"math64::mul_div(" returning u64 when I need decimals (ex: calculate percentage)

[NaDasai]

Discord user ID

ibuidl

Describe your question in detail.

Let's say I have an amount of coins (ex: 1 APT) and I want to calculate the percentage of fees (1%).
So the value is 0.01 APT. I'm using math64 to calculate the percentage:
use aptos_std::math64;
let fees = math64::mul_div(amount, 1, 100)};

The issue is that mul_div returns an u64. So the value of fees will be 0 instead of 0.01.
And here I'm not withdrawing anything:
let coin = coin::withdraw<AptosCoin>(account, fees);

(I guess APT has 8 decimals so we can do a workaround with but I don't know how. Is there a solution for this?)

What error, if any, are you getting?

No response

What have you tried or looked at? Or how can we reproduce the error?

No response

Which operating system are you using?

macOS

Which SDK or tool are you using? (if any)

Aptos CLI

Describe your environment or tooling in detail

No response

[MoonShiesty]

the usual pattern for doing decimals math with integer is to using a scaling factor:

let scaling_factor: u64 = 10_000_000;
let fees: u64 = 1_000; // 1bps

let fees = muldiv(amount, fees, scaling_factor)

this example would have zero fees for values between E-8 and E-4, if thats an issue you could play around with rounding, a different scaling factor or using u128 for scaling

you could also using trying fixed_point64 but i find integer math is generally cleaner if you reason through it