模乘

#include"iostream"
using namespace std;
long long mul1(long long a,long long b,long long mod)
{
   long double x;
   long long c;
   long long r;
   if (a >= mod) a %= mod;
   if (b >= mod) b %= mod;
   x = a;
   c = x * b / mod;
   r = (a * b - c * mod) % mod;
   return r < 0 ? r + mod : r;
}

long long mul2(long long a,long long b,long long mod)
{
    a%=mod;
    b%=mod;
    long long res=0,base=a;
    while(b)
    {
        if(b&1)res=(res+base)%mod;
        base=(base*2)%mod;
        b>>=1;
    }
    return res;
} 

long long mul3(long long a,long long b,long long mod)///tao shen de gong shi
{
    return (a*b-(long long)(a/(long double) mod * b + 1e-3)* mod + mod) % mod;

}
int main()
{
    long long a=1e18+7,b=1e17+38,mod=1e18+79;
// for(int i=1;i<=100000000;i++)
    cout<<mul1(a,b,mod)<<"\n"; 
    cout<<mul2(a,b,mod)<<"\n"; 
    cout<<mul3(a,b,mod)<<"\n"; 
}