## Copyright (C) 1996, 1998 Auburn University.  All rights reserved.
##
## This file is part of Octave.
##
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## 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} {} __freqresp__ (@var{sys}, @var{USEW}, @var{w})
## Frequency response function - used internally by @command{bode}, @command{nyquist}.
## minimal argument checking; ``do not attempt to do this at home''.
##
## @strong{Inputs}
## @table @var
## @item sys
## system data structure
## @item USEW
## returned by @code{freqchkw}
## @item optional
## must be present if @var{USEW} is true (nonzero)
## @end table
## @strong{Outputs}
## @table @var
## @item @var{out}
## vector of finite @math{G(j*w)} entries (or @math{||G(j*w)||} for @acronym{MIMO})
## @item w
## vector of corresponding frequencies
## @end table
## @end deftypefn

## Author: R. Bruce Tenison <btenison@eng.auburn.edu>
## Created: July 11, 1994

function [ff, w] = __freqresp__ (sys, USEW, w);

  ## SYS_INTERNAL accesses members of system data structure

  save_warn_empty_list_elements = warn_empty_list_elements;
  unwind_protect
    warn_empty_list_elements = 0;

    ## Check Args
    if ((nargin < 2) || (nargin > 4))
      usage ("[ff, w] = __freqresp__ (sys, USEW, w)");
    elseif (USEW & (nargin < 3) )
      error ("USEW = 1 but w was not passed.");
    elseif (USEW & isempty(w))
      warning("USEW = 1 but w is empty; setting USEW=0");
      USEW = 0;
    endif

    DIGITAL = is_digital(sys);

    ## compute default w if needed
    if(!USEW)
      if(is_siso(sys))
	sys = sysupdate(sys,"zp");
	[zer,pol] = sys2zp(sys);
      else
	zer = tzero(sys);
	pol = eig(sys2ss(sys));
      endif

      ## get default frequency range
      [wmin,wmax] = bode_bounds(zer,pol,DIGITAL,sysgettsam(sys));
      w = logspace(wmin,wmax,50);
    else
      w = reshape(w,1,length(w));         # make sure it's a row vector
    endif

    ## now get complex values of s or z
    if(DIGITAL)
      jw = exp(i*w*sysgettsam(sys));
    else
      jw = i*w;
    endif

    [nn,nz,mm,pp] = sysdimensions(sys);

    ## now compute the frequency response - divide by zero yields a warning
    if (strcmp(sysgettype(sys),"zp"))
      ## zero-pole form (preferred)
      [zer,pol,sysk] = sys2zp(sys);
      ff = ones(size(jw));
      l1 = min(length(zer)*(1-isempty(zer)),length(pol)*(1-isempty(pol)));
      for ii=1:l1
	ff = ff .* (jw - zer(ii)) ./ (jw - pol(ii));
      endfor

      ## require proper  transfer function, so now just get poles.
      for ii=(l1+1):length(pol)
	ff = ff ./ (jw - pol(ii));
      endfor
      ff = ff*sysk;

    elseif (strcmp(sysgettype(sys),"tf"))
      ## transfer function form
      [num,den] = sys2tf(sys);
      ff = polyval(num,jw)./polyval(den,jw);
    elseif (mm==pp)
      ## The system is square; do state-space form bode plot
      [sysa,sysb,sysc,sysd,tsam,sysn,sysnz] = sys2ss(sys);
      n = sysn + sysnz;
      for ii=1:length(jw);
	ff(ii) = det(sysc*((jw(ii).*eye(n)-sysa)\sysb)+sysd);
      endfor;
    else
      ## Must be state space... bode
      [sysa,sysb,sysc,sysd,tsam,sysn,sysnz] = sys2ss(sys);
      n = sysn + sysnz;
      for ii=1:length(jw);
	ff(ii) = norm(sysc*((jw(ii)*eye(n)-sysa)\sysb)+sysd);
      endfor

    endif

    w = reshape(w,1,length(w));
    ff = reshape(ff,1,length(ff));

  unwind_protect_cleanup
    warn_empty_list_elements = save_warn_empty_list_elements;
  end_unwind_protect

endfunction