Euler's totient function

uint64_t phiFunction(uint64_t n) {
    uint64_t result = n;
    for (uint64_t i = 2; i * i <= n; i++) {
        if (n % i == 0) {
            while (n % i == 0) {
                n /= i;
            }
            result -= result / i;
        }
    }
    if (n > 1)
        result -= result / n;
    return result;
}