uzyn/bigdenary

🦕🧮 Arbitrary-length decimal library, implemented with ES2020's native BigInt.

bigdenary's Issues

Rounding issues and unsafe operations

I think this line in the implementation is not safe it could cause rounding issues:

https://github.com/uzyn/bigdenary/blob/master/mod.ts#L31C7-L31C60

For example the following number will fail: 0.0023537

You can see the problem with this if you try the following calculation:

// 23536.999999999996 instead of 23537
0.0023537 * Math.pow(10, 7)

The response will be a float instead of an integer so it will fail with BigInt.

(100 / 3) * 3 != 100 and (100 / 3) + (100 / 3) + (100 / 3) != 100

Hi,
I found an issue I want to highlight, curious about the position of the maintainer of this library on it.

I found that for bigdenary (100 / 3) * 3 != 100 and (100 / 3) + (100 / 3) + (100 / 3) != 100

the failing test are the following

Deno.test("BigDenary: (100 / 3) + (100 / 3) + (100 / 3) == 100", () => {
	const b_d100 = new BigDenary(100);
	const b_d33 = b_d100.div(3);
	const b_d100sum = new BigDenary(0).add(b_d33).add(b_d33).add(b_d33);
	assert(b_d100.eq(b_d100sum));
});

Deno.test("BigDenary: (100 / 3) * 3 == 100", () => {
	const b_d100 = new BigDenary(100);
	const b_d100mul = new BigDenary(100).div(3).mul(3);
	assert(b_d100.eq(b_d100mul));
});

The tests succeed when a BigDenary is converted to number with valueOf(), because valueOf() internally convert to string and then uses parseFloat() for the conversion to number, and because the converted string has more than 14 decimals, the returned number is rounded.

I decided then to test also decimal.js and number object, and I found that: number process them correctly (failing on others) while decimal.js fails at them as this library.

Test file attached.

For completeness, I also tested C #, and using the decimal type the problem is not present:

	decimal d100 = 100;
	decimal d33 = d100/ 3;
	decimal d100sum = d33 + d33 + d33;
	decimal d100mul = d33 * 3;
	Console.WriteLine(d33);
	Console.WriteLine(d100 == d100sum);
	Console.WriteLine(d100sum);
	Console.WriteLine(d100 == d100mul);
	Console.WriteLine(d100mul);

test.txt

This module name has been reserved for a repository

Please take a look at https://deno.land/x/bigdenary