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Some time passed. The client came back to me: "You've been calculating the invoices wrong! Our accountant has had to work an extra few days because about one in five of your invoices has an error of £0.01, and our accounts don't balance!"
Did you spot my error? So, what's the moral of this story? I'm not sure, because it's quite subtle, but I could claim that it's a case of "make impossible states unrepresentable". The challenge was in recognising that we were representing impossible states. Suggestions welcome.
Implement some kind of double booking type, where money can never disappear. I.e. the sum of the values is preserved across all operations?
.... all amounts in bitcoin, no rounding. Done!
I actually really like the idea of arbitrary-precision money values, independent of bitcoin.
A division and a subtraction would avoid both rounding errors going the same direction and summing to a lost penny.
Actually have faced it in many projects, my workaround define a rounding context to freeze max permissible valid digits in double. e.g. in current work it is 4 decimal with roundup beyond 4. We can also encapsulate it into a class(Type).
Also I work in java, have been meaning to develop a Money lib to handle this. I would ideally want a fractional denomination separator, like it might have interface Money and an Implementation like dollar which can represent 1 $ and 56cents without float.
an accountant I worked with recommended adding a "fudge factor" column to ledgers...
Yes, this sounds like an accountants' solution. And that's not meant to sound entirely dismissive: a few pennies lost in accounting doesn't indicate the same sort of problem or solution as an off-by-one error in software engineering.
I do not get it... Have you used `double` for monetary calculation? But that would not be subtle...
Certain whole numbers can't ever be the result of adding 20% to another whole number. If VAT were 100% instead of 20%, you couldn't have a VAT-inclusive price be odd, for example.
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