How to do bitwise AND in javascript on variables that are longer than 32 bit?

Question

I have 2 numbers in javascript that I want to bit and. They both are 33bit long

in C#:

 ((4294967296 & 4294967296 )==0) is false

but in javascript:

 ((4294967296 & 4294967296 )==0) is true

4294967296 is ((long)1) << 32

As I understand it, it is due to the fact that javascript converts values to int32 when performing bit wise operations.

How do I work around this? Any suggestions on how to replace bit and with a set of other math operations so that bits are not lost?

Answer 1

You could split each of the vars into 2 32-bit values (like a high word and low word), then do a bitwise operation on both pairs.

The script below runs as a Windows .js script. You can replace WScript.Echo() with alert() for Web.

var a = 4294967296;
var b = 4294967296;

var w = 4294967296; // 2^32

var aHI = a / w;
var aLO = a % w;
var bHI = b / w;
var bLO = b % w;

WScript.Echo((aHI & bHI) * w + (aLO & bLO));

Answer 2

In Java/C/C#/C++, the bitwise operators work with integers. JavaScript doesn't have integers. It only has double precision floating-point numbers. So, the bitwise operators convert their number operands into integers, do their business, and then convert them back. In most languages, these operators are very close to the hardware and very fast. In JavaScript, they are very far from the hardware and very slow. JavaScript is rarely used for doing bit manipulation.

As a result, in JavaScript programs, it is more likely that & is a mistyped && operator. The presence of the bitwise operators reduces some of the language's redundancy, making it easier for bugs to hide. So I advise not to resort to using bit-wise in Javascript.

Answer 3

There are several BigInteger librairy in Javascript, but none of them offer bitwise operation you need at the moment. If you are motivated and really need that functionality you can modify one of those librairy and add a method to do so. They already offer a good code base to work with huge number.

You can find a list of the BigInteger librairy in Javascript in this question :

http://stackoverflow.com/questions/964139/huge-integer-javascript-library

Answer 4

Here's a fun function for arbitrarily large integers:

function BitwiseAndLarge(val1, val2) {
    var shift = 0, result = 0;
    var mask = ~((~0) << 30); // Gives us a bit mask like 01111..1 (30 ones)
    var divisor = 1 << 30; // To work with the bit mask, we need to clear bits at a time
    while( (val1 != 0) && (val2 != 0) ) {
        var rs = (mask & val1) & (mask & val2);
        val1 = Math.floor(val1 / divisor); // val1 >>> 30
        val2 = Math.floor(val2 / divisor); // val2 >>> 30
        for(var i = shift++; i--;) {
            rs *= divisor; // rs << 30
        }
        result += rs;
    }
    return result;
}

Assuming that the system handles at least 30-bit bitwise operations properly.