Contract Source Code:

// SPDX-License-Identifier: MIT

pragma solidity >=0.7.6;
pragma abicoder v2;

// helper methods for interacting with ERC20 tokens and sending ETH that do not consistently return true/false
library TransferHelper {
  function safeApprove(
    address token,
    address to,
    uint256 value
  ) internal {
    // bytes4(keccak256(bytes('approve(address,uint256)')));
    (bool success, bytes memory data) = token.call(abi.encodeWithSelector(0x095ea7b3, to, value));
    require(success && (data.length == 0 || abi.decode(data, (bool))), "TransferHelper: APPROVE_FAILED");
  }

  function safeTransfer(
    address token,
    address to,
    uint256 value
  ) internal {
    // bytes4(keccak256(bytes('transfer(address,uint256)')));
    (bool success, bytes memory data) = token.call(abi.encodeWithSelector(0xa9059cbb, to, value));
    require(success && (data.length == 0 || abi.decode(data, (bool))), "TransferHelper: TRANSFER_FAILED");
  }

  function safeTransferFrom(
    address token,
    address from,
    address to,
    uint256 value
  ) internal {
    // bytes4(keccak256(bytes('transferFrom(address,address,uint256)')));
    (bool success, bytes memory data) = token.call(abi.encodeWithSelector(0x23b872dd, from, to, value));
    require(success && (data.length == 0 || abi.decode(data, (bool))), "TransferHelper: TRANSFER_FROM_FAILED");
  }

  function safeTransferETH(address to, uint256 value) internal {
    (bool success, ) = to.call{value: value}(new bytes(0));
    require(success, "TransferHelper: ETH_TRANSFER_FAILED");
  }
}

interface IERC20 {
  event Approval(address indexed owner, address indexed spender, uint256 value);
  event Transfer(address indexed from, address indexed to, uint256 value);

  function name() external view returns (string memory);

  function symbol() external view returns (string memory);

  function decimals() external view returns (uint8);

  function totalSupply() external view returns (uint256);

  function balanceOf(address owner) external view returns (uint256);

  function allowance(address owner, address spender) external view returns (uint256);

  function approve(address spender, uint256 value) external returns (bool);

  function transfer(address to, uint256 value) external returns (bool);

  function transferFrom(
    address from,
    address to,
    uint256 value
  ) external returns (bool);
}

// a library for performing overflow-safe math, courtesy of DappHub (https://github.com/dapphub/ds-math)
library SafeMath {
  function add(uint256 x, uint256 y) internal pure returns (uint256 z) {
    require((z = x + y) >= x, "ds-math-add-overflow");
  }

  function sub(uint256 x, uint256 y) internal pure returns (uint256 z) {
    require((z = x - y) <= x, "ds-math-sub-underflow");
  }

  function mul(uint256 x, uint256 y) internal pure returns (uint256 z) {
    require(y == 0 || (z = x * y) / y == x, "ds-math-mul-overflow");
  }

  function div(uint256 a, uint256 b) internal pure returns (uint256 c) {
    require(b > 0, "ds-math-division-by-zero");
    c = a / b;
  }
}

interface IWETH {
  function deposit() external payable;

  function transfer(address to, uint256 value) external returns (bool);

  function withdraw(uint256) external;

  function balanceOf(address account) external view returns (uint256);
}

interface IAggregationExecutor {
  function callBytes(bytes calldata data, address srcSpender) external payable; // 0xd9c45357
}

interface IERC20Permit {
  function permit(
    address owner,
    address spender,
    uint256 amount,
    uint256 deadline,
    uint8 v,
    bytes32 r,
    bytes32 s
  ) external;
}

library RevertReasonParser {
  function parse(bytes memory data, string memory prefix) internal pure returns (string memory) {
    // https://solidity.readthedocs.io/en/latest/control-structures.html#revert
    // We assume that revert reason is abi-encoded as Error(string)

