FFT的一种迭代实现

struct Complex
{
    double x,y;
    Complex(double x1=0.0 ,double y1=0.0)
    {
        x=x1;
        y=y1;
    }
    Complex operator -(const Complex &b)const
    {
        return Complex(x-b.x,y-b.y);
    }
    Complex operator +(const Complex &b)const
    {
        return Complex(x+b.x,y+b.y);
    }
    Complex operator *(const Complex &b)const
    {
        return Complex(x*b.x-y*b.y,x*b.y+y*b.x);
    }
};

void change(Complex y[],int len)
{
    int i,j,k;
    for(i=1,j=len/2;i<len-1;i++)
    {
        if(i<j)swap(y[i],y[j]);
        k=len/2;
        while(j>=k)
        {
            j-=k;
            k/=2;
        }
        j+=k;
    }
}

void FFT(Complex y[],int len,int on)
{
    change(y,len);
    for(int h=2;h<=len;h<<=1)
    {
        Complex wn(cos(on*2*pi/h),sin(on*2*pi/h));
        for(int j=0;j<=len;j+=h)
        {
            Complex w(1,0);
            for(int k=j;k<j+h/2;k++)
            {
                Complex u=y[k];
                Complex t=w*y[k+h/2];
                y[k]=u+t;
                y[k+h/2]=u-t;
                w=w*wn;
            }
        }
    }
    if(on==-1)
        for(int i=0;i<len;i++)
        y[i].x/=len;
}