// File: contracts/bill/TBillSimple.sol



// SPDX-License-Identifier: GPL-3.0-or-later
pragma solidity 0.6.12;
pragma experimental ABIEncoderV2;

import {DTBT} from './DTBT.sol';
import {IERC20Detailed} from '../dependencies/openzeppelin/contracts/IERC20Detailed.sol';
import {SafeERC20} from '../dependencies/openzeppelin/contracts/SafeERC20.sol';
import {Ownable} from '../dependencies/openzeppelin/contracts/Ownable.sol';

interface IETF is IERC20Detailed {
  enum Etypes {
    OPENED,
    CLOSED
  }

  function execute(
    address _target,
    uint _value,
    bytes calldata _data,
    bool isUnderlying
  ) external returns (bytes memory _returnValue);

  function isCompletedCollect() external view returns (bool);

  function etype() external view returns (Etypes);

  function adminList(address) external view returns (bool);

  function getController() external view returns (address);

  function etfStatus()
    external
    view
    returns (
      uint256 collectPeriod,
      uint256 collectEndTime,
      uint256 closurePeriod,
      uint256 closureEndTime,
      uint256 upperCap,
      uint256 floorCap,
      uint256 managerFee,
      uint256 redeemFee,
      uint256 issueFee,
      uint256 perfermanceFee,
      uint256 startClaimFeeTime
    );

  function bPool() external view virtual returns (LiquidityPoolActions);
}

interface LiquidityPoolActions {
  function getCurrentTokens() external view virtual returns (address[] memory);

  function isBound(address t) external view returns (bool);
}

interface IFactory {
  function isPaused() external view returns (bool);
}

contract TBillSimple is Ownable {
  using SafeERC20 for IERC20Detailed;

  address public stableCoinReceiver = 0x5a47DF2aaec5ad2F95A6a353c906559075f94186; // return stbt
  address public sTBTReceiver = 0xDEE9Ed3B19d104ADBbE255B6bEFC680b4eaAAda3; // return usdc

  IETF public etf;
  IFactory public factory;

  DTBT public dtbt;
  IERC20Detailed stbt = IERC20Detailed(0x530824DA86689C9C17CdC2871Ff29B058345b44a);
  IERC20Detailed stableCoin = IERC20Detailed(0xA0b86991c6218b36c1d19D4a2e9Eb0cE3606eB48);

  event SendStaleCoin(address caller, address etf, address sendToken, uint sendAmount);
  event SendSTBT(address caller, address etf, address snedToken, uint sendAmount);
  event UpdateDTBTAmount(address caller, address etf, address dtbt, uint newAmount);

  event UpdateETFAddr(address caller, address oldValue, address newValue);
  event UpdateStbtAddr(address caller, address oldValue, address newValue);
  event UpdateStableCoinAddr(address caller, address oldValue, address newValue);
  event UpdateStableCoinReceiverAddr(address caller, address oldValue, address newValue);
  event UpdateStbtReceiverAddr(address caller, address oldValue, address newValue);

  constructor(IETF etf_, IFactory factory_, address dtbt_) public {
    require(etf_.etype() == IETF.Etypes.CLOSED, 'ERR_ONLY_SUPPORT_CLOSED_ETF');
    require(
      address(etf_) != address(0) && address(factory_) != address(0) && dtbt_ != address(0),
      'ERR_ZERO_ADDRESS'
    );
    factory = factory_;
    etf = etf_;
    dtbt = DTBT(dtbt_);
  }

  enum CurrentPeriod {
    WaitSendStableCoin,
    WaitSendSTBT
  }

  CurrentPeriod public currentPeriod = CurrentPeriod.WaitSendStableCoin;

  // only entry to make stable coin and stbt swap
  function swap() external {
    if (currentPeriod == CurrentPeriod.WaitSendStableCoin) {
      (, uint256 collectEndTime, , uint256 closureEndTime, , , , , , , ) = etf.etfStatus();
      require(collectEndTime < block.timestamp && closureEndTime > block.timestamp, 'ERR_PERIOD');
      _sendStableCoin();
      currentPeriod = CurrentPeriod.WaitSendSTBT;
    } else {
      _sendSTBT();
      currentPeriod = CurrentPeriod.WaitSendStableCoin;
    }
  }

