Method iwt

Math/FWHT.cpp:33–43

    }
  }
  void iwt(int* a, int n, int flag = XOR) {
    if (n == 0) return;
    int m = n / 2;
    iwt(a, m, flag); iwt(a + m, m, flag);
    for (int i = 0; i < m; i++){
      int x = a[i], y = a[i + m];
      if (flag == OR) a[i] = x, a[i + m] = (y - x + mod) % mod;
      if (flag == AND) a[i] = (x - y + mod) % mod, a[i + m] = y;
      if (flag == XOR) a[i] = 1LL * (x + y) * inv2 % mod, a[i + m] = 1LL * (x - y + mod) * inv2 % mod; // replace inv2 by >>1 if not required
    }
  }
 vector<int> multiply(int n, vector<int> A, vector<int> B, int flag = XOR) {
    assert(__builtin_popcount(n) == 1);
    A.resize(n); B.resize(n);