1 int reverse(int x, int len){
2 int t = 0;
3 for (int i = 1; i < len; i <<= 1){
4 t <<= 1;
5 if (x & i) t |= 1;
6 }
7 return t;
8 }
9
10 Complex A[200010];
11 void FFT(Complex *a, int n, int DFT){
12 for (int i = 0; i < n; ++i) A[reverse(i, n)] = a[i];
13 for (int i = 2; i <= n; i <<= 1){
14 Complex wn = Complex(cos(2 * pi / i), DFT * sin(2 * pi / i));
15 for (int j = 0; j < n; j += i){
16 Complex w = Complex(1, 0);
17 for (int k = 0; k < (i >> 1); ++k){
18 Complex x = A[j + k];
19 Complex y = w * A[j + k + (i >> 1)];
20 A[j + k] = x + y;
21 A[j + k + (i >> 1)] = x - y;
22 w = w * wn;
23 }
24 }
25 }
26 if (DFT == -1) for (int i = 0; i < n; ++i) A[i] = A[i] / n;
27 for (int i = 0; i < n; ++i) a[i] = A[i];
28 }