  // update stable coin on etf and burn dtbt
  function rebalance() external _checkTx {
    require(currentPeriod == CurrentPeriod.WaitSendStableCoin, 'ERR_NOT_WAIT_SEND_STABLECOIN');
    (, , , uint256 closureEndTime, , , , , , , ) = etf.etfStatus();
    require(closureEndTime < block.timestamp, 'ERR_ETF_HAS_NOT_CLOSURE');

    uint bal = stableCoin.balanceOf(address(etf.bPool()));
    require(bal > 0, 'ERR_STABLECOIN_AMOUNT_IS_ZERO');
    _updateDtbtAmount(0);
    _invokeRebind(address(stableCoin), bal, 50e18);
  }

  // send stable coin to swap stbt coin
  function _sendStableCoin() internal _checkTx {
    uint bal = stableCoin.balanceOf(address(etf.bPool()));
    require(bal > 0, 'ERR_STABLECOIN_AMOUNT_IS_ZERO');

    bytes memory callData = abi.encodeWithSignature(
      'transfer(address,uint256)',
      stableCoinReceiver,
      bal
    );
    etf.execute(address(stableCoin), 0, callData, true);

    uint decimalsDtbt = dtbt.decimals();
    uint decimalsStableCoin = stableCoin.decimals();
    uint tbtNewBal = _decimalsHandle(bal, decimalsStableCoin, decimalsDtbt);

    _updateDtbtAmount(tbtNewBal);
    _invokeUnbind(etf, address(stableCoin));
    _invokeRebind(address(stbt), tbtNewBal, 50e18);

    emit SendStaleCoin(msg.sender, address(etf), address(stableCoin), bal);
  }

  // send stbt coin to swap stable coin
  function _sendSTBT() internal _checkTx {
    uint bal = stbt.balanceOf(address(etf.bPool()));
    require(bal > 0, 'ERR_STBT_AMOUNT_IS_ZERO');

    bytes memory callData = abi.encodeWithSignature('transfer(address,uint256)', sTBTReceiver, bal);
    etf.execute(address(stbt), 0, callData, true);

    uint decimalsStbt = stbt.decimals();
    uint decimalsStableCoin = stableCoin.decimals();
    uint stableCoinNewBal = _decimalsHandle(bal, decimalsStbt, decimalsStableCoin);

    _updateDtbtAmount(0);
    _invokeUnbind(etf, address(stbt));
    _invokeRebind(address(stableCoin), stableCoinNewBal, 50e18);

    emit SendSTBT(msg.sender, address(etf), address(stbt), bal);
  }

  function _updateDtbtAmount(uint amount) internal {
    address bpool = address(etf.bPool());
    uint balDtbt = dtbt.balanceOf(bpool);

    if (balDtbt > 0) dtbt.burn(bpool, balDtbt);
    if (amount > 0) dtbt.mint(bpool, amount);

    emit UpdateDTBTAmount(msg.sender, address(etf), address(dtbt), amount);
  }

  // convert token amount based on decimals
  function _decimalsHandle(
    uint256 currentValue,
    uint currentDecimals,
    uint targetDecimals
  ) internal pure returns (uint256) {
    if (currentDecimals >= targetDecimals)
      return currentValue / 10 ** (currentDecimals - targetDecimals);
    if (currentDecimals < targetDecimals)
      return currentValue * 10 ** (targetDecimals - currentDecimals);
  }

  function _invokeRebind(address _token, uint256 _balance, uint256 _weight) internal {
    bool isBound = etf.bPool().isBound(_token);
    bytes memory callData = abi.encodeWithSignature(
      'rebindPure(address,uint256,uint256,bool)',
      _token,
      _balance,
      _weight,
      isBound
    );

    etf.execute(address(etf.bPool()), 0, callData, false);
  }

  function _invokeUnbind(IETF _etf, address _token) internal {
    bool isBound = etf.bPool().isBound(_token);
    if (isBound) {
      bytes memory callData = abi.encodeWithSignature('unbindPure(address)', _token);
      _etf.execute(address(_etf.bPool()), 0, callData, false);
    }
  }

