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Comment by jandrese | Hacker News Reader

Adding and subtracting numbers of wildly different magnitudes is always cursed. If you're doing this I'd say the majority of the time you are making an error.

Your equation ends up being like "We calculated the diameter of the observable universe, but what if we added the Empire State Building to one side, how does that affect the result?"

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I don't remember the exact numbers, but I remember seeing a CAD file that was not working well a few years ago, and it turned out the coordinate system set up for the part was roughly equivalent to building a part the size of a man's fist but measuring it from the center of the Earth (in meters) while putting the part in Earth orbit.

It’s also what we do when incrementing a counter, if it’s a very big counter. Operations like that don’t have a physical meaning, but can happen in things like random number generators where it’s a way of mixing the bits. Or maybe allocating unique ids, if you started from a random number.

Of course it is just an example so we shouldn’t nitpick you too much, but that sounds more like a job for an integer.

Yes, absolutely. Though in JavaScript we often make do with the safe integer range. (I don’t know how fast BigInt is for 64 bit integers.)

I don’t know JavaScript so I’m sure to shoot my own foot here, but surely a BigInt who’s value can fit in 64 bits is just a normal 64 bit int (from hardware) with some extra bookkeeping around it on most platforms, right?

I can't speak for any other engines, but in SpiderMonkey a BigInt is a GC-managed object with a one-word header containing the sign bit, the length, and some GC bookkeeping bits, along with a second word that is either a single inline 64-bit digit, or a pointer to an array of 64-bit digits. So the memory overhead of a BigInt that fits in 64 bits is "only" 2x, but doing math with BigInt requires more work than a native integer because it needs to handle larger numbers, manually manage sign bits, and so on.

(https://searchfox.org/mozilla-central/source/js/src/vm/BigIn...)

If all you’ve got is JS then every problem looks like a double.

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