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## Copyright (C) 1995-2013 Friedrich Leisch
##
## 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} {} spectral_xdf (@var{x})
## @deftypefnx {Function File} {} spectral_xdf (@var{x}, @var{win})
## @deftypefnx {Function File} {} spectral_xdf (@var{x}, @var{win}, @var{b})
## Return the spectral density estimator given a data vector @var{x},
## window name @var{win}, and bandwidth, @var{b}.
##
## The window name, e.g., @qcode{"triangle"} or @qcode{"rectangle"} is
## used to search for a function called @code{@var{win}_sw}.
##
## If @var{win} is omitted, the triangle window is used. If @var{b} is
## omitted, @code{1 / sqrt (length (@var{x}))} is used.
## @seealso{spectral_adf}
## @end deftypefn
## Author: FL <Friedrich.Leisch@ci.tuwien.ac.at>
## Description: Spectral density estimation
function retval = spectral_xdf (x, win, b)
if (nargin < 1 || nargin > 3)
print_usage ();
endif
xr = length (x);
if (columns (x) > 1)
x = x';
endif
if (nargin < 3)
b = 1 / ceil (sqrt (xr));
endif
if (nargin == 1)
w = triangle_sw (xr, b);
elseif (! ischar (win))
error ("spectral_xdf: WIN must be a string");
else
win = str2func ([win "_sw"]);
w = feval (win, xr, b);
endif
x = x - sum (x) / xr;
retval = (abs (fft (x)) / xr).^2;
retval = real (ifft (fft (retval) .* fft (w)));
retval = [(zeros (xr, 1)), retval];
retval(:, 1) = (0 : xr-1)' / xr;
endfunction
%% Test input validation
%!error spectral_xdf ();
%!error spectral_xdf (1, 2, 3, 4);
%!error spectral_xdf (1, 2);
%!error spectral_xdf (1, "invalid");
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