Sanity check on FixedNumber use :-) Trying to use small percentages

[iangregsondev]

Hi,

I was trying to workout a slippage of 0.1%, I had so many problems with BigNumber and then I found FixedNumber that everyone was talking about but I couldn't find anything concrete. So I have managed to do it - I think.

But I would appreciate if anybody can comment if I am doing it right ? Am I doing too much, re-inventing any wheel :-)

I thought it might serve as an example to others, if its correct :-) Or even better an example of what not to do, if I am completely off the mark.

Code is below, and here is my output from the code (i used a really small percentage to ensure I had no issues with rounding etc, probably no real world, 0.1% would be sufficient)

amountOut as BigNumber:  1,500,000,000
amountOut as FixedNumber:  1,500,000,000

-----------------------------

Percent to use 0.04242423222%
Here is the percent as a fractional and in a FixedNumber:  0
0.04242423222% of 1,500,000,000 = 636,363.483
1,500,000,000 - 636,363.483 = 1,499,363,636.517  (fixed number)
Here is the result (fixed number) converted to a BigNumber :  1,499,363,637

and code...

import { BigNumber, ethers, FixedNumber } from "ethers"

const conversionPOC = async () => {
  const amountOut = ethers.utils.parseUnits("1.5", 9)
  console.log("amountOut as BigNumber: ", formatIt(amountOut))

  const amountOutFn = FixedNumber.fromValue(amountOut)

  console.log("amountOut as FixedNumber: ", formatIt(amountOutFn))

  console.log("\n-----------------------------\n")

  // Standard Percent of 0.5%
  // const percent = 0.5
  const percent = 0.04242423222
  console.log(`Percent to use ${percent}%`)

  // Convert to fractional by dividing by 100
  const percentage = percent / 100

  // Convert fractional percentage to a FixedNumber
  const fnPercentage = FixedNumber.fromString(percentage.toString())   // fromString ? ??
  console.log("Here is the percent as a fractional and in a FixedNumber: ", fnPercentage.toString())

  // Work out the percentage of the value
  const percentageOf = amountOutFn.mulUnsafe(fnPercentage)
  console.log(`${percent}% of ${formatIt(amountOut)} = ${formatIt(percentageOf)}`)

  // Finally, subtract the value gained from the percentage FROM the original value
  const result = amountOutFn.subUnsafe(percentageOf)
  console.log(`${formatIt(amountOutFn)} - ${formatIt(percentageOf)} = ${formatIt(result)}  (fixed number)`)

  // Attempt to convert FixedNumber to BigNumber
  const converted = BigNumber.from(result.round().toFormat("fixed128x0"))

  console.log(`Here is the result (fixed number) converted to a BigNumber :  ${formatIt(converted)}`)

  console.log("\n--------------------\n")
}

const formatIt = (bn: BigNumber | FixedNumber) => {
  return Number(bn.toString()).toLocaleString("en")
}

void conversionPOC()