Rescaling can overflow a number during standard operations about rust-decimal

[paupino]

I've taken a further look and am going to change approach slightly. Returning the overflowing bit is relatively easy however there is not much that can be done with it. I think a much more gracious solution is to round the number upon rescaling - namely because it can't be represented in greater detail. In the example above, 0.6386554621848739495798319328 should be scaled down to 0.638655462184873949579831933 since the alternative can't be scaled up without overflowing giving a final result of 12.453781512605042016806722690. This seems consistent with other libraries.

[rkusa]

I think a much more gracious solution is to round the number upon rescaling - namely because it can't be represented in greater detail. In the example above, 0.6386554621848739495798319328 should be scaled down to 0.638655462184873949579831933 since the alternative can't be scaled up without overflowing giving a final result of 12.453781512605042016806722690.

Sounds good! Thanks @paupino for having a look and also for providing details and reasoning 👍

[rkusa]

Out of curiosity, for a consistent behavior, couldn't you round to the max possible precision in the multiplication case as well, instead of giving a panic?

rust-decimal/src/decimal.rs

Lines 1811 to 1817 in 7a3728a

	if final_scale > MAX_PRECISION {
	panic!(
	"Multiplication underflowed: {} > {}",
	final_scale,
	MAX_PRECISION
	);
	}

(I've already encountered this panic multiple times)

[paupino]

Yes; good catch. Let’s open a new ticket for this case and I’ll see if I can get to it today - I believe the fix should be relatively straight forward. If possible, can you provide some failure cases for this?

[paupino]

Issue created for the multiplication case at #96.