光速乘

x383494 / 2023-05-24 / 原文

ll qmul(ll a, ll b, ll c){
	a%=c, b%=c;
	ll w = (ld)a/c*b;
	ll r = (ull)a*b-(ull)w*c;
	return r<0?r+c:r;
}