Slippage setting

[daron4ever]

Describe your Issue

We normally use the following snippets to get slippage.

const amounts = await router.getAmountsOut(amountIn, [
    tokenA,
    tokenB,
]);
amountOutMin = amounts[1].sub(amounts[1].div(100).mul(slippage));

But I have the error when I use decimal slippage such as 0.5%

Error: underflow [ See: https://links.ethers.org/v5-errors-NUMERIC_FAULT-underflow ] (fault="underflow", operation="BigNumber.from", value=99.5, code=NUMERIC_FAULT, version=bignumber/5.6.0)
    ... {
  reason: 'underflow',
  code: 'NUMERIC_FAULT',
  fault: 'underflow',
  operation: 'BigNumber.from',
  value: 99.5

Anyways to pass the slippage with a decimal point?

This is an exception issue

• I understand this form is for exceptional issues and believe my issue qualifies.

[ricmoo]

This should have been opened as a discussion, not an exceptional issue.

You can either re-arrange your maths to prevent underflow (or mitigate it earlier in evaluation) or use fixed-point maths, using the FixedNumber class. Keep in mind BigNumber only operates on integers, so 4 / 5 == 0.

For 0.5% slipage, I would use a formula akin to:

amountOutMin = amounts[1].sub(amounts[1].mul(995).div(1000));

But the FixedNumber class, while more complex is also more advanced if you need more dynamic values and don't want to deal with figuring out the significant digits yourself. It is also useful if you wish to compound other operations.