Comment by bigiain | Hacker News Reader

Comment by bigiain | Hacker News Reader

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> Granularity of your financial amounts should ...

... comply with relevant regulations.

I worked on a gambling webapp a long time back - we were bound by government regulations on gambling licences to perform all calculations at a precision of one ten thousandth of a cent, and were only permitted to round to cents once at the very end just before displaying a dollar amount to a person.

(This suited me down to the ground because I pointed out to everybody that Javascript couldn't be guaranteed to treat numbers and calculations in a way that would keep the regulators auditors happy, and managed to push _every_ calculation involving dollar amounts off to the Java backend team.)

We also used integer UTC milliseconds for time, so we'd have good enough for the auditors timestamps showing who placed a valid bet at (say) 12:34:59.9997 and who else place an invalid bet at 12:35:00.0003 for a wager with a cut off time of 12:35. (We asked for and got a ruling from the regulators that the time that mattered was the time the API call was processed by the backend, since network latencies between the webapp and the backend platform were impossible to predict. I have no idea if the backend had millisecond accurate time, that wasn't my problem and I _so_ didn't want to have them make it my problem by asking the question.)

> This suited me down to the ground because I pointed out to everybody that Javascript couldn't be guaranteed to treat numbers and calculations in a way that would keep the regulators auditors happy,

Javascript can represent dollar values up to $2.25 billion at a resolution of ten-thousandths of a cent without any loss of precision.

I would want all money calculations done by the backend team as a matter of policy. But it's not a technical limitation.

While you are correct that it can _represent_ these numbers with that precision... it cannot operate on them and retain that precision, so DO NOT USE FLOATS TO REPRESENT MONEY. Thank you for your time.

    let a = 35
    let b = -34.99

    console.log(a+b)

    // output: 0.00999999999999801

Brr almost got the shivers until I read your comment

You wrote "a long time back", so maybe my point can be ignored.

Did you not consider to use decimal math? According to MDN, the largest int in Javascript is 2^53 – 1. If you steal four digits for your fraction, that still leaves a huge number. I will assume that no one was gambling more than one billion currency units, so you had plenty of space. Do all your calcs with ints, then divide by 10,000 for a final result.

Ref: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...

> Do all your calcs with ints

I suspect there's an important set of gotchas lurking in here around the difference between actually doing calculations with integers---which is not possible in Javascript--versus doing calculations with floating-points that happen to represent (safe) integers.

Luckily javascript has bigint now, so you can do this stuff safely in js now (just don't do it in the frontend ever, because you cannot trust the frontend).

> We asked for and got a ruling from the regulators that the time that mattered was the time the API call was processed by the backend

I haven't been in that situation, but I imagine I would reach for a "what if it was snail-mail" analogy. "It's dangerous to honor whatever time the bettor wrote by hand, the postmark-time may not be fair if the mailman got delayed because of a half-illegible address or random USPS delays... So how about we go for when it was actually delivered to us?"

I wonder if that's the sort of ruling that just got thrown out by the Supreme Court in favor of having courts decide.

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