function y = sig_hassager2016(x,B,fs)
%SIG_HASSAGER2016 spectral smoothing algorithm from Hassager et al. (2016)
%   Usage:    y = sig_hassager2016(x,B,fs)
%
%   Input parameters:
%     x   : binaural impulse response(s); SOFA object or matrix with time 
%           as first matrix dimension.
%     B   : bandwidth factor. B=1 represents 1 ERB
%     fs  : sampling rate; required only of x is not a SOFA object
%
%   Output parameters:
%     y   : smoothed version of x.
%
%   SIG_HASSAGER2016(...) is an algortihm for spectral
%   smoothing based on gammatone filters with a variable bandwidth B.
%
%   References:
%     H. G. Hassager, F. Gran, and T. Dau. The role of spectral detail in the
%     binaural transfer function on perceived externalization in a
%     reverberant environment. J. Acoust. Soc. Am., 139(5):2992--3000, 2016.
%     
%
%   Url: http://amtoolbox.org/amt-1.5.0/doc/signals/sig_hassager2016.php


%   #Author: Robert Baumgartner (2016), Acoustics Research Institute, Vienna, Austria

% This file is licensed unter the GNU General Public License (GPL) either 
% version 3 of the license, or any later version as published by the Free Software 
% Foundation. Details of the GPLv3 can be found in the AMT directory "licences" and 
% at <https://www.gnu.org/licenses/gpl-3.0.html>. 
% You can redistribute this file and/or modify it under the terms of the GPLv3. 
% This file is distributed without any warranty; without even the implied warranty 
% of merchantability or fitness for a particular purpose. 

flags.do_plot = false;

if isstruct(x) % input: SOFA object
  Obj = x;
  fs = Obj.Data.SamplingRate;
  x = shiftdim(Obj.Data.IR,2);
end

[~,excessPhaseTF] = RB_minphase(x,1,'freq');
Nfft = 2.^nextpow2(size(x,1));
X = fftreal(x,Nfft);
f = linspace(0,fs/2,Nfft/2+1);
b = B*1.149*24.7*(4.37*f/1000+1);
Yabs = zeros(size(X));
for idf = 1:length(f)
  Habs2 = abs((1./(1+1i*(f-f(idf))/b(idf))).^4).^2;
  Habs2 = repmat(Habs2(:),[1,size(X,2),size(X,3)]);
  Yabs(idf,:,:) = sqrt(sum(abs(X).^2.*Habs2)./sum(Habs2));
end
% YrandPhase = Yabs.*exp(1i*angle(X));
yRandPhase = ifftreal(Yabs,Nfft);
YminPhase = RB_minphase(yRandPhase,1,'freq');
y = ifft(YminPhase.*excessPhaseTF,Nfft);

if exist('Obj','var') % output: SOFA object
  Obj.Data.IR = shiftdim(y,1);
  y = Obj;
end

if flags.do_plot
  idpos = 7; % 0° azimuth for ARI HRTFs
  for ii = 1:2
    subplot(1,2,ii)
    semilogx(f,20*log10(Yabs(:,idpos,ii)))
    xlabel('Frequency (Hz)')
    ylabel('Magnitude (dB)')
    hold on
  end
end

end

function [minPhase,excessPhase] = RB_minphase(IR,dim,TFdomainFlag)
% RB_minphase - create minimum-phase filter via causal cepstrum
%
% Usage: [minPhase,excessPhase] = RB_minphase(IR,dim,TFdomainFlag)

% RB, 2016/6/3

Nfft = 2.^nextpow2(size(IR,dim));

TF = fft(IR,Nfft,dim);
logTF = log(abs(TF)+eps);

cep = ifft(logTF,Nfft,dim);
Nshift = mod(dim-1,ndims(cep));
cep1 = shiftdim(cep,Nshift);
cep1(Nfft/2+2:Nfft,:,:,:,:) = 0;    % set non-causal part to zero and 
cep1(2:Nfft/2,:,:,:,:) = 2*cep1(2:Nfft/2,:,:,:,:);    % multiply causal part by 2 (due to symmetry)
cepMinPhase = shiftdim(cep1,ndims(cep)-Nshift);

logTFminPhase = fft(cepMinPhase,Nfft,dim);
TFminPhase = exp(logTFminPhase);

switch TFdomainFlag
  case 'freq'
    minPhase = TFminPhase;
  case 'time'
    minPhase = ifft(TFminPhase,Nfft,dim);
end


if nargout == 2
  switch TFdomainFlag
    case 'freq'
      excessPhase = TF./TFminPhase;
    case 'time'
      excessPhase = ifft(TF./TFminPhase,Nfft,dim);
  end
end

end