function [X]=bfft(x,m)
N=2^m;
y=daoxu(x,N);
for L=1:m
    B=2^L;
    w=1;
    wnp=exp(-i*2*pi/B);
    for J=1:B/2
        for k=J:B:N
            y(k+B/2)=y(k)-y(k+B/2)*w;
            y(k)=y(k+B/2)+y(k+B/2)*w;
        end
        w=w*wnp;
    end
end