两大数相乘 -- javascript 实现

(function (){

var addLarge = function(n1,n2){
var carry = 0;
var ret = "";
n1=n1.toString();
n2=n2.toString();

var len = Math.min(n1.length,n2.length);
var sln1 = n1.substr(n1.length - len,n1.length );
var sln2 = n2.substr(n2.length - len,n2.length );

for(var i = len;i > 0; i--)
{

var di = parseInt(sln1[i-1]);
var dj = parseInt(sln2[i-1]);

ret = (carry + di + dj)>= 10 ? ((carry + di + dj) - 10 + ret) : ((carry+di+dj) + ret);

carry = ((carry + di + dj) /10 ) | 0;

}

if(n1.length != n2.length)
{
if(n1.length > n2.length){
ret = addLarge(n1.substr(0,n1.length-len),carry.toString()) + ret;
}
else{
ret = addLarge(n2.substr(0,n2.length - len),carry.toString()) + ret;
}

return ret;
}

else{
if(carry> 0){
ret = carry+ ret;
}
return ret.toString();
}

};

var multiply = function(n1,n2){
var nstr1 = n1.toString();
var nstr2 = n2.toString();

var carry = 0;
var ret = "";

var di = 0;
var dj = 0;
var dig = "";
for(var i = nstr1.length - 1 ;i >= 0;i--)
{

var tmp = "";
var di = parseInt(nstr1[i]) ;
for(var j = nstr2.length - 1;j >= 0;j--)
{
var dj = parseInt(nstr2[j]);
var r = di*dj + carry;

if(r >= 10)  {
carry =(r / 10) | 0;
tmp = (r%10) + tmp;
}else{
carry=0;
tmp = r + tmp;
}

}

if(carry > 0){
tmp = carry.toString() + tmp;
carry = 0;
}

if(i < nstr1.length -1)
{
ret = addLarge (ret ,tmp + dig);
}
else{
ret = tmp;
}

dig += "0";
}

return ret;
};

var a = "1543843275857859592482498432243255";
var b = "1554335546546546662";

console.log(multiply(a,b));
console.log(parseInt(a)*parseInt(b));

})();