## Uniswap V2-R2: Flash Swap In this lesson, we'll write some Solidity code for a Uniswap V2 arbitrage bot using the `flashSwap` function. This function allows us to borrow tokens from a Uniswap V2 pair, execute a trade on another pair, and repay the borrowed tokens all within the same transaction. We'll write code for both the `flashSwap` function and the `uniswapV2Call` function, which is the callback function for the flash swap. Let's get started! First, we'll import the `UniswapV2Pair` interface: ```javascript import { UniswapV2Pair } from "uniswap-v2-core/contracts/UniswapV2Pair.sol"; ``` Next, we'll create a `struct` to represent the data that we'll send to the `flashSwap` function: ```javascript struct FlashSwapData { // Caller of flashSwap (msg.sender inside flashSwap) address caller; // Pair to flash swap from address pair; // Pair to swap from address pair1; // True if flash swap is token0 in and token1 out bool isZeroForOne; // Amount in to repay flash swap uint256 amountIn; // Amount to borrow from flash swap uint256 amountOut; // Amount out of profit is less than this minimum uint256 minProfit; } ``` Now, we'll write the `flashSwap` function: ```javascript function flashSwap( address pair0, address pair1, bool isZeroForOne, uint256 amountIn, uint256 minProfit ) external { // Write your code here // Don't change any other code // Hint - use getAmountOut to calculate amountOut to borrow uint256 amountOut = getAmountOut(amountIn, reserve0, reserve1); bytes memory data = abi.encode( FlashSwapData({ caller: msg.sender, pair: pair0, pair1: pair1, isZeroForOne: isZeroForOne, amountIn: amountIn, amountOut: amountOut, minProfit: minProfit, }) ); UniswapV2Pair(pair0).swap( amountOut, amount0Out, amount1Out, address(this), data ); } ``` Finally, we'll write the `uniswapV2Call` function, which will execute the arbitrage and repay the borrowed tokens: ```javascript function uniswapV2Call( address sender, uint256 amount0Out, uint256 amount1Out, bytes calldata data ) external { // Write your code here // Don't change any other code FlashSwapData memory params = abi.decode(data, (FlashSwapData)); address token0 = IUniswapV2Pair(params.pair0).token0(); address token1 = IUniswapV2Pair(params.pair0).token1(); (address tokenIn, address tokenOut) = params.isZeroForOne ? (token0, token1) : (token1, token0); IERC20(tokenOut).transfer(params.pair1, params.amountOut); UniswapV2Pair(params.pair1).swap( amountOut, amount0Out, amount1Out, address(this), data ); IERC20(tokenIn).transfer(params.pair0, params.amountIn); uint256 profit = amountOut - params.amountIn; require(profit >= params.minProfit, "profit < min"); IERC20(tokenIn).transfer(params.caller, profit); } ``` Now, we can execute the tests in our terminal. We'll start by setting up the fork URL, then execute the tests: ```bash forge test --fork-url $FORK_URL --match-path test/uniswap-v2/exercises/UniswapV2arb2.sol:UniswapV2arb2Test ``` We should see the tests pass. We can copy the profit from the terminal and divide it by 1e18 to see our profit in ETH. Let me know if you have any questions.