## 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{dsys}, @var{adc}, @var{cdc}] =} sysdisc (@var{sys})
##
## @strong{Input}
## @table @var
## @item sys
## System data structure.
## @end table
##
## @strong{Outputs}
## @table @var
## @item dsys
## Purely discrete portion of sys (returned empty if there is
## no purely discrete path from inputs to outputs).
## @item    adc
## @itemx   cdc
## Connections from continuous states to discrete states and discrete.
## outputs, respectively.
## @end table
## @end deftypefn

function [dsys, Adc, Cdc] = sysdisc (sys)

  save_warn_empty_list_elements = warn_empty_list_elements;
  unwind_protect
    warn_empty_list_elements = 0;

    if (nargin != 1)
      usage("[dsys,Adc,Cdc] = sysdisc(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
    Add = Adc = Bdd = Cdd = Cdc = Ddd = [];

    if(isempty(st_d) & isempty(y_d))
      error("sysdisc: expecting discrete states and/or continous outputs");
    elseif (isempty(st_d))
      warning("sysdisc: no discrete states");
    elseif(isempty(y_d))
      warning("sysdisc: no discrete outputs");
    endif

    [aa,bb,cc,dd] = sys2ss(sys);
    if(!isempty(st_d) )
      Add = aa( st_d , st_d);
      stname = sysgetsignals(sys,"st",st_d);
      Bdd = bb( st_d , :);
      if(!isempty(st_c))
	Adc = aa( st_d , st_c);
      endif
      if(!isempty(y_d))
	Cdd = cc(y_d , st_d);
      endif
    else
      stname = [];
    endif
    if(!isempty(y_d))
      Ddd = dd(y_d , :);
      outname = sysgetsignals(sys,"out",y_d);
      if(!isempty(st_c))
	Cdc = cc(y_d , st_c);
      endif
    else
      outname=[];
    endif
    inname = sysgetsignals(sys,"in");
    outlist = 1:rows(outname);

    if(!isempty(outname))
      tsam = sysgettsam(sys);
      [nc,nz] = sysdimensions(sys);
      dsys = ss(Add,Bdd,Cdd,Ddd,tsam,0,nz,stname,inname,outname,outlist);
    else
      dsys=[];
    endif

  unwind_protect_cleanup
    warn_empty_list_elements = save_warn_empty_list_elements;
  end_unwind_protect

endfunction