CODEWITHSUNDEEP
javascriptstringifyarraybufferuint64
danilabagroff
danilabagroff

Reputation: 656

Uint8Array(8) stringification

I'm getting unsigned 64-bit integer from the backend into ArrayBuffer, due to IEEE 754 I can't get&keep this sequence as a Number. Is there any way to "stringify" this huge number?

In a nutshell:

var buffer = new ArrayBuffer(8);
var raw = new Uint8Array(buffer);

raw[0] = 255;
raw[1] = 255;
raw[2] = 255;
raw[3] = 255;
raw[4] = 255;
raw[5] = 255;
raw[6] = 255;
raw[7] = 255;

var view = new DataView(buffer);
var result = view.getUint64(0); /// :-(

Thank you in advance.

Upvotes: 1

Views: 1176

Answers (2)

anthumchris
anthumchris

Reputation: 9072

The DataView.getUint64() implementation below will get you a number value with as much precision as currently possible:

How to Read 64-bit Integer from an ArrayBuffer / DataView in JavaScript

Upvotes: 0

Benjamin Gruenbaum
Benjamin Gruenbaum

Reputation: 276306

Yes, you can stringify an Int64, but there is no "built in" way to do that since like you said JavaScript numbers are double precision floats. 

// ...
function readInt64(view){
    var small = view.getUint32(0).toString(16); // present as base 16 string
    var large = view.getUint32(1).toString(16);
    // in a power-of-2  base it's just a string concat to add them
    var bigNumber = large + small; 
    return h2d(bigNumber);
}

Where h2d is taken from this answer here.

function h2d(s) {
    // https://stackoverflow.com/a/12533838/1348195
    function add(x, y) {
        var c = 0, r = [];
        var x = x.split('').map(Number);
        var y = y.split('').map(Number);
        while(x.length || y.length) {
            var s = (x.pop() || 0) + (y.pop() || 0) + c;
            r.unshift(s < 10 ? s : s - 10); 
            c = s < 10 ? 0 : 1;
        }
        if(c) r.unshift(c);
        return r.join('');
    }

    var dec = '0';
    s.split('').forEach(function(chr) {
        var n = parseInt(chr, 16);
        for(var t = 8; t; t >>= 1) {
            dec = add(dec, dec);
            if(n & t) dec = add(dec, '1');
        }
    });
    return dec;
}
function readInt64(view, i){
    var small = view.getUint32(0 + 2 * i).toString(16); // present as base 16 string
    var large = view.getUint32(1 + 2 * i).toString(16);
    // in a power-of-2  base it's just a string concat to add them
    var bigNumber = large + small; 
    return h2d(bigNumber);
}

var buffer = new ArrayBuffer(8);
var raw = new Uint8Array(buffer);

raw[0] = 255;
raw[1] = 255;
raw[2] = 255;
raw[3] = 255;
raw[4] = 255;
raw[5] = 255;
raw[6] = 255;
raw[7] = 255;

var view = new DataView(buffer);
var result = readInt64(view, 0);
document.body.innerHTML = result;

Upvotes: 1

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