UVA10225 Discrete Logging

题意

BSGS 。

思路

BSGS 。

代码

ll a,b,mod;
ll block;
unordered_map<ll,ll>m;
ll bsgs(ll x,ll y){
//	if(y==1)return 0;
	ll bas=1;
	for(ll i=0;i<block;i++){
		m[bas*y%mod]=i;
		bas=bas*x%mod;
	}
	x=bas;
	bas=1;
	if(!x)return y==0?1:-1;
	for(ll i=1;i<=block;i++){
		bas=bas*x%mod;
		if(m.count(bas)){
			ll res=i*block-m[bas];
			if(res>=0)return res;
		}
	}
	return -1;
}
void solve(){
	//reading had finished in the main fuc. 
	m.clear();
	block=sqrt(mod);
	if(b==1){
		cout<<0<<endl;
		return ;
	}
	ll res=bsgs(a,b);
	if(res==-1)cout<<"no solution"<<endl;
	else cout<<res<<endl;
	return ;
}