## Copyright (C) 1996, 2000, 2002, 2003, 2004, 2005, 2006, 2007
##               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 3 of the License, 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, see
## <http://www.gnu.org/licenses/>.

## -*- 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)

  if (nargin != 1)
    print_usage ();
  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 continuous 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

endfunction