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