  // config: ETF coin address
  function updateETFaddr(IETF etf_) external onlyOwner _config_ {
    require(address(etf_) != address(0), 'ERR_ZERO_ADDRESS');
    emit UpdateETFAddr(msg.sender, address(etf), address(etf_));
    etf = etf_;
  }

  // config: stbt coin address
  function updateStbtAddr(IERC20Detailed stbt_) external onlyOwner _config_ {
    require(address(stbt_) != address(0), 'ERR_ZERO_ADDRESS');
    emit UpdateStbtAddr(msg.sender, address(stbt), address(stbt_));
    stbt = stbt_;
    dtbt.updateDecimals(stbt.decimals());
  }

  // config: stable coin address
  function updateStableCoinAddr(IERC20Detailed stableCoin_) external onlyOwner _config_ {
    require(address(stableCoin_) != address(0), 'ERR_ZERO_ADDRESS');
    emit UpdateStableCoinAddr(msg.sender, address(stableCoin), address(stableCoin_));
    stableCoin = stableCoin_;
  }

  // config: stable coin receiver address
  function updateStableCoinReceiverAddr(address stableCoinReceiver_) external onlyOwner _config_ {
    require(stableCoinReceiver_ != address(0), 'ERR_ZERO_ADDRESS');
    emit UpdateStableCoinReceiverAddr(msg.sender, stableCoinReceiver, stableCoinReceiver_);
    stableCoinReceiver = stableCoinReceiver_;
  }

  // config: stbt coin receiver address
  function updateStbtReceiverAddr(address sTBTReceiver_) external onlyOwner _config_ {
    require(sTBTReceiver_ != address(0), 'ERR_ZERO_ADDRESS');
    emit UpdateStbtReceiverAddr(msg.sender, sTBTReceiver, sTBTReceiver_);
    sTBTReceiver = sTBTReceiver_;
  }

  modifier _checkTx() {
    require(!factory.isPaused(), 'PAUSED');
    require(etf.isCompletedCollect(), 'COLLECTION_FAILED');
    require(etf.adminList(msg.sender) || msg.sender == etf.getController(), 'NOT_CONTROLLER');
    _;
  }

  modifier _config_() {
    (, uint256 collectEndTime, , , , , , , , , ) = etf.etfStatus();
    require(block.timestamp < collectEndTime, 'ERR_ETF_HAS_BEEN_CREATE');
    _;
  }
}

// File: contracts/bill/DTBT.sol



// SPDX-License-Identifier: GPL-3.0-or-later
pragma solidity 0.6.12;

// Core contract; can't be changed. So disable solhint (reminder for v2)

/* solhint-disable private-vars-leading-underscore */
/* solhint-disable reason-string */
/* solhint-disable func-order */
contract DTBT {
  string private constant _name = 'DTBT';
  string private constant _symbol = 'DTBT';
  uint8 private _decimals = 18;

  address private _owner = msg.sender;

  uint internal _totalSupply;

  mapping(address => uint) private _balance;
  mapping(address => mapping(address => uint)) private _allowance;
  mapping(address => bool) _tbills;

  modifier _onlyOwner_() {
    require(msg.sender == _owner || _tbills[msg.sender], 'ERR_NOT_OWNER');
    _;
  }

  event Approval(address indexed src, address indexed dst, uint amt);
  event Transfer(address indexed src, address indexed dst, uint amt);
  event UpdateDecimals(uint8 oldDecmals, uint8 newDecmals);
  event AdminsUpdate(address indexed caller, address newAdmin);
  event AdminRemoved(address indexed caller, address newAdmin);

