【新人求助】求该高精度算法的原理和四则运算的编法

rt，代码如下
struct Bigint
{
static const int BASE=100000000;//节省时间，每个int中存储8位
static const int WIDTH=8;//每位长度
vector<int> s;//保存
Bigint(long long num=0)
{
*this=num;
}
Bigint operator = (long long num)
{
s.clear();
do
{
s.push_back(num%BASE);
num/=BASE;
}
while(num>0);
return *this;
}
Bigint operator = (const string & str)
{
s.clear();
int x,len=(str.length()-1)/WIDTH+1;
for(int i=0;i<len;i++)
{
int end=str.length()-i*WIDTH;
int start=max(0,end-WIDTH);
sscanf(str.substr(start,end-start).c_str(),"%d",&x);
s.push_back(x);
}
return *this;
}
ostream& operator << (ostream &out,const Bigint& x)
{
out<<x.s.back();
for(int i=x.s.size()-2;i>=0;i--)
{
char buf[20];
sprintf(buf,"%08d",x.s[i]);
for(int j=0;j<strlen(buf);j++)
{
out<<buf[j];
}
}
return out;
}
istream& operator >> (istream &in,Bigint& x)
{
string s;
if(!(in>>s))
{
return in;
}
x=s;
return in;
}
Bigint operator + (const Bigint& b)const
{
Bigint c;
c.s.clear();
for(int i=0,g=0;;i++)
{
if(g==0&&i>=s.size()&&i>=b.s.size())
{
break;
}
int x=g;
if(i<s.size())
{
x+=s[i];
}
if(i<b.s.size())
{
x+=b.s[i];
}
c.s.push_back(x%BASE);
g=x/BASE;
}
return c;
}
};