How to Handle Rounding with BigNumber?

[ernanirst]

I have a use case where I need to divide 105 with 100 using ether.js BigNumber (it's not really 105, it's a much larger number than that). The result is 1.05 but because I'm using BigNumber that rounds down to 1.

The issue I'm facing is that I need this division to always round up, instead of rounding down, so I implemented the following workaround:

function divRoundUp(bn1, bn2) {
  const result = bn1.div(bn2)
  return bn1.mod(bn2).isZero() ? result : result.add(BigNumber.from('1'))
}

Essentially, this function uses the mod function to check for a remainder, and if there is any, it increases the result in 1.

I understand that ether.js BigNumber uses bn.js under the hood, does it make sense to expose bn.js function divRound to support rounded divisions?

[ricmoo]

Are you trying to round or take the ceiling? 1.05 will always round to one. 1.45 will round to 1. 1.5 will round to 2 in numeric systems that support rounding, and for the correct rounding strategy.

What you are interested in is using the ethers.FixedNumber class, which supports operation that preserve some fixed number of decimal places using an internal fixed-precision representation.

See: https://docs.ethers.io/v5/api/utils/fixednumber/