## Copyright (C) 1996, 1997, 1998, 1999, 2000, 2005, 2006, 2007, 2008
##               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 3 of the License, 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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## You should have received a copy of the GNU General Public License
## along with Octave; see the file COPYING.  If not, see
## <http://www.gnu.org/licenses/>.

## -*- texinfo -*-
## @deftypefn {Function File} {} mahalanobis (@var{x}, @var{y})
## Return the Mahalanobis' D-square distance between the multivariate
## samples @var{x} and @var{y}, which must have the same number of
## components (columns), but may have a different number of observations
## (rows).
## @end deftypefn

## Author: Friedrich Leisch <leisch@ci.tuwien.ac.at>
## Created: July 1993
## Adapted-By: jwe

function retval = mahalanobis (X, Y)

  if (nargin != 2)
    print_usage ();
  endif

  [xr, xc] = size (X);
  [yr, yc] = size (Y);

  if (xc != yc)
    error ("mahalanobis: X and Y must have the same number of columns");
  endif

  Xm = sum (X) / xr;
  Ym = sum (Y) / yr;

  X = X - ones (xr, 1) * Xm;
  Y = Y - ones (yr, 1) * Ym;

  W = (X' * X + Y' * Y) / (xr + yr - 2);

  Winv = inv (W);

  retval = (Xm - Ym) * Winv * (Xm - Ym)';

endfunction

%!error mahalanobis ();

%!error mahalanobis (1, 2, 3);