C++: Fast power of number modulo x

C++: Fast power modulo

inline long long fastmodpow(long long a, long long b, long long x) {
    if (!x || !a) {
        return 0;
    }
    
    if (1 == a) {
        return 1;
    }
    
    a %= x;
    if (1 == b) {
        return a;
    }
    
    long long pow = 1;
    long long res = 1;
    long long last_pw = a;
    long long last_i = 1;
    while (b) {
        if (b & 1) {
            long long pw = last_pw;
            long long i = last_i;
            while (i < pow) {
                pw *= pw;
                pw %= x;
                i <<= 1;
            }
            last_pw = pw;
            last_i = i;
            res *= pw;
            res %= x;
        }
        pow <<= 1;
        b >>= 1;
    }
    return int(res % x);
}