FFT卷积相加模板

struct Complex
{
    double r,i;
    Complex(double _r,double _i):r(_r),i(_i){}
    Complex(){}
    Complex operator +(const Complex &b)
    {
        return Complex(r+b.r,i+b.i);
    }
    Complex operator -(const Complex &b)
    {
        return Complex(r-b.r,i-b.i);
    }
    Complex operator *(const Complex &b)
    {
        return Complex(r*b.r-i*b.i,r*b.i+i*b.r);
    }
};
void change(Complex y[],int len)
{
    int i,j,k;
    for(i = , j = len/;i < len-;i++)
    {
        if(i < j)swap(y[i],y[j]);
        k = len/;
        while( j >= k)
        {
            j -= k;
            k /= ;
        }
        if(j < k)j += k;
    }
}
void fft(Complex y[],int len,int on)
{
    change(y,len);
    for(int h = ;h <= len;h <<= )
    {
        Complex wn(cos(-on**pi/h),sin(-on**pi/h));
        for(int j = ;j < len;j += h)
        {
            Complex w(,);
            for(int k = j;k < j+h/;k++)
            {
                Complex u = y[k];
                Complex t = w*y[k+h/];
                y[k] = u+t;
                y[k+h/] = u-t;
                w = w*wn;
            }
        }
    }
    if(on == -)
        for(int i = ;i < len;i++)
            y[i].r /= len;
}