# Copyright (C) 1997 Kai P. Mueller
#
# 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 Qs = ctrb(sys, b)
  # ------------------------------------------------------
  # Qs = ctrb(sys [, b])
  # Build controllability matrix
  #
  #                  2       n-1
  #     Qs = [ B AB A B ... A   B
  #
  # of a system data structure or the pair (A, B).
  #
  # Note: ctrb() forms the controllability matrix.
  #       The numerical properties of is_controllable()
  #       are much better for controllability tests.
  # See also: obsv, is_observable, is_controllable
  # ------------------------------------------------------

  # Written by Kai P. Mueller November 4, 1997
  # based on is_controllable.m of Scottedward Hodel
  # modified by

  if (nargin == 2)
    a = sys;
  elseif (nargin == 1 && is_struct(sys))
    sysupdate(sys,"ss");
    [a,b] = sys2ss(sys);
  else
    usage("ctrb(sys [, b])")
  endif

  if (!is_abcd(a,b))
    Qs = [];
  else
    # no need to check dimensions, we trust is_abcd().
    [na, ma] = size(a);
    # using imb avoids name conflict with the "mb" function
    [inb, imb] = size(b);
    Qs = zeros(na, ma*imb);
    for i = 1:na
      Qs(:, (i-1)*imb+1:i*imb) = b;
      b = a * b;
    endfor
  endif
endfunction