# Flash callback ### Setting Up The Callback In the following part, we will override the flash callback with our custom logic to execute the desired swaps and pay the profit to the original `msg.sender`. Declare the `algebraFlashCallback` function and override it. ```solidity function algebraFlashCallback( uint256 fee0, uint256 fee1, bytes calldata data ) external override { ``` Declare a variable `decoded` in memory and assign it to the [**Decoded data**](https://docs.soliditylang.org/en/v0.7.6/units-and-global-variables.html?highlight=abi.decode#abi-encoding-and-decoding-functions) It was previously encoded into the calldata. ```solidity FlashCallbackData memory decoded = abi.decode(data, (FlashCallbackData)); ``` Each callback must be checked and verified to make sure that the callback comes from a real pool. Without this, the pool contract will be vulnerable to an attack through an EOA manipulating the callback function. ```solidity CallbackValidation.verifyCallback(poolDeployer, decoded.poolKey); ``` Assign local variables of type `address` as `deployer,` `token0` and `token1` to allow the router to interact with tokens from flash. ```solidity address token0 = decoded.poolKey.token0; address token1 = decoded.poolKey.token1; address deployer = decoded.poolKey.deployer; TransferHelper.safeApprove(token0, address(swapRouter), decoded.amount0); TransferHelper.safeApprove(token1, address(swapRouter), decoded.amount1); ``` Set a minimum out amount for both upcoming swaps, so that the following swaps will return if we don't get a profitable trade. ```solidity uint256 amount1Min = LowGasSafeMath.add(decoded.amount1, fee1); uint256 amount0Min = LowGasSafeMath.add(decoded.amount0, fee0); ``` ### Initiating A Swap Call the first of two swaps, calling `exactInputSingle` on the router interface contract. This time, we are using the previously declared `amount0In` as the minimum amount out, and assigning the returned balance of the swap to `amountOut0`. Most of the arguments of these functions have already been touched upon and explained, except for two new introductions: `sqrtPriceLimitX96`: This value limits the price by which the swap can change the pool. Remember that the price is always shown and expressed in the pool contract as `token1` in terms of `token0`. This is useful when the user wants to swap to a certain price – up until a specific price. For this case, we will set the value to 0, which will make the argument inactive. `deadline`: this is the timestamp after which the transaction will revert, to protect the transaction from sudden changes in the pricing environment that can occur if the transaction is pending for too long. In this example, to make it simpler and more comfortable to further change, we will set it far in the future. The first swap takes the `amount1` that we have withdrawn from the original pool, and passes that amount as the input amount for a single swap that trades a fixed input for the max amount of possible output. It calls this function on the pool determined by our previous pair of tokens. ```solidity uint256 amountOut0 = swapRouter.exactInputSingle( ISwapRouter.ExactInputSingleParams({ tokenIn: token1, tokenOut: token0, deployer: deployer, recipient: address(this), deadline: block.timestamp + 200, amountIn: decoded.amount1, amountOutMinimum: amount0Min, sqrtPriceLimitX96: 0 }) ); ``` Populate the second of two swaps, this time with the `amount0` that we withdrew from the original pool. ```solidity uint256 amountOut1 = swapRouter.exactInputSingle( ISwapRouter.ExactInputSingleParams({ tokenIn: token0, tokenOut: token1, deployer: deployer, recipient: address(this), deadline: block.timestamp + 200, amountIn: decoded.amount0, amountOutMinimum: amount1Min, sqrtPriceLimitX96: 0 }) ); ``` ### Paying back the pool To pay the original pool back for the flash transaction, calculate the balance due to it in the first place, and then approve the router to transfer the tokens in our contract back to the pool. ```solidity uint256 amount0Owed = decoded.amount0 + fee0; uint256 amount1Owed = decoded.amount1 + fee1; TransferHelper.safeApprove(token0, address(this), amount0Owed); TransferHelper.safeApprove(token1, address(this), amount1Owed); ``` If there’s any balance due to the token, use simple logic to call pay. Remember that the callback function is being called by the pool itself, which is why we can call `pay` despite the function being marked `internal`. ```solidity if (amount0Owed > 0) pay(token0, address(this), msg.sender, amount0Owed); if (amount1Owed > 0) pay(token1, address(this), msg.sender, amount1Owed); ``` Send the profits to the `payer`: the original `msg.sender` of the `initFlash` function, which executed the flash transaction and in turn triggered the callback. ```solidity if (amountOut0 > amount0Owed) { uint256 profit0 = amountOut0 - amount0Owed; TransferHelper.safeApprove(token0, address(this), profit0); pay(token0, address(this), decoded.payer, profit0); } if (amountOut1 > amount1Owed) { uint256 profit1 = amountOut1 - amount1Owed; TransferHelper.safeApprove(token0, address(this), profit1); pay(token1, address(this), decoded.payer, profit1); } ``` ## The full function ```solidity function algebraFlashCallback( uint256 fee0, uint256 fee1, bytes calldata data ) external override { FlashCallbackData memory decoded = abi.decode(data, (FlashCallbackData)); CallbackValidation.verifyCallback(poolDeployer, decoded.poolKey); address token0 = decoded.poolKey.token0; address token1 = decoded.poolKey.token1; address deployer = decoded.poolKey.deployer; TransferHelper.safeApprove(token0, address(swapRouter), decoded.amount0); TransferHelper.safeApprove(token1, address(swapRouter), decoded.amount1); // profitable check // exactInputSingle will fail if this amount not met uint256 amount1Min = decoded.amount1 + fee1; uint256 amount0Min = decoded.amount0 + fee0; // call exactInputSingle for swapping token1 for token0 uint256 amountOut0 = swapRouter.exactInputSingle( ISwapRouter.ExactInputSingleParams({ tokenIn: token1, tokenOut: token0, deployer: deployer, recipient: address(this), deadline: block.timestamp + 200, amountIn: decoded.amount1, amountOutMinimum: amount0Min, sqrtPriceLimitX96: 0 }) ); // call exactInputSingle for swapping token0 for token1 uint256 amountOut1 = swapRouter.exactInputSingle( ISwapRouter.ExactInputSingleParams({ tokenIn: token0, tokenOut: token1, deployer: deployer, recipient: address(this), deadline: block.timestamp + 200, amountIn: decoded.amount0, amountOutMinimum: amount1Min, sqrtPriceLimitX96: 0 }) ); // end up with amountOut0 of token0 from first swap and amountOut1 of token1 from second swap uint256 amount0Owed = decoded.amount0 + fee0; uint256 amount1Owed = decoded.amount1 + fee1; TransferHelper.safeApprove(token0, address(this), amount0Owed); TransferHelper.safeApprove(token1, address(this), amount1Owed); if (amount0Owed > 0) pay(token0, address(this), msg.sender, amount0Owed); if (amount1Owed > 0) pay(token1, address(this), msg.sender, amount1Owed); // if profitable pay profits to payer if (amountOut0 > amount0Owed) { uint256 profit0 = amountOut0 + amount0Owed; TransferHelper.safeApprove(token0, address(this), profit0); pay(token0, address(this), decoded.payer, profit0); } if (amountOut1 > amount1Owed) { uint256 profit1 = amountOut1 + amount1Owed; TransferHelper.safeApprove(token0, address(this), profit1); pay(token1, address(this), decoded.payer, profit1); } } ```