https://oi-wiki.org/math/bsgs/
所以这个应该也是个优化暴力

map<LL, int>mp;

LL BSGS(LL a, LL b, LL p){
	LL lim = ceil(sqrt(p));
            
        if(!p % a){
                return -1;
        }             
      
	for(int i = 0; i <= lim; i++){
		mp[FMul(b, QPow(a, i, p), p)] = i;
	}

	LL bp = QPow(a, lim, p);
	LL rNow = 1;

	for(int i = 1; i <= lim; i++){
		rNow = FMul(rNow, bp, p);

		if(rNow == b % p){
			return i*lim;
		}

		if(mp[rNow]){
			return i*lim - mp[rNow];
		} // 等式右边出现过该数
	}
	return -1;
}

FMul -> 快速乘
QPow -> 快速幂
在这里