function d = inv2x2(x)

% INV2X2 computes inverse of matrix x, using explicit analytic definition
% if size(x,1) < 4, otherwise use MATLAB inv-function

% Copyright (C) 2012, Donders Centre for Cognitive Neuroimaging, Nijmegen, NL
%
% This file is part of FieldTrip, see http://www.fieldtriptoolbox.org
% for the documentation and details.
%
%    FieldTrip 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 3 of the License, or
%    (at your option) any later version.
%
%    FieldTrip 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 FieldTrip. If not, see <http://www.gnu.org/licenses/>.
%
% $Id$

persistent warning_once
if isempty(warning_once) || ~warning_once
  % the mex file is many times faster than the matlab implementation, hence that is prefered
  % but now we use the matlab implementation as a fallback
  warning_once = true;
  warning('Could not locate the MEX file "%s.%s"', mfilename, mexext);
end

siz = size(x);
if all(siz(1:2)==2)
  adjx  = [x(2,2,:,:) -x(1,2,:,:); -x(2,1,:,:) x(1,1,:,:)];
  denom = det2x2(x);
  d     = adjx./denom([1 1],[1 1],:,:);
elseif all(siz(1:2)==3)
  adjx = [ det2x2(x([2 3],[2 3],:,:)) -det2x2(x([1 3],[2 3],:,:))  det2x2(x([1 2],[2 3],:,:)); ...
          -det2x2(x([2 3],[1 3],:,:))  det2x2(x([1 3],[1 3],:,:)) -det2x2(x([1 2],[1 3],:,:)); ...
	   det2x2(x([2 3],[1 2],:,:)) -det2x2(x([1 3],[1 2],:,:))  det2x2(x([1 2],[1 2],:,:))];
  denom = det3x3(x);
  d     = adjx./denom([1 1 1],[1 1 1],:,:);
elseif numel(siz)==2
  d = inv(x);
else
  ft_error('cannot compute slicewise inverse');
  % write for loop for the higher dimensions, using normal inv
end
