/*

Copyright (C) 1996, 1997 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.

*/

#ifdef HAVE_CONFIG_H
#include <config.h>
#endif

#include "CmplxHESS.h"
#include "dbleHESS.h"

#include "defun-dld.h"
#include "error.h"
#include "gripes.h"
#include "oct-obj.h"
#include "utils.h"

DEFUN_DLD (hess, args, nargout,
  "-*- texinfo -*-\n\
@deftypefn {Loadable Function} {@var{h} =} hess (@var{a})\n\
@deftypefnx {Loadable Function} {[@var{p}, @var{h}] =} hess (@var{a})\n\
@cindex Hessenberg decomposition\n\
Compute the Hessenberg decomposition of the matrix @var{a}.\n\
\n\
The Hessenberg decomposition is usually used as the first step in an\n\
eigenvalue computation, but has other applications as well (see Golub,\n\
Nash, and Van Loan, IEEE Transactions on Automatic Control, 1979).  The\n\
Hessenberg decomposition is\n\
@iftex\n\
@tex\n\
$$\n\
A = PHP^T\n\
$$\n\
where $P$ is a square unitary matrix ($P^HP = I$), and $H$\n\
is upper Hessenberg ($H_{i,j} = 0, \\forall i \\ge j+1$).\n\
@end tex\n\
@end iftex\n\
@ifinfo\n\
@code{p * h * p' = a} where @code{p} is a square unitary matrix\n\
(@code{p' * p = I}, using complex-conjugate transposition) and @code{h}\n\
is upper Hessenberg (@code{i >= j+1 => h (i, j) = 0}).\n\
@end ifinfo\n\
@end deftypefn")
{
  octave_value_list retval;

  int nargin = args.length ();

  if (nargin != 1 || nargout > 2)
    {
      print_usage ();
      return retval;
    }

  octave_value arg = args(0);

  octave_idx_type nr = arg.rows ();
  octave_idx_type nc = arg.columns ();

  int arg_is_empty = empty_arg ("hess", nr, nc);

  if (arg_is_empty < 0)
    return retval;
  else if (arg_is_empty > 0)
    return octave_value_list (2, Matrix ());

  if (nr != nc)
    {
      gripe_square_matrix_required ("hess");
      return retval;
    }

  if (arg.is_real_type ())
    {
      Matrix tmp = arg.matrix_value ();

      if (! error_state)
	{
	  HESS result (tmp);

	  if (nargout == 0 || nargout == 1)
	    {
	      retval.resize (1);
	      retval(0) = result.hess_matrix ();
	    }
	  else
	    {
	      retval.resize (2);
	      retval(0) = result.unitary_hess_matrix ();
	      retval(1) = result.hess_matrix ();
	    }
	}
    }
  else if (arg.is_complex_type ())
    {
      ComplexMatrix ctmp = arg.complex_matrix_value ();

      if (! error_state)
	{
	  ComplexHESS result (ctmp);

	  if (nargout == 0 || nargout == 1)
	    {
	      retval.resize (1);
	      retval(0) = result.hess_matrix ();
	    }
	  else
	    {
	      retval.resize (2);
	      retval(0) = result.unitary_hess_matrix ();
	      retval(1) = result.hess_matrix ();
	    }
	}
    }
  else
    {
      gripe_wrong_type_arg ("hess", arg);
    }

  return retval;
}

/*
;;; Local Variables: ***
;;; mode: C++ ***
;;; End: ***
*/