  // tbill modules
  function addAdmin(address tbill) public returns (bool) {
    require(msg.sender == _owner, 'ERR_NOT_OWNER');
    _tbills[tbill] = true;
    emit AdminsUpdate(msg.sender, tbill);
    return true;
  }

  function removeAdmin(address tbill) public returns (bool) {
    require(msg.sender == _owner, 'ERR_NOT_OWNER');
    require(_tbills[tbill], 'NOT_APPROVED');
    _tbills[tbill] = false;
    emit AdminRemoved(msg.sender, tbill);
    return true;
  }

  // Math
  function add(uint a, uint b) internal pure returns (uint c) {
    require((c = a + b) >= a, 'SafeMath: addition overflow');
  }

  function sub(uint a, uint b) internal pure returns (uint c) {
    require((c = a - b) <= a, 'SafeMath: subtraction overflow');
  }

  function name() public view returns (string memory) {
    return _name;
  }

  function symbol() public view returns (string memory) {
    return _symbol;
  }

  function decimals() public view returns (uint8) {
    return _decimals;
  }

  function _move(address src, address dst, uint amt) internal {
    require(_balance[src] >= amt, 'ERR_INSUFFICIENT_BAL');
    _balance[src] = sub(_balance[src], amt);
    _balance[dst] = add(_balance[dst], amt);
    emit Transfer(src, dst, amt);
  }

  function _mint(address dst, uint amt) internal {
    _balance[dst] = add(_balance[dst], amt);
    _totalSupply = add(_totalSupply, amt);
    emit Transfer(address(0), dst, amt);
  }

  function updateDecimals(uint8 newDecimals) public _onlyOwner_ {
    emit UpdateDecimals(_decimals, newDecimals);
    _decimals = newDecimals;
  }

  function allowance(address src, address dst) external view returns (uint) {
    return _allowance[src][dst];
  }

  function balanceOf(address whom) external view returns (uint) {
    return _balance[whom];
  }

  function totalSupply() public view returns (uint) {
    return _totalSupply;
  }

  function approve(address dst, uint amt) external returns (bool) {
    _allowance[msg.sender][dst] = amt;
    emit Approval(msg.sender, dst, amt);
    return true;
  }

  function mint(address dst, uint amt) public _onlyOwner_ returns (bool) {
    _mint(dst, amt);
    return true;
  }

  function burn(address dst, uint amt) public _onlyOwner_ returns (bool) {
    require(_balance[dst] >= amt, 'ERR_INSUFFICIENT_BAL');
    _balance[dst] = sub(_balance[dst], amt);
    _totalSupply = sub(_totalSupply, amt);
    emit Transfer(dst, address(0), amt);
    return true;
  }

  function transfer(address dst, uint amt) external returns (bool) {
    _move(msg.sender, dst, amt);
    return true;
  }

  function transferFrom(address src, address dst, uint amt) external returns (bool) {
    require(msg.sender == src || amt <= _allowance[src][msg.sender], 'ERR_BTOKEN_BAD_CALLER');
    _move(src, dst, amt);
    if (msg.sender != src && _allowance[src][msg.sender] != uint(-1)) {
      _allowance[src][msg.sender] = sub(_allowance[src][msg.sender], amt);
      emit Approval(msg.sender, dst, _allowance[src][msg.sender]);
    }
    return true;
  }
}

// File: contracts/dependencies/openzeppelin/contracts/IERC20Detailed.sol



// SPDX-License-Identifier: agpl-3.0
pragma solidity 0.6.12;

import {IERC20} from './IERC20.sol';

interface IERC20Detailed is IERC20 {
  function name() external view returns (string memory);

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

  function decimals() external view returns (uint8);
}

// File: contracts/dependencies/openzeppelin/contracts/Ownable.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.6.0;

import './Context.sol';

/**
 * @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.
 */
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 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;
  }
}

// File: contracts/dependencies/openzeppelin/contracts/SafeERC20.sol



// SPDX-License-Identifier: MIT

pragma solidity 0.6.12;

import {IERC20} from './IERC20.sol';
import {SafeMath} from './SafeMath.sol';
import {Address} from './Address.sol';

