👨‍💻 Code

You can find all source code on GitHub: https://github.com/AleRapchan/flash-swap-arbitrage-bot

FlashLoaner.sol

pragma solidity =0.6.6;

import './UniswapV2Library.sol';
import './interfaces/IUniswapV2Router02.sol';
import './interfaces/IUniswapV2Pair.sol';
import './interfaces/IERC20.sol';

contract FlashLoaner {
  address immutable factory;
  uint constant deadline = 10 days;
  IUniswapV2Router02 immutable sushiRouter;

  constructor(address _factory, address _uniRouter, address _sushiRouter) public {
    factory = _factory;  
    sushiRouter = IUniswapV2Router02(_sushiRouter);
  }

  function uniswapV2Call(address _sender, uint _amount0, uint _amount1, bytes calldata _data) external {
      address[] memory path = new address[](2);
      uint amountToken = _amount0 == 0 ? _amount1 : _amount0;
      
      address token0 = IUniswapV2Pair(msg.sender).token0();
      address token1 = IUniswapV2Pair(msg.sender).token1();

      require(msg.sender == UniswapV2Library.pairFor(factory, token0, token1), "Unauthorized"); 
      require(_amount0 == 0 || _amount1 == 0);

      path[0] = _amount0 == 0 ? token1 : token0;
      path[1] = _amount0 == 0 ? token0 : token1;

      IERC20 token = IERC20(_amount0 == 0 ? token1 : token0);
      
      token.approve(address(sushiRouter), amountToken);

      // no need for require() check, if amount required is not sent sushiRouter will revert
      uint amountRequired = UniswapV2Library.getAmountsIn(factory, amountToken, path)[0];
      uint amountReceived = sushiRouter.swapExactTokensForTokens(amountToken, amountRequired, path, msg.sender, deadline)[1];

      // YEAHH PROFIT
      token.transfer(_sender, amountReceived - amountRequired);
    
  }
}

Flash Loan Function from AAVE

function flashLoan(address _receiver, address _reserve, uint256 _amount, bytes memory _params)
        public
        nonReentrant
        onlyActiveReserve(_reserve)
        onlyAmountGreaterThanZero(_amount)
    {
        //check that the reserve has enough available liquidity
        //we avoid using the getAvailableLiquidity() function in LendingPoolCore to save gas
        uint256 availableLiquidityBefore = _reserve == EthAddressLib.ethAddress()
            ? address(core).balance
            : IERC20(_reserve).balanceOf(address(core));

        require(
            availableLiquidityBefore >= _amount,
            "There is not enough liquidity available to borrow"
        );

        (uint256 totalFeeBips, uint256 protocolFeeBips) = parametersProvider
            .getFlashLoanFeesInBips();
        //calculate amount fee
        uint256 amountFee = _amount.mul(totalFeeBips).div(10000);

        //protocol fee is the part of the amountFee reserved for the protocol - the rest goes to depositors
        uint256 protocolFee = amountFee.mul(protocolFeeBips).div(10000);
        require(
            amountFee > 0 && protocolFee > 0,
            "The requested amount is too small for a flashLoan."
        );

        //get the FlashLoanReceiver instance
        IFlashLoanReceiver receiver = IFlashLoanReceiver(_receiver);

        address payable userPayable = address(uint160(_receiver));

        //transfer funds to the receiver
        core.transferToUser(_reserve, userPayable, _amount);

        //execute action of the receiver
        receiver.executeOperation(_reserve, _amount, amountFee, _params);

        //check that the actual balance of the core contract includes the returned amount
        uint256 availableLiquidityAfter = _reserve == EthAddressLib.ethAddress()
            ? address(core).balance
            : IERC20(_reserve).balanceOf(address(core));

        require(
            availableLiquidityAfter == availableLiquidityBefore.add(amountFee),
            "The actual balance of the protocol is inconsistent"
        );

        core.updateStateOnFlashLoan(
            _reserve,
            availableLiquidityBefore,
            amountFee.sub(protocolFee),
            protocolFee
        );

        //solium-disable-next-line
        emit FlashLoan(_receiver, _reserve, _amount, amountFee, protocolFee, block.timestamp);
    }

More information:

aave-protocol/contracts/lendingpool/LendingPool.sol at master · aave/aave-protocolGitHub