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%% Copyright: 2010 Integrated Sytems Laboratory, ETH Zurich
%%            http://www.iis.ee.ethz.ch/~sha3
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function y = butterfly(y, i, j , n)

u = y(i+1);
v = y(j+1);


y(i+1) =  u+v;

if u-v < 0
   u = u; 
end

y(j+1) = bitshift(u-v, 2*n);