% calculate xcorr across the 1st dimension, gives same result as matlab% bulit-in function, but much much fasterfunction y = xcorryy(x)% x is 2 dimension.  calculate xcorr across 1st dimensiony = zeros(size(x));for k = 1:size(x,1)    tmp = [zeros(k-1,size(x,2));x];    x2 = tmp(1:size(x,1),:);    y(k,:) = sum(x.*x2,1);end