JavaScript BigNum

multiply.js

function reverse(s){
    return s.split("").reverse().join("");
}
function add(left, right) {
    if(left.length < right.length) {
      var swp = left;
    left = right;
    right = swp;
  }
  rleft = reverse(left);
  rright = reverse(right);
  
  var result = "";
  var uebertrag = 0;  
  for(var i = 0; i < rleft.length; i++) {
    var tmp = Number(rleft[i]) + Number(rright[i] === undefined ? "0" : rright[i]);
    tmp += uebertrag;
    uebertrag = Math.floor(tmp / 10);
    result += "" + (tmp % 10);
  }
  
  if(uebertrag == 1) {
      return "1" + reverse(result);
  }
  
  return reverse(result);
}
function multiplyDigit(num, digit) {
    var rnum = reverse(num);
  
  var result = "";
  
  var uebertrag = 0;
  for(var i = 0; i < rnum.length; i++) {
    var tmp = rnum[i] * digit + uebertrag;
    result += "" + (tmp % 10);
    uebertrag = Math.floor(tmp / 10);
  }
  
  if(uebertrag > 0) {
      return "" + uebertrag + reverse(result);
  }
  
  return reverse(result);
}
function multiply(left, right) {
    if(left.length > right.length) {
      var swp = left;
    left = right;
    right = swp;
  }
  
  var rleft = reverse(left);
  
  var result = "";
  var wertigkeit = "";
  for(var i = 0; i < rleft.length; i++) {
    var tmp = multiplyDigit(right, Number(rleft[i])) + wertigkeit;
    result = add(tmp, result);
    wertigkeit += "0";
  }
  
  return result;
}

function benchmark(num) {
 var startTime = window.performance.now();
 multiply(num, num);
 var endTime = window.performance.now();
 console.log(num.length, endTime - startTime);
}

benchmark("0");
benchmark("12390128930");
benchmark("1239012893012390128930");
benchmark("12390128930123901289301239012893012390128930");
benchmark("1239012893012390128930123901289301239012893012390128930123901289301239012893012390128930");
benchmark("12390128930123901289301239012893012390128930123901289301239012893012390128930123901289301239012893012390128930123901289301239012893012390128930123901289301239012893012390128930");