First Flight #18: T-Swap

[H-02] Wrong Fee Calculations in TSwapPool::getInputAmountBasedOnOutput Causes users to pay more than intended

Description:
The getInputAmountBasedOnOutput function uses hardcoded values for fees instead of constant defined variables. This in itself is not a good practice, but to add to the problem, the used values are wrong and instead of 1000 basis points, it uses 10000.

Impact:
Users that intend to swap for an exact output will have to pay 100 - 100*997/10000 = 90.03% fees instead of 100 - 100*997/1000 = 0.3% fees.

Proof of Concept:
If x is the inputReserves and y is the outputReserves, the ∆x is calculated like this:

Another way to write this, inputting β = (∆y / y) and assuming γ = 997/1000:

showing the value should be 1000 instead of 10000.

Recommended Mitigation:

Fix the value and also use constant variables for fees.