UniswapV2Arb2 solution note : no need to calculate flash swap fee?

[SiewLin-AIDeaBuddy]

In test/uniswap-v2/solutions/UniswapV2Arb2.test.sol:UniswapV2Arb2Test, there’s a note in the solution that says :
// NOTE - no need to calculate flash swap fee

Why is the flash swap fee calculation not needed in this case?
Shouldn't we be repaying the borrowed amount plus the 0.3% fee when arbitraging back to the pair address?

[EngrPips]

That is because did the arbitrage in a way that we won't be paying back fees, we basically swapped token but instead of first sending the tokens we wanted to swap we first received the token we wanted from the pool and the do an arbitrage using that token and then send back to the pool the token we wanted to swap. Doing that makes us not pay the flash-loan swap fee.

Ask questions if anything is confusing.

[SiewLin-AIDeaBuddy]

Thanks for the reply. I found it a bit difficult to understand the difference in context between UniswapV2Arb2 and UniswapV2Arb1 until I took a closer look at the test case for UniswapV2Arb2.

It was initially confusing because the comments in UniswapV2Arb2.test.sol are exactly the same as those in UniswapV2Arb1.test.sol. That made it a bit tricky to immediately grasp the unique goal or setup of the second exercise. Perhaps the comments in UniswapV2Arb2.test.sol could be updated to reflect the slightly different scenario more accurately.

In UniswapV2Arb2, we’re not performing a typical flash loan in the same way as in UniswapV2Arb1. Instead of choosing an amount to borrow and then calculating how to repay it, we start with the repayment amount (amountIn) and use the getAmountOut function to determine how much we can borrow:

    function getAmountOut(
        uint256 amountIn,
        uint256 reserveIn,
        uint256 reserveOut
    ) internal pure returns (uint256 amountOut) {
        uint256 amountInWithFee = amountIn * 997;
        uint256 numerator = amountInWithFee * reserveOut;
        uint256 denominator = reserveIn * 1000 + amountInWithFee;
        amountOut = numerator / denominator;
    }

The swap fee is already factored in using the *997/1000 multiplier, so the amountOut we borrow is net of fees. Then we use that value directly in the swap:

        IUniswapV2Pair(pair0).swap({
            amount0Out: isZeroForOne ? 0 : amountOut,
            amount1Out: isZeroForOne ? amountOut : 0,
            to: address(this),
            data: data
        });

Seeing it from this reverse perspective—starting with repayment and working backwards, helped me understand the logic behind UniswapV2Arb2 better. These examples are a great way to learn about different flash loan strategies. Appreciate the thoughtful exercise design!

[EngrPips]

Fair enough. Keep crushing it!