## Copyright (C) 1999 David M. Doolin <doolin@ce.berkeley.edu>
##
## 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.

## -*- texinfo -*-
## @deftypefn {Function File} {} polyarea (@var{x}, @var{y})
## @deftypefnx {Function File} {} polyarea (@var{x}, @var{y}, @var{dim})
##
## Determines area of a polygon by triangle method. The variables
## @var{x} and @var{y} define the vertex pairs, and must therefore have
## the same shape. They can be either vectors or arrays. If they are
## arrays then the columns of @var{x} and @var{y} are treated separately
## and an area returned for each.
##
## If the optional @var{dim} argument is given, then @code{polyarea}
## works along this dimension of the arrays @var{x} and @var{y}.
##
## @end deftypefn

## todo:  Add moments for centroid, etc.
##
## bugs and limitations:  
##        Probably ought to be an optional check to make sure that
##        traversing the vertices doesn't make any sides cross 
##        (Is simple closed curve the technical definition of this?). 

## Author: David M. Doolin <doolin@ce.berkeley.edu>
## Date: 1999-04-14
## Modified-by: 
##    2000-01-15 Paul Kienzle <pkienzle@kienzle.powernet.co.uk>
##    * use matlab compatible interface
##    * return absolute value of area so traversal order doesn't matter
##    2005-10-13 Torsten Finke
##    * optimization saving half the sums and multiplies

function a = polyarea (x, y, dim)
  if (nargin != 2 && nargin != 3)
    print_usage ();
  elseif (size_equal (x, y))
    if (nargin == 2)
      a = abs (sum (x .* (shift (y, -1) - shift (y, 1)))) / 2;
    else
      a = abs (sum (x .* (shift (y, -1, dim) - shift (y, 1, dim)), dim)) / 2;
    endif
  else
    error ("polyarea: x and y must have the same shape");
  endif
endfunction

%!shared x, y
%! x = [1;1;3;3;1];
%! y = [1;3;3;1;1];
%!assert (polyarea(x,y), 4, eps)
%!assert (polyarea([x,x],[y,y]), [4,4], eps)
%!assert (polyarea([x,x],[y,y],1), [4,4], eps)
%!assert (polyarea([x,x]',[y,y]',2), [4;4], eps)