## Copyright (C) 1996, 1999 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{retval} =} is_digital ( @var{sys})
## Return nonzero if system is digital;
##  inputs:
##    sys: system data structure
##    eflg: 0 [default] exit with an error if system is mixed (continuous and
##            discrete components)
##        : 1 print a warning if system is mixed (continuous and discrete)
##        : 2 silent operation
##  outputs:
##    DIGITAL:  0: system is purely continuous
##           :  1: system is purely discrete
##           : -1: system is mixed continuous and discrete
## Exits with an error of sys is a mixed (continuous and discrete) system
## @end deftypefn

function DIGITAL = is_digital(sys,eflg)

  ## a s hodel July 1996

  switch(nargin)
  case(1),  eflg = 0;
  case(2),  
    if( isempty(find(eflg == [0, 1, 2])) )
      error("Illegal value of eflg=%d (%e)",eflg,eflg);
    endif
  otherwise,
    usage("DIGITAL = is_digital(sys{,eflg})");
  endswitch

  ## checked for sampled data system (mixed)
  ## discrete system
  sysyd = sysgetsignals(sys,"yd");
  [nn,nz] = sysdimensions(sys);
  cont = sum(sysyd == 0) + nn;
  tsam = sysgettsam(sys);
  dig = sum(sysyd != 0) + nz + tsam;

  ## check for mixed system
  if( cont*dig != 0)
   switch(eflg)
   case(0),
     error("continuous/discrete system; use syscont, sysdisc, or c2d first");
   case(1),
     warning("is_digital: mixed continuous/discrete system");
   endswitch
   dig_sign = -1;
  else
   dig_sign = 1;
  endif

  DIGITAL = dig_sign*(tsam > 0);
 
endfunction