FFT algorithm matlab help input ordering

[goldriver]

I am having problems understanding the following code in matlab which implements an FFT algorithm using butterfly.

N1=8;
% disp('Enter the sample values using ENTER KEY');
% fs=2*N1;
% n=0:1/fs:1;
% x=sin(2*pi*100*n);
x=[1 2 3 4 5 6 7 8];
m=ceil(log2(N1));
% to ensure we compute an FFT that is in
% powers of 2 we use the above and below 
% command
N=2^m;
% p=N-N1;
%zero-padding
x2=x;
% if (p~=0)
%     x1=zeros(1,p);
%     x2=[x2 x1];
% end
p=N-length(x2);
if(p>0)
    x1=zeros(1,p);
    x2=[x2 x1]
end

I am stuck at the input ordering part which arranges the indices together in such a way that a butterfly could be implemented. Is there a way to possibly arrange the indices in the way we require to implement a butterfly?

 

[goldriver]

https://cnx.org/content/m12016/latest/ i have checked so many websites for an appropriate explanation of bit reversal or input re-ordering but have failed to found one.