## Copyright (C) 1995, 1996  Kurt Hornik
##
## This program 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.
##
## This program 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 this file.  If not, write to the Free Software Foundation,
## 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.

## -*- texinfo -*-
## @deftypefn {Function File} {} cross (@var{x}, @var{y})
## Computes the vector cross product of the two 3-dimensional vectors
## @var{x} and @var{y}.
##
## A row vector is returned if @var{x} and @var{y} are both row vectors;
## otherwise, a column vector is returned.
##
## If @var{x} and @var{y} are two - dimensional matrices the
## cross product is applied along the first dimension with 3 elements.
##
## @example
## @group
## cross ([1,1,0], [0,1,1])
##      @result{} [ 1; -1; 1 ]
## @end group
## @end example
## @end deftypefn

## Author: KH <Kurt.Hornik@ci.tuwien.ac.at>
## Created: 15 October 1994
## Adapted-By: jwe

function z = cross (x, y)
	
  if (nargin != 2)
    usage ("cross (x, y)");
  endif
 
  if (is_vector(x) && is_vector(y))
    if (length (x) == 3 && length (y) == 3)

      z = [x(2)*y(3) - x(3)*y(2);\
           x(3)*y(1) - x(1)*y(3);\
           x(1)*y(2) - x(2)*y(1)];

      x_nr = rows (x);
      y_nr = rows (y);

      if (x_nr == y_nr && x_nr == 1)
        z = z.';
      endif

    else
      error ("cross: both x and y must be 3-dimensional vectors");
    endif
  elseif (size(x) == size(y))

    [xr,xc]=size(x);

    if (xr == 3)
      x=x';y=y';
	elseif (xc != 3)
      error ("cross: x and y must have one dimension with 3 elements")
    endif

	z = [x(:,2).*y(:,3) .- x(:,3).*y(:,2)\
         x(:,3).*y(:,1) .- x(:,1).*y(:,3)\
         x(:,1).*y(:,2) .- x(:,2).*y(:,1)];

    if (xr == 3)
      z=z';
    endif
  else
    error ("cross: for matrix arguments x and y must have same size");
  endif

endfunction