## Copyright (C) 1993, 1994, 1995 John W. Eaton
##
## 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} {} dcgain (@var{sys}, @var{tol})
## Returns dc-gain matrix. If dc-gain is infinite
## an empty matrix is returned.
## The argument @var{tol} is an optional tolerance for the condition
## number of the @math{A} Matrix in @var{sys} (default @var{tol} = 1.0e-10)
## @end deftypefn

## Author: Kai P. Mueller <mueller@ifr.ing.tu-bs.de>
## Created: October 1, 1997

function gm = dcgain (sys, tol)

  if((nargin < 1) || (nargin > 2) || (nargout > 1))
    print_usage ();
  endif
  if(!isstruct(sys))
    error("dcgain: first argument is not a system data structure.")
  endif
  sys = sysupdate(sys, "ss");
  [aa,bb,cc,dd] = sys2ss(sys);
  if (is_digital(sys))  aa = aa - eye(size(aa));  endif
  if (nargin == 1)  tol = 1.0e-10;  endif
  r = rank(aa, tol);
  if (r < rows(aa))
    gm = [];
  else
    gm = -cc / aa * bb + dd;
  endif
  if(!is_stable(sys))
    [nn,nz,mm,pp] = sysdimensions(sys);
    warning("dcgain: unstable system; dimensions [nc=%d,nz=%d,mm=%d,pp=%d]", ...
      nn,nz,mm,pp);
  endif

endfunction