    // 68 = 4-byte selector 0x08c379a0 + 32 bytes offset + 32 bytes length
    if (data.length >= 68 && data[0] == "\x08" && data[1] == "\xc3" && data[2] == "\x79" && data[3] == "\xa0") {
      string memory reason;
      // solhint-disable no-inline-assembly
      assembly {
        // 68 = 32 bytes data length + 4-byte selector + 32 bytes offset
        reason := add(data, 68)
      }
      /*
                revert reason is padded up to 32 bytes with ABI encoder: Error(string)
                also sometimes there is extra 32 bytes of zeros padded in the end:
                https://github.com/ethereum/solidity/issues/10170
                because of that we can't check for equality and instead check
                that string length + extra 68 bytes is less than overall data length
            */
      require(data.length >= 68 + bytes(reason).length, "Invalid revert reason");
      return string(abi.encodePacked(prefix, "Error(", reason, ")"));
    }
    // 36 = 4-byte selector 0x4e487b71 + 32 bytes integer
    else if (data.length == 36 && data[0] == "\x4e" && data[1] == "\x48" && data[2] == "\x7b" && data[3] == "\x71") {
      uint256 code;
      // solhint-disable no-inline-assembly
      assembly {
        // 36 = 32 bytes data length + 4-byte selector
        code := mload(add(data, 36))
      }
      return string(abi.encodePacked(prefix, "Panic(", _toHex(code), ")"));
    }

    return string(abi.encodePacked(prefix, "Unknown(", _toHex(data), ")"));
  }

  function _toHex(uint256 value) private pure returns (string memory) {
    return _toHex(abi.encodePacked(value));
  }

  function _toHex(bytes memory data) private pure returns (string memory) {
    bytes16 alphabet = 0x30313233343536373839616263646566;
    bytes memory str = new bytes(2 + data.length * 2);
    str[0] = "0";
    str[1] = "x";
    for (uint256 i = 0; i < data.length; i++) {
      str[2 * i + 2] = alphabet[uint8(data[i] >> 4)];
      str[2 * i + 3] = alphabet[uint8(data[i] & 0x0f)];
    }
    return string(str);
  }
}

contract Permitable {
  event Error(string reason);

  function _permit(
    IERC20 token,
    uint256 amount,
    bytes calldata permit
  ) internal {
    if (permit.length == 32 * 7) {
      // solhint-disable-next-line avoid-low-level-calls
      (bool success, bytes memory result) = address(token).call(abi.encodePacked(IERC20Permit.permit.selector, permit));
      if (!success) {
        string memory reason = RevertReasonParser.parse(result, "Permit call failed: ");
        if (token.allowance(msg.sender, address(this)) < amount) {
          revert(reason);
        } else {
          emit Error(reason);
        }
      }
    }
  }
}

/*
 * @dev Provides information about the current execution context, including the
 * sender of the transaction and its data. While these are generally available
 * via msg.sender and msg.data, they should not be accessed in such a direct
 * manner, since when dealing with GSN meta-transactions the account sending and
 * paying for execution may not be the actual sender (as far as an application
 * is concerned).
 *
 * This contract is only required for intermediate, library-like contracts.
 */
abstract contract Context {
  function _msgSender() internal view virtual returns (address payable) {
    return msg.sender;
  }

  function _msgData() internal view virtual returns (bytes memory) {
    this; // silence state mutability warning without generating bytecode - see https://github.com/ethereum/solidity/issues/2691
    return msg.data;
  }
}

/**
 * @dev Contract module which provides a basic access control mechanism, where
 * there is an account (an owner) that can be granted exclusive access to
 * specific functions.
 *
 * By default, the owner account will be the one that deploys the contract. This
 * can later be changed with {transferOwnership}.
 *
 * This module is used through inheritance. It will make available the modifier
 * `onlyOwner`, which can be applied to your functions to restrict their use to
 * the owner.
 */
abstract contract Ownable is Context {
  address private _owner;

  event OwnershipTransferred(address indexed previousOwner, address indexed newOwner);

  /**
   * @dev Initializes the contract setting the deployer as the initial owner.
   */
  constructor() internal {
    address msgSender = _msgSender();
    _owner = msgSender;
    emit OwnershipTransferred(address(0), msgSender);
  }

  /**
   * @dev Returns the address of the current owner.
   */
  function owner() public view virtual returns (address) {
    return _owner;
  }

  /**
   * @dev Throws if called by any account other than the owner.
   */
  modifier onlyOwner() {
    require(owner() == _msgSender(), "Ownable: caller is not the owner");
    _;
  }

  /**
   * @dev Leaves the contract without owner. It will not be possible to call
   * `onlyOwner` functions anymore. Can only be called by the current owner.
   *
   * NOTE: Renouncing ownership will leave the contract without an owner,
   * thereby removing any functionality that is only available to the owner.
   */
  function renounceOwnership() public virtual onlyOwner {
    emit OwnershipTransferred(_owner, address(0));
    _owner = address(0);
  }

