Get Fraction of SOL Balance

Question

I have a u128 total and a u128 user_total. I also have a balance and want the user to be able to withdraw (balance * user_total) / (total * multiplier)

What is the best way to do this to avoid overflow, since balance is a u64 and both totals are u128s?

I still want the calculation to happen in the case of an overflow as shown by u64::MAX

Currently, I have:

// user_total: u128, total: u128, balance: u64, multiplier: u8 ; multiplier > 0 and user_total < total

let divisor: Option<u128> = total.checked_mul(multiplier as u128);

let amount_won = match (balance as u128).checked_mul(user_total) {
    Some(result) => match divisor {
        Some(div) => result.checked_div(div).ok_or(StakeError::Overflow)?,
        None => {
            let intermediate = result.checked_div(multiplier as u128).ok_or(StakeError::Overflow)?;
            intermediate.checked_div(total).ok_or(StakeError::Overflow)?
        }
    },
    None => {
        let fallback_result = (balance as u128).checked_mul(u64::MAX as u128).unwrap_or_default();
        match divisor {
            Some(div) => fallback_result.checked_div(div).ok_or(StakeError::Overflow)?,
            None => {
                let intermediate = fallback_result.checked_div(multiplier as u128).ok_or(StakeError::Overflow)?;
                intermediate.checked_div(total).ok_or(StakeError::Overflow)?
            }
        }
    }
} as u64;

It's a bit complicated and I haven't tested all edge cases of this piece of code, but am wondering if there is just a better way to do this.

Answer 1

You can certainly do something like this, or use a 256-bit number using a crate like uint https://github.com/paritytech/parity-common/tree/master/uint, ie:

construct_uint! {
    /// 256-bit unsigned integer.
    pub struct U256(4);
}

And then perform math on it as you need.