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## -*- texinfo -*-
## @deftypefn  {} {} spectral_adf (@var{c})
## @deftypefnx {} {} spectral_adf (@var{c}, @var{win})
## @deftypefnx {} {} spectral_adf (@var{c}, @var{win}, @var{b})
## Return the spectral density estimator given a vector of autocovariances
## @var{c}, 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}_lw}.
##
## 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_xdf}
## @end deftypefn

function retval = spectral_adf (c, win, b)

  if (nargin < 1 || nargin > 3)
    print_usage ();
  endif

  cr = length (c);

  if (columns (c) > 1)
    c = c';
  endif

  if (nargin < 3)
    b = 1 / ceil (sqrt (cr));
  endif

  if (nargin == 1)
    w = triangle_lw (cr, b);
  elseif (! ischar (win))
    error ("spectral_adf: WIN must be a string");
  else
    win = str2func ([win "_lw"]);
    w = feval (win, cr, b);
  endif

  c .*= w;

  retval = 2 * real (fft (c)) - c(1);
  retval = [(zeros (cr, 1)), retval];
  retval(:, 1) = (0 : cr-1)' / cr;

endfunction


## Test input validation
%!error spectral_adf ()
%!error spectral_adf (1, 2, 3, 4)
%!error spectral_adf (1, 2)
%!error spectral_adf (1, "invalid")