  /**
   * @dev Transfers ownership of the contract to a new account (`newOwner`).
   * Can only be called by the current owner.
   */
  function transferOwnership(address newOwner) public virtual onlyOwner {
    require(newOwner != address(0), "Ownable: new owner is the zero address");
    emit OwnershipTransferred(_owner, newOwner);
    _owner = newOwner;
  }
}

contract AggregationRouter is Ownable, Permitable {
  using SafeMath for uint256;
  address public immutable WETH;
  address private constant ETH_ADDRESS = address(0xEeeeeEeeeEeEeeEeEeEeeEEEeeeeEeeeeeeeEEeE);

  uint256 private constant _PARTIAL_FILL = 0x01;
  uint256 private constant _REQUIRES_EXTRA_ETH = 0x02;
  uint256 private constant _SHOULD_CLAIM = 0x04;
  uint256 private constant _BURN_FROM_MSG_SENDER = 0x08;
  uint256 private constant _BURN_FROM_TX_ORIGIN = 0x10;

  struct SwapDescription {
    IERC20 srcToken;
    IERC20 dstToken;
    address srcReceiver;
    address dstReceiver;
    uint256 amount;
    uint256 minReturnAmount;
    uint256 flags;
    bytes permit;
  }

  event Swapped(address sender, IERC20 srcToken, IERC20 dstToken, address dstReceiver, uint256 spentAmount, uint256 returnAmount);
  event Exchange(address pair, uint256 amountOut, address output);

  modifier ensure(uint256 deadline) {
    require(deadline >= block.timestamp, "Router: EXPIRED");
    _;
  }

  constructor(address _WETH) public {
    WETH = _WETH;
  }

  receive() external payable {}

  function swap(
    IAggregationExecutor caller,
    SwapDescription calldata desc,
    bytes calldata data
  ) external payable returns (uint256 returnAmount) {
    require(desc.minReturnAmount > 0, "Min return should not be 0");
    require(data.length > 0, "data should be not zero");

    uint256 flags = desc.flags;
    uint256 amount = desc.amount;
    IERC20 srcToken = desc.srcToken;
    IERC20 dstToken = desc.dstToken;

    if (flags & _REQUIRES_EXTRA_ETH != 0) {
      require(msg.value > (isETH(srcToken) ? amount : 0), "Invalid msg.value");
    } else {
      require(msg.value == (isETH(srcToken) ? amount : 0), "Invalid msg.value");
    }

    if (flags & _SHOULD_CLAIM != 0) {
      require(!isETH(srcToken), "Claim token is ETH");
      _permit(srcToken, amount, desc.permit);
      TransferHelper.safeTransferFrom(address(srcToken), msg.sender, desc.srcReceiver, amount);
    }

    address dstReceiver = (desc.dstReceiver == address(0)) ? msg.sender : desc.dstReceiver;
    uint256 initialSrcBalance = (flags & _PARTIAL_FILL != 0) ? getBalance(srcToken, msg.sender) : 0;
    uint256 initialDstBalance = getBalance(dstToken, dstReceiver);

    {
      // solhint-disable-next-line avoid-low-level-calls
      (bool success, bytes memory result) = address(caller).call{value: msg.value}(abi.encodeWithSelector(caller.callBytes.selector, data, msg.sender));
      if (!success) {
        revert(RevertReasonParser.parse(result, "callBytes failed: "));
      }
    }

    uint256 spentAmount = amount;
    returnAmount = getBalance(dstToken, dstReceiver).sub(initialDstBalance);

    if (flags & _PARTIAL_FILL != 0) {
      spentAmount = initialSrcBalance.add(amount).sub(getBalance(srcToken, msg.sender));
      require(returnAmount.mul(amount) >= desc.minReturnAmount.mul(spentAmount), "Return amount is not enough");
    } else {
      require(returnAmount >= desc.minReturnAmount, "Return amount is not enough");
    }

    emit Swapped(msg.sender, srcToken, dstToken, dstReceiver, spentAmount, returnAmount);
    emit Exchange(address(caller), returnAmount, isETH(dstToken) ? WETH : address(dstToken));
  }

  function getBalance(IERC20 token, address account) internal view returns (uint256) {
    if (isETH(token)) {
      return account.balance;
    } else {
      return token.balanceOf(account);
    }
  }

  function isETH(IERC20 token) internal pure returns (bool) {
    return (address(token) == ETH_ADDRESS);
  }

  function rescueFunds(address token, uint256 amount) external onlyOwner {
    if (isETH(IERC20(token))) {
      TransferHelper.safeTransferETH(msg.sender, amount);
    } else {
      TransferHelper.safeTransfer(token, msg.sender, amount);
    }
  }
}