Project Euler #27 : Quadratic primes

vector<llong> alist = { 2, 7, 61 };
llong mul(llong a, llong b, llong mod) {
    return (a * b) % mod;
}
llong bin_pow(llong a, llong p, llong mod) {
    if (p == 0) return 1 % mod;
    if (p & 1) return mul(a, bin_pow(a, p - 1, mod), mod);
    return bin_pow(mul(a, a, mod), p / 2, mod);
}
bool miller_rabin(llong n, llong a) {
    llong d = n - 1;
    while (d % 2 == 0) {
        if (bin_pow(a, d, n) == n - 1) return true;
        d /= 2;
    }
    llong tmp = bin_pow(a, d, n);
    return tmp == n - 1 || tmp == 1;
}
bool is_prime(llong n) {
    if (n <= 1) return false;
    if (n <= 10000) {
        for (int i = 2; i * i <= n; i++) {
            if (n % i == 0) return false;
        }
        return true;
    }
    for (llong a : alist) {
        if (!miller_rabin(n, a)) return false;
    }
    return true;
}
 
int main() {
    IOS;
    int ansIndex;
    int ans = 0;
    for (int a = -1000; a <= 1000; a++) {
        for (int b = -1000; b <= 1000; b++) {
            int cnt = 0;
            for (int n = 1; ; n++) {
                if (is_prime(n * n + a * n + b)) cnt++;
                else break;
            }
            if (ans < cnt) {
                ansIndex = a * b;
                ans = cnt;
            }
        }
    }
    cout << ansIndex;
}