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up vote 8 down vote favorite

JavaScript can handle the following Math just fine:

var result = (20000000 * 48271) % 0x7FFFFFFF;

But in some programming languages, that first int*int multiplication results in a value too large to hold in a standard 32 bit integer. Is there any way to "simulate" this in JavaScript, and see what the resulting calculation would be if the multiplication resulted in an integer overflow?

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possible duplicate of Does Javascript handle integer overflow and underflow? If yes, how? –  Félix Saparelli May 10 at 6:30
    
@FélixSaparelli It's not a duplicate, 20000000 * 48271 is still well within the JavaScript Number's 52 bit accuracy; it will not overflow. I'm trying to simulate a 32 bit overflow. –  IQAndreas May 10 at 6:33
    
I cheated by subtracting 2^32 enough times from the result, but I don't think that's very efficient or very smart :P –  BoltClock♦ May 10 at 6:34
    
if Math.abs(int * int) is greater than (2^32)/2 then log value and continue? –  Xotic750 May 10 at 6:45
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2 Answers 2

up vote 1 down vote accepted

It is possible to simulate 32-bit integer by "abusing" the bitwise operators available in JavaScript (since they can only return integers within that range).

To convert to a signed 32-bit integer:

x = (a * b) | 0;

To convert to an unsigned 32-bit integer:

x = (a * b) >>> 0;
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The source of the code is from an different question: StackOverflow: JavaScript Integer math incorrect results –  IQAndreas May 27 at 1:26
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up vote 1 down vote

In newer browsers, Math.imul(a,b) will give you an actual 32-bit integer multiplied result, with overflow resulting the way you would expect (it gives the lower half of the 64-bit result as what it returns).

However, as far as I know there's no way to actually get the overflow, (the upper 32 bits) but the modulus you showed in your answer gets rid of that information, so I figure that's not what you want. If they were going to do overflow, they'd have to separate it based on signed and unsigned anyway.

I know this works in Chrome, Firefox, and Opera, not sure about the rest, though pretty sure IE doesn't have it (typical). You'd need to fall back to a shim such as this one.

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I think you're confusing 32-bit/64-bit with unsigned/signed. –  BoltClock♦ May 10 at 16:53
    
No, when you multiply two 32 bit numbers in two's complement, it produces a 64 bit result, though the lower half will be correct regardless of whether the multiplication is signed or not- it's the upper half, the overflow, that varies based on signedness. See stackoverflow.com/questions/14063599/… –  TND May 10 at 21:07
    
Ah OK, thanks for the correction. –  BoltClock♦ May 11 at 6:34
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