Function gcd

test/unittest/math_test.cpp:12–16

using ull = unsigned long long;

ll gcd(ll a, ll b) {
    assert(0 <= a && 0 <= b);
    if (b == 0) return a;
    return gcd(b, a % b);
}

ll pow_mod_naive(ll x, ull n, uint mod) {
    ull y = (x % mod + mod) % mod;