Javascript bigint division

Question

I am trying to decide whether I need a bigint lib or the normal js numbers will be enough. I want to divide file sizes with chunk sizes in bits up to 100GB to get how many chunks I have: howManyChunks = Math.ceil(fileSize/chunkSize). I remember that js has problems with precise division.

What is the chance I get x+1 = Math.ceil(x.00000000000..01) instead of x = Math.ceil(x.0) in the result?

As your value may be `2.5` you will probably loose precision... — Jonas Wilms, Nov 19 '17 at 20:28
@JonasW. It does not matter by 2.5, since the result of `ceil` will be the same. — inf3rno, Nov 19 '17 at 23:17
In nowadays, 2019, you can use [BigInt primitive type](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/BigInt). — Peter Krauss, Mar 10 '19 at 12:14

inf3rno · Accepted Answer · 2017-11-20T03:53:33.340

I decided to try out. Precision of normal js numbers starts to be not enough around 3-4PB file size. So in Firefox js integers have around 55bits precision. I guess this might differ by browsers, but I need this for nodejs.

console.log("start");

var B = 8;
var KB = 1024*B;
var MB = 1024*KB;
var GB = 1024*MB;
var TB = 1024*GB;
var PB = 1024*TB;
var EB = 1024*PB;

var maxBits = 4 * PB;
var bitSteps = 100 * TB;
var maxChunkSize = 333 * B;

for (var bits = 0; bits <= maxBits; bits += bitSteps)
    for (var chunkSize = 1; chunkSize <= maxChunkSize; ++chunkSize) {
        var calculatedChunkCount = Math.ceil(bits/chunkSize);
        var calculatedBits = calculatedChunkCount * chunkSize;
        var difference = calculatedBits - bits;
        var error = difference>= chunkSize || difference<0;
        if (error)
            console.log({
                chunkSize: chunkSize,
                calculatedChunkCount: calculatedChunkCount,
                bits: bits,
                calculatedBits: calculatedBits,
                difference: difference
            });
    }

console.log("end");

output:

{ chunkSize: 97, calculatedChunkCount: 290180388361501, bits: 28147497671065600, calculatedBits: 28147497671065596, difference: -4 }
{ chunkSize: 1579, calculatedChunkCount: 20611490932343, bits: 32545544182169600, calculatedBits: 32545544182169596, difference: -4 }

I'll start a parallel nodejs script for night just to be sure, but I think that precision is enough in the 0 - 100GB range.

According to another answer: https://stackoverflow.com/a/2803010/607033 js integers are accurate up to 53 bits. So this division limit might be close to that value, maybe a few bits lower.

Javascript bigint division

1 Answers1