Dcc,0,sys_n,0,stname,inname,outname);

  empty_list_elements_ok = save_empty;
 
endfunction