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Divide before multiply

Description​

In Rust, the order of operations can influence the precision of the result, especially in integer arithmetic.

Why is this bad?​

Performing a division operation before a multiplication can lead to a loss of precision as division between integers might return zero.

Issue example​

Consider the following Soroban contract:


 pub fn split_profit(percentage: u64, total_profit: u64) -> u64 {
    (percentage / 100) * total_profit
}

In this contract, the split_profit function divides the percentage by 100 before multiplying it with total_profit. This could lead to a loss of precision if percentage is less than 100 as the division would return 0. This could lead to incorrect calculations and potential financial loss in a real-world smart contract.

The code example can be found here.

Remediated example​

Reverse the order of operations to ensure multiplication occurs before division.


 pub fn split_profit(&self, percentage: u64, total_profit: u64) -> u64 {
            (percentage * total_profit) / 100
        }

The remediated code example can be found here.

How is it detected?​

Checks the existence of a division before a multiplication.

References​

Rust documentation: Integer Division