function [coef]=comp_dwiltiii(f,g,M)
%COMP_DWILTIII  Compute Discrete Wilson transform type III.
%   
%
%   Url: http://ltfat.github.io/doc/comp/comp_dwiltiii.html

% Copyright (C) 2005-2015 Peter L. Soendergaard <peter@sonderport.dk>.
% This file is part of LTFAT version 2.1.1
%
% 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/>.

L = size(f,1);
a=M;
N=L/a;
W=size(f,2);

fwasreal = isreal(f);
coef=zeros(M,N,W,assert_classname(f,g));

halfmod=exp(-pi*i*(0:L-1).'/(2*M));
f=f.*repmat(halfmod,1,W);

coef2=comp_sepdgt(f,g,a,2*M,0);
  
if (isreal(g) && fwasreal)
   % --- m is even ---------
   coef(1:2:M,1:2:N,:)= real(coef2(1:2:M,1:2:N,:)) + imag(coef2(1:2:M,1:2:N,:));
   coef(1:2:M,2:2:N,:)= real(coef2(1:2:M,2:2:N,:)) - imag(coef2(1:2:M,2:2:N,:));

   % --- m is odd ----------
   coef(2:2:M,1:2:N,:)= real(coef2(2:2:M,1:2:N,:)) - imag(coef2(2:2:M,1:2:N,:));
   coef(2:2:M,2:2:N,:)= real(coef2(2:2:M,2:2:N,:)) + imag(coef2(2:2:M,2:2:N,:));    
else
   % --- m is even ---------
   coef(1:2:M,1:2:N,:)= 1/sqrt(2)*(exp(-i*pi/4)*coef2(1:2:M,1:2:N,:)+exp(i*pi/4)*coef2(2*M:-2:M+1,1:2:N,:));
   coef(1:2:M,2:2:N,:)= 1/sqrt(2)*(exp(i*pi/4)*coef2(1:2:M,2:2:N,:)+exp(-i*pi/4)*coef2(2*M:-2:M+1,2:2:N,:));

   % --- m is odd ----------
   coef(2:2:M,1:2:N,:)= 1/sqrt(2)*(exp(i*pi/4)*coef2(2:2:M,1:2:N,:)+exp(-i*pi/4)*coef2(2*M-1:-2:M+1,1:2:N,:));
   coef(2:2:M,2:2:N,:)= 1/sqrt(2)*(exp(-i*pi/4)*coef2(2:2:M,2:2:N,:)+exp(i*pi/4)*coef2(2*M-1:-2:M+1,2:2:N,:));
end

