You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: spm_autocorr.m
    path of file: ./repos/PsPM/src/ext/VBA/stats&plots
    
    the code of the file until where you have to start completion: function R= spm_autocorr(y)
% computes simple autocorrelation function of signal y
% function R = spm_autocorr(y)
% Note: the autocorrelation is computed from lag 0 to lag t (length of the
% times series), and back. The symmetry of the autocorrelation function can
% be eyeballed using the following command:
% figure,plot(fftshift(R)')

    
function R= spm_autocorr(y)
% computes simple autocorrelation function of signal y
% function R = spm_autocorr(y)
% Note: the autocorrelation is computed from lag 0 to lag t (length of the
% times series), and back. The symmetry of the autocorrelation function can
% be eyeballed using the following command:
% figure,plot(fftshift(R)')
% This way, the 0-lag correlation (always 1) is in the centre of the axes.
% IN:
%   - y: nXt time series matrix
% OUT:
%   - R: nXt autocorrelation matrix

[n,t] = size(y);
R = zeros(n,t*2);
% standardize y
my = nanmean(y,2);
sy = nanstd(y,[],2);
sy(sy==0) = 1; % correct for identically constant time series
y = y - repmat(my,1,t);
y = diag(1./sy)*y;
% compute auto-correlation using FFT
hasNan = any(any(isnan(y)));
if hasNan
     disp('= Warning: Autocorrelation with missing data could yield unreliable results'); 
end