# 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, 675 Mass Ave, Cambridge, MA 02139, USA. 
 
function [dsys,Adc,Cdc] = sysdisc(sys)
# function [dsys,Adc,Cdc] = sysdisc(sys)
# inputs: sys = system data structure
# outputs:
#    dsys: purely discrete portion of sys (returned empty if there is
#          no purely discrete path from inputs to outputs)
#    Adc, Cdc: connections from continuous states to discrete states/discrete
#    outputs, respectively.
#

  save_val = implicit_str_to_num_ok;	# save for later
  save_empty = empty_list_elements_ok;
  empty_list_elements_ok = implicit_str_to_num_ok = 1;


  if (nargin != 1)
    usage("[dsys,Adc,Cdc] = sysdisc(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
  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
  implicit_str_to_num_ok = save_val;	# restore value
  empty_list_elements_ok = save_empty;

endfunction