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function c = comp_gga(f,indvec)
%-*- texinfo -*-
%@deftypefn {Function} comp_gga
%@verbatim
%COMP_GGA Generalized Goertzel Algorithm
%
%@end verbatim
%@strong{Url}: @url{http://ltfat.github.io/doc/comp/comp_gga.html}
%@end deftypefn
% Copyright (C) 2005-2016 Peter L. Soendergaard <peter@sonderport.dk>.
% This file is part of LTFAT version 2.3.1
%
% This program 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.
%
% This program 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 this program. If not, see <http://www.gnu.org/licenses/>.
%% Initialization
L = size(f,1);
W = size(f,2);
no_freq = length(indvec); %number of frequencies to compute
classname = assert_classname(f);
c = zeros(no_freq,W,classname); %memory allocation for the output coefficients
%% Computation via second-order system
% loop over the particular frequencies
for cnt_freq = 1:no_freq
%for a single frequency:
%a/ precompute the constants
pik_term = 2*pi*(indvec(cnt_freq))/(L);
cos_pik_term2 = cos(pik_term) * 2;
cc = exp(-1i*pik_term); % complex constant
for w=1:W
%b/ state variables
s0 = 0;
s1 = 0;
s2 = 0;
%c/ 'main' loop
for ind = 1:L-1 %number of iterations is (by one) less than the length of signal
%new state
s0 = f(ind,w) + cos_pik_term2 * s1 - s2; % (*)
%shifting the state variables
s2 = s1;
s1 = s0;
end
%d/ final computations
s0 = f(L,w) + cos_pik_term2 * s1 - s2; %correspond to one extra performing of (*)
c(cnt_freq,w) = s0 - s1*cc; %resultant complex coefficient
%complex multiplication substituting the last iteration
%and correcting the phase for (potentially) non-integer valued
%frequencies at the same time
c(cnt_freq,w) = c(cnt_freq,w) * exp(-1i*pik_term*(L-1));
end
end
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