## Copyright (C) 1995, 1996  Kurt Hornik
##
## 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.
##
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## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
## General Public License for more details.
##
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## along with Octave; see the file COPYING.  If not, write to the Free
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## 02110-1301, USA.

## -*- texinfo -*-
## @deftypefn {Function File} {} cross (@var{x}, @var{y}, @var{dim})
## Computes the vector cross product of the two 3-dimensional vectors
## @var{x} and @var{y}.
##
## @example
## @group
## cross ([1,1,0], [0,1,1])
##      @result{} [ 1; -1; 1 ]
## @end group
## @end example
##
## If @var{x} and @var{y} are matrices, the cross product is applied 
## along the first dimension with 3 elements. The optional argument 
## @var{dim} is used to force the cross product to be calculated along
## the dimension defined by @var{dim}.
## @end deftypefn

## Author: Kurt Hornik <Kurt.Hornik@wu-wien.ac.at>
## Created: 15 October 1994
## Adapted-By: jwe

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

  if (ndims (x) < 3 && ndims (y) < 3 && nargin < 3)
    ## COMPATIBILITY -- opposite behaviour for cross(row,col)
    ## Swap x and y in the assignments below to get the matlab behaviour.
    ## Better yet, fix the calling code so that it uses conformant vectors.
    if (columns (x) == 1 && rows (y) == 1)
      warning ("cross: taking cross product of column by row");
      y = y.';
    elseif (rows (x) == 1 && columns (y) == 1)
      warning ("cross: taking cross product of row by column");
      x = x.';
    endif
  endif

  if (nargin == 2)
     dim = find (size (x) == 3, 1);
     if (isempty (dim)) 
       error ("cross: must have at least one dimension with 3 elements");
     endif
   else
     if (size (x) != 3)
       error ("cross: dimension dim must have 3 elements");
     endif
  endif

  nd = ndims (x);
  sz = size (x);
  idx1 = cell (1, nd);
  for i = 1:nd
    idx1{i} = 1:sz(i);
  endfor
  idx2 = idx3 = idx1;
  idx1(dim) = 1;
  idx2(dim) = 2;
  idx3(dim) = 3;

  if (size (x) == size (y))
    z = cat (dim, 
	     (x(idx2{:}) .* y(idx3{:}) - x(idx3{:}) .* y(idx2{:})),
             (x(idx3{:}) .* y(idx1{:}) - x(idx1{:}) .* y(idx3{:})),
             (x(idx1{:}) .* y(idx2{:}) - x(idx2{:}) .* y(idx1{:})));
  else
    error ("cross: x and y must have the same dimensions");
  endif

endfunction