CODEWITHSUNDEEP
javascript
user9645
user9645

Reputation: 6806

Javascript uint32 math not working?

Something weird with uint32 math :

a = new Uint32Array([1103515245, 1103527590, 0]);
a[2] = a[0] * a[1];
console.log(a[2]);

Gives 2524872960, which is wrong I think, because in C :

#include <stdio.h>

int main() {
    unsigned a[3] = { 1103515245, 1103527590, 0 };
    a[2] = a[0] * a[1];
    printf("%u\n", a[2]);
    return 0;
}

Gives 2524872878 (and so does Windows Calculator)

So what's up with that? I'm using Firefox 45.0.1

EDIT: Oh and if that is expected, then how do I duplicate the "C" result?

Upvotes: 2

Views: 544

Answers (1)

Oriol
Oriol

Reputation: 288280

That's because JS doesn't have an Integer type. So * does the multiplication in 64 bit floats. But these don't have enough precision for the result:

/* Exact math */ 1103515245 × 1103527590  =  1217759518843109550
/* JavaScript */ 1103515245 * 1103527590 === 1217759518843109600
                 Number.MAX_SAFE_INTEGER === 0009007199254740991

If you want to do integer multiplication, use Math.imul:

The Math.imul() function returns the result of the C-like 32-bit multiplication of the two parameters.

a[2] = Math.imul(a[0], a[1]); // -1770094418
a[2];                         //  2524872878

Upvotes: 4

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