## Copyright (C) 1996, 2000, 2002, 2004, 2005, 2006, 2007
##               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 3 of the License, or (at
## your option) any later version.
##
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## 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, see
## <http://www.gnu.org/licenses/>.

## -*- texinfo -*-
## @deftypefn {Function File} {[@var{y}, @var{x}] =} lsim (@var{sys}, @var{u}, @var{t}, @var{x0})
## Produce output for a linear simulation of a system; produces 
## a plot for the output of the system, @var{sys}.
##
## @var{u} is an array that contains the system's inputs.  Each row in @var{u}
## corresponds to a different time step.  Each column in @var{u} corresponds to a
## different input.  @var{t} is an array that contains the time index of the
## system; @var{t} should be regularly spaced.  If initial conditions are required
## on the system, the @var{x0} vector should be added to the argument list.
##
## When the lsim function is invoked a plot is not displayed; 
## however, the data is returned in @var{y} (system output)
## and @var{x} (system states).
## @end deftypefn

## Author: David Clem
## Author: A. S. Hodel <a.s.hodel@eng.auburn.edu>
## Created: July 1995
## modified by John Ingram for system format August 1996

function [y, x] = lsim (sys, u, t, x0)

  if (nargin < 3 || nargin > 4)
    print_usage ();
  endif

  if (! isstruct (sys))
    error ("sys must be in system data structure");
  endif

  sys = sysupdate (sys,"ss");

  [ncstates, ndstates, nin, nout] = sysdimensions (sys);
  [a, b, c, d] = sys2ss (sys);

  if (nargin == 3)
    x0 = zeros (columns (a), 1);
  endif

  if (rows (u) != length (t))
    error ("lsim: There should be an input value (row) for each time instant");
  endif
  if (columns (u) != columns (d))
    error ("lsim: U and d should have the same number of inputs");
  endif
  if (columns (x0) > 1)
    error ("lsim: Initial condition vector should have only one column");
  endif
  if (rows (x0) > rows (a))
    error ("lsim: Initial condition vector is too large");
  endif

  Ts = 0;
  t(2)-t(1);
  u=u';
  n = max (size (t));

  for ii = 1:(n-1)

    ## check if step size changed
    ## FIXME -- this is probably not the best test, but it is
    ## better than a test for exact equality.
    if (abs (t(ii+1) - t(ii) - Ts) > 10 * eps)
      Ts = t(ii+1) - t(ii);
      ## [F,G] = c2d(a,b,Ts);
      dsys = c2d (sys, Ts);
      [F, G] = sys2ss (dsys);
    endif

    x(:,ii) = x0;
    x0 = F*x0 + G*u(:,ii);
  endfor

  ## pick up last point
  x(:,n) = x0;

  y = c*x + d*u;
  if (nargout == 0)
   plot (t, y);
   y = [];
   x = [];
  endif

endfunction