FFT

Module

#define L(x) (1 << (x))
const double PI = acos(-1.0);
const int N = 17, Maxn = L(N + 1);
double ax[Maxn], ay[Maxn], bx[Maxn], by[Maxn]
struct FFT
{
    private :
    int revv(int x, int bits) {
        int ret = 0;
        for (int i = 0; i < bits; i++) {
            ret <<= 1;
            ret |= x & 1;
            x >>= 1;
        }
        return ret;
    }
    void fft(double * a, double * b, int n, bool rev) 
    {
        int bits = 0;
        while (1 << bits < n) ++bits;
        for (int i = 0; i < n; i++) 
        {
            int j = revv(i, bits);
            if (i < j) 
            {
                double t = a[i];
                a[i] = a[j];
                a[j] = t;
                t = b[i];
                b[i] = b[j];
                b[j] = t;
            }
        }
        for (int len = 2; len <= n; len <<= 1) 
        {
            int half = len >> 1;
            double wmx = cos(2 * PI / len);
            double wmy = sin(2 * PI / len);
            if (rev)
                wmy = -wmy;
            for (int i = 0; i < n; i += len) 
            {
                double wx = 1;
                double wy = 0;
                for (int j = 0; j < half; j++) 
                {
                    double cx = a[i + j];
                    double cy = b[i + j];
                    double dx = a[i + j + half];
                    double dy = b[i + j + half];
                    double ex = dx * wx - dy * wy;
                    double ey = dx * wy + dy * wx;
                    a[i + j] = cx + ex;
                    b[i + j] = cy + ey;
                    a[i + j + half] = cx - ex;
                    b[i + j + half] = cy - ey;
                    double wnx = wx * wmx - wy * wmy;
                    double wny = wx * wmy + wy * wmx;
                    wx = wnx;
                    wy = wny;
                }
            }
        }
        if (rev) 
        {
            for (int i = 0; i < n; i++) 
            {
                a[i] /= n;
                b[i] /= n;
            }
        }
    }
    public:
    int solve(int a[],int na,int b[],int nb,long long ans[])
    {
        int len = max(na, nb), ln;
        for(ln=0; L(ln)<len; ++ln);
        len=L(++ln);
        for (int i = 0; i < len ; ++i)
        {    
            if (i >= na) ax[i] = 0, bx[i] =0;
            else ax[i] = a[i], bx[i] = 0;
        }
        fft(ax, ay, len, 0);
        for (int i = 0; i < len; ++i)
        {
            if (i >= nb) bx[i] = 0, by[i] = 0;
            else bx[i] = b[i], by[i] = 0;
        }
        fft(bx, by, len, 0);
        for (int i = 0; i < len; ++i)
        {
            double cx = ax[i] * bx[i] - ay[i] * by[i];
            double cy = ax[i] * by[i] + ay[i] * bx[i];
            ax[i] = cx, ay[i] = cy;
        }
        fft(ax, ay, len, 1);
        for (int i = 0; i < len; ++i)
            ans[i] = ax[i] + 0.01;
        return len;
    }
    int solve(int a[], int na, long long ans[])
    {
        int len = na, ln;
        for(ln = 0; L(ln) < na; ++ln);
        len=L(++ln);
        for(int i = 0;i < len; ++i)
        {
            if (i >= na) ax[i] = 0, ay[i] = 0;
            else ax[i] = a[i], ay[i] = 0;
        }
        fft(ax, ay, len, 0);
        for(int i=0;i<len;++i)
        {
            double cx = ax[i] * ax[i] - ay[i] * ay[i];
            double cy = 2 * ax[i] * ay[i];
            ax[i] = cx, ay[i] = cy;
        }
        fft(ax, ay, len, 1);

        for(int i=0;i<len;++i)
            ans[i] = ax[i] + 0.5;
        return len;
    }
 };