Avoiding rounding pitfalls in payment systems

This title was summarized by AI from the post below.

a common pitfall in payment systems is not handling rounding correctly. you must ensure that all components of a payment collection are duly accounted for. things like taxes, fees, etc. this is often specified in percentages. if you would charge 1% of $123.45 in fees and 7.5% of the resulting value in VAT, you can see how this becomes a real problem. if your payment works across multiple markets, the problem gets more interesting. you may have different pricing and the taxes may be different. one thing I learned recently is that even the rounding laws for taxes may be different. for example, some countries like the US may require you to round to the nearest whole unit of the currency (a dollar in the US, vs cents). Malaysia requires you to round to the nearest 5 sen (a cent). the UK allows you to round income down to the nearest pound and round expenses up to the nearest pound. this means that rounding is something that requires context of the transaction such as the nature of the transaction and the applicable regulations. a generic rounding function will fail. but it gets even more interesting, after computing individual parts and rounding correctly, you must ensure that it all sums up to the correct amount. this means you can't round parts in isolation. a flexible part must exist to ensure the total balances up. otherwise, you may end up 'creating' new money or 'dropping' existing money in your system.

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