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function r=acorrl(X,maxlag,varargin) 
% returns the biased autocorrelation of X in r. r is a row vector.  
% 
% r=acorrl(X,maxlag,'half') 
%          X--> input data 
%    maxlag --> maximum lag,which should be less than N-1.N is the length of X 
%     'half'--> returns the r(m) ,whose m is from 0 to maxlag 
% 
%   r=acorrl(X,maxlag,'whole'), returns the r(m) ,whose m is from -maxlag 
%   to maxlag. If not determinate the third argument, namely r=acorrl(X,maxlag), 
%   the function returns the same result. 
% 
%   The algorithm is follow as: 
%           1  N-m 
%    r(m)= --- sum conj(x(n))*x(n+m)      0<=m<=N-1 
%           N  n=1 
 
%  Copyright by Zhilin Zhang, Hanlin Laboratory of UESTC 
%  $ Revision: 1.1 $  $ Date: 2003/04/02  01:29 $  
% 
%  Reference: 
%  [1] 胡广书，数字信号处理——理论、算法与实现，清华大学出版社，1997 
 
LenX=length(X);  
 
% make sure the maxlag is correct 
if maxlag<0 | maxlag>=LenX 
    error('The max lag should be nonnegative and less than the length of input data\n'); 
end 
 
% computer the r(m),which is from 0 lag to maxlag 
rm=zeros(1,maxlag+1); 
for m=0:maxlag 
    rsum=0; 
    for n=1:LenX-m 
        rsum=rsum+conj(X(n))*X(n+m); 
    end 
    rm(m+1)=rsum/LenX; 
end 
 
% If the third argument is 'whole' or default ,returns the r(m) from 
% -maxlag to maxlag. If 'half',returns only the r(m) from 0 lag to  
% maxlag. 
if nargin==2 | strcmp(varargin{1},'whole') 
    r=[conj(rm(maxlag+1:-1:2)),rm]; 
elseif strcmp(varargin{1},'half') 
    r=rm; 
else 
    error('The third argument is wrong\n'); 
end