function ApEn = ApproxEntropy( data, m, r)
% OVERVIEW: This function calculates the approximate entropy for the input
% vector data
%
% INPUTS
%   data : time-series data
%   m    : Template length
%   r    : tolerance (typically 0.2 * std)
% OUTPUT
%   ApEn : approximate entropy value
%
% Written by Giulia Da Poian <giulia.dap@gmail.com>
%	REPO:       
%       https://github.com/cliffordlab/PhysioNet-Cardiovascular-Signal-Toolbox
%	COPYRIGHT (C) 2016 
%   LICENSE:    
%       This software is offered freely and without warranty under 
%       the GNU (v3 or later) public license. See license file for
%       more information
%

N = length(data);
result = zeros(1,2);

for j = 1:2
    m = m+j-1;
    phi = zeros(1,N-m+1);
    dataMat = zeros(m,N-m+1);
    
    % setting up data matrix
    for i = 1:m
        dataMat(i,:) = data(i:N-m+i);
    end
    
    % counting similar patterns using distance calculation
    for i = 1:N-m+1
        tempMat = abs(dataMat - repmat(dataMat(:,i),1,N-m+1));
        boolMat = any( (tempMat > r),1);
        phi(i) = sum(~boolMat)/(N-m+1);
    end
    
    % summing over the counts
    result(j) = sum(log(phi))/(N-m+1);
end

ApEn = result(1)-result(2);

end