Chillee

template <int maxn> struct FFT {
    constexpr static int lg2(int n) { return 32 - __builtin_clz(n - 1); }
    const static int MAXN = 1 << lg2(maxn);
    typedef complex<double> cpx;
    int rev[MAXN];
    cpx rt[MAXN];
    FFT() {
        rt[1] = cpx{1, 0};
        for (int k = 2; k < MAXN; k *= 2) {
            cpx z[] = {1, polar(1.0, M_PI / k)};