## Copyright (C) 1995-2011 Andreas Weingessel
##
## 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.
##
## 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, see
## <http://www.gnu.org/licenses/>.

## -*- texinfo -*-
## @deftypefn {Function File} {} blackman (@var{m})
## Return the filter coefficients of a Blackman window of length @var{m}.
##
## For a definition of the Blackman window, see e.g., A. V. Oppenheim &
## R. W. Schafer, @cite{Discrete-Time Signal Processing}.
## @end deftypefn

## Author: AW <Andreas.Weingessel@ci.tuwien.ac.at>
## Description: Coefficients of the Blackman window

function c = blackman (m)

  if (nargin != 1)
    print_usage ();
  endif

  if (! (isscalar (m) && (m == fix (m)) && (m > 0)))
    error ("blackman: M has to be an integer > 0");
  endif

  if (m == 1)
    c = 1;
  else
    m = m - 1;
    k = (0 : m)' / m;
    c = 0.42 - 0.5 * cos (2 * pi * k) + 0.08 * cos (4 * pi * k);
  endif

endfunction

%!assert (blackman (1), 1);
%!assert (blackman (2), zeros(2,1), 1e-6);
%!assert (blackman (16), fliplr (blackman (16)));
%!assert (blackman (15), fliplr (blackman (15)));
%!test
%! N = 9;
%! A = blackman (N);
%! assert (A (ceil (N/2)), 1, 1e-6);
%! assert ([A(1), A(length (A))], zeros (1, 2), 1e-6);

%!error blackman ();
%!error blackman (0.5);
%!error blackman (-1);
%!error blackman (ones(1,4));