## 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{dsys}, @var{adc}, @var{cdc}] =} sysdisc (@var{sys})
##
## @strong{Inputs}
## @var{sys} = system data structure
##
## @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_empty = empty_list_elements_ok;
  empty_list_elements_ok = 1;

  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 = ss2sys(Add,Bdd,Cdd,Ddd,tsam,0,nz,stname,inname,outname,outlist);
  else
    dsys=[];
  endif

  empty_list_elements_ok = save_empty;

endfunction