Or just work in floats and round as the last step. Or do both selectively depending on which rounding error works in your favor, but don't tell anyone that's what you're doing.
The real problem is that nobody at the company seems to have looked at the parts of their software that everyone sees, so what else have they not looked at?
That's not as easy to get right as one might like. Let's say we have an amount in dollars, represented as an IEEE 754 double, and a tax rate, also represented as a double. Let's assume that the amount is always some integer multiple of 0.01, and the tax rate is always some integer multiple of 0.00001.
Let's say we want the tax in cents. A first try might be:
unsigned long tax_in_cents(double subtotal, double rate)
{
return (unsigned long)round(subtotal * rate * 100);
}
That will sometimes fail. The problem is it is rounding at the wrong place. It's logically rounding to the nearest multiple of 0.01, which is too crude. The rounding has to be much farther to the right.
This will do the trick:
unsigned long tax_in_cents(double amt, double rate)
{
const unsigned long M = 100 * 100000;
return (unsigned long)round((round(amt * rate * M) / (M/100)));
}
I'm not at all sure that if I had come across that first try in real life I would have noticed that it is not adequate.