Question

The Twitter API returns IDs that are 64 bit integers, such as 276403573577891842. I want to see if one Tweet is newer than another by comparing their ID’s.

Is there a clever way to compare two 64 bit integers to see which is greater in JavaScript which only supports 32 bit integers?

Answer 1

Assuming the API is returning strings which are then parsed into ints, there are two ways to do this – a split way and a pad way;

Split.
The idea here is to split the data into a size the environment can handle. Largest 32-bit number is 0xFFFFFFFF or 4294967295. Now 4294967295..toString().length === 10. So all dec with str length <= 9 should be 32-bit safe.

function compare(a,b){
    var aa = [0,0,0], ba = [0,0,0], i = 3;
    while (a.length) aa[--i] = parseInt(a.slice(-9)), a = a.slice(0,-9);
    i = 3;
    while (b.length) ba[--i] = parseInt(b.slice(-9)), b = b.slice(0,-9);
    if (aa[0] === ba[0]){
        if (aa[1] === ba[1]) {
            if (aa[2] === ba[2]) return 0;
            else if (aa[2] > ba[2]) return 1;
            return -1;
        } else if (aa[1] > ba[1]) return 1;
        return -1;
    } else if (aa[0] > ba[0]) return 1;
    return -1;
}
// a > b: 1, a = b: 0, a < b: -1

Pad.
The idea this time is to do a string comparison, but to do that we must first make sure the (string) numbers have the same length; i.e. pad them with 0s. This time '18446744073709551615'.length === 20 so

function pad(str, len) { // Your fav. padding fn
    var pre = '0';
    len = len - str.length;
    while (len > 0) {
        if (len & 1) str = pre + str;
        len >>= 1;
        pre += pre;
    }
    return str;
};

a = pad('276403573577891842',20);
b = pad('50',20);

Now you can use a === b, a < b, a > b so normal (except you’re using strings and not int).