## 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, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
## 02110-1301 USA.

## -*- texinfo -*-
## @deftypefn {Function File} {[@var{csys}, @var{acd}, @var{ccd}] =} syscont (@var{sys})
## Extract the purely continuous subsystem of an input system.
##
## @strong{Input}
## @table @var
## @item sys
## system data structure.
## @end table
##
## @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_warn_empty_list_elements = warn_empty_list_elements;
  unwind_protect
    warn_empty_list_elements = 0;

    if (nargin != 1)
      usage("[csys,Acd,Ccd,Dcd] = syscont(sys)");
    elseif (!isstruct(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 = ss(Acc,Bcc,Ccc,Dcc,0,sys_n,0,stname,inname,outname);

  unwind_protect_cleanup
    warn_empty_list_elements = save_warn_empty_list_elements;
  end_unwind_protect

endfunction