long sumXor(long n) {
    if (n == 0) {
        return 1;
    }
    
    int num_bits = std::floor(std::log2(n));
    int num_one_bits = __builtin_popcountl(n) - 1; // -1 excludes highest power of 2
    int num_zero_bits = num_bits - num_one_bits;
    
    // # of ways of choosing num_zero_bits
    return 1L << num_zero_bits; // pow(2, num_zero_bits)
}