# Liquidation Exercises In this exercise, you'll write a contract that will 1. Get a flash loan from Aave V3. 2. Liquidate an under-collateralized loan on Aave V3. 3. Use the [`UniversalRouter`](https://github.com/Uniswap/universal-router/blob/main/contracts/UniversalRouter.sol) contract to swap collateral received from liquidation into the token that was borrowed. 4. Repay the flash loan and send profit to `msg.sender`. The starter code for this exercise is provided in [`foundry/src/exercises/Liquidate.sol`](https://github.com/Cyfrin/defi-uniswap-v4/blob/main/foundry/src/exercises/Liquidate.sol) Solution is in [`foundry/src/solutions/Liquidate.sol`](https://github.com/Cyfrin/defi-uniswap-v4/blob/main/foundry/src/solutions/Liquidate.sol) > **Note** > This exercise uses a flash loan from **Aave V3** because it’s not possible to obtain a flash loan from **Uniswap V4** and then perform a swap on Uniswap V4 via the `UniversalRouter`. > When you get a flash loan from Uniswap V4, the `PoolManager` contract becomes locked, which prevents the `UniversalRouter` from acquiring a lock. > In practice, a more efficient approach would be to take the flash loan directly from Uniswap V4 (since its flash loan fee is **0**) and perform the swap directly through the `PoolManager` contract. > However, in this exercise, the focus is on learning how to interact with the `UniversalRouter` contract. ## Task 1 - Initiate liquidation ```solidity function liquidate( // Token to flash loan address tokenToRepay, // User to liquidate address user, // V4 pool to swap collateral PoolKey calldata key ) external {} ``` This function initiates a liquidation by obtaining a flash loan. - Call `liquidator.getDebt` to get the amount of the token needed to repay the under-collateralized loan. - Call `flash.flash` to obtain a flash loan. `IFlash` will call back into the `flashCallback` function. - Refund any excess `tokenToRepay` to `msg.sender`. ## Task 2 - Liquidate, swap and repay ```solidity function flashCallback( address tokenToRepay, uint256 amount, uint256 fee, bytes calldata data ) external { // Write your code here } ``` This function performs the liquidation, swaps the collateral received into the repayment token, and repays the flash loan. - Call `liquidator.liquidate` to liquidate the under-collateralized loan of `user`. - Call `swap` to convert the collateral into the token needed to repay the flash loan. - Repay the flash loan. ## Test ```shell forge test --fork-url $FORK_URL --match-path test/Liquidate.test.sol -vvv ```