/**
 * @title SafeERC20
 * @dev Wrappers around ERC20 operations that throw on failure (when the token
 * contract returns false). Tokens that return no value (and instead revert or
 * throw on failure) are also supported, non-reverting calls are assumed to be
 * successful.
 * To use this library you can add a `using SafeERC20 for IERC20;` statement to your contract,
 * which allows you to call the safe operations as `token.safeTransfer(...)`, etc.
 */
library SafeERC20 {
  using SafeMath for uint256;
  using Address for address;

  function safeTransfer(
    IERC20 token,
    address to,
    uint256 value
  ) internal {
    callOptionalReturn(token, abi.encodeWithSelector(token.transfer.selector, to, value));
  }

  function safeTransferFrom(
    IERC20 token,
    address from,
    address to,
    uint256 value
  ) internal {
    callOptionalReturn(token, abi.encodeWithSelector(token.transferFrom.selector, from, to, value));
  }

  function safeApprove(
    IERC20 token,
    address spender,
    uint256 value
  ) internal {
    require(
      (value == 0) || (token.allowance(address(this), spender) == 0),
      'SafeERC20: approve from non-zero to non-zero allowance'
    );
    callOptionalReturn(token, abi.encodeWithSelector(token.approve.selector, spender, value));
  }

  function callOptionalReturn(IERC20 token, bytes memory data) private {
    require(address(token).isContract(), 'SafeERC20: call to non-contract');

    // solhint-disable-next-line avoid-low-level-calls
    (bool success, bytes memory returndata) = address(token).call(data);
    require(success, 'SafeERC20: low-level call failed');

    if (returndata.length > 0) {
      // Return data is optional
      // solhint-disable-next-line max-line-length
      require(abi.decode(returndata, (bool)), 'SafeERC20: ERC20 operation did not succeed');
    }
  }
}

// File: contracts/dependencies/openzeppelin/contracts/Context.sol



// SPDX-License-Identifier: MIT
pragma solidity 0.6.12;

/*
 * @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;
  }
}

// File: contracts/dependencies/openzeppelin/contracts/IERC20.sol



// SPDX-License-Identifier: agpl-3.0
pragma solidity 0.6.12;

/**
 * @dev Interface of the ERC20 standard as defined in the EIP.
 */
interface IERC20 {
  /**
   * @dev Returns the amount of tokens in existence.
   */
  function totalSupply() external view returns (uint256);

  /**
   * @dev Returns the amount of tokens owned by `account`.
   */
  function balanceOf(address account) external view returns (uint256);

  /**
   * @dev Moves `amount` tokens from the caller's account to `recipient`.
   *
   * Returns a boolean value indicating whether the operation succeeded.
   *
   * Emits a {Transfer} event.
   */
  function transfer(address recipient, uint256 amount) external returns (bool);

  /**
   * @dev Returns the remaining number of tokens that `spender` will be
   * allowed to spend on behalf of `owner` through {transferFrom}. This is
   * zero by default.
   *
   * This value changes when {approve} or {transferFrom} are called.
   */
  function allowance(address owner, address spender) external view returns (uint256);

  /**
   * @dev Sets `amount` as the allowance of `spender` over the caller's tokens.
   *
   * Returns a boolean value indicating whether the operation succeeded.
   *
   * IMPORTANT: Beware that changing an allowance with this method brings the risk
   * that someone may use both the old and the new allowance by unfortunate
   * transaction ordering. One possible solution to mitigate this race
   * condition is to first reduce the spender's allowance to 0 and set the
   * desired value afterwards:
   * https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
   *
   * Emits an {Approval} event.
   */
  function approve(address spender, uint256 amount) external returns (bool);

  /**
   * @dev Moves `amount` tokens from `sender` to `recipient` using the
   * allowance mechanism. `amount` is then deducted from the caller's
   * allowance.
   *
   * Returns a boolean value indicating whether the operation succeeded.
   *
   * Emits a {Transfer} event.
   */
  function transferFrom(
    address sender,
    address recipient,
    uint256 amount
  ) external returns (bool);

  /**
   * @dev Emitted when `value` tokens are moved from one account (`from`) to
   * another (`to`).
   *
   * Note that `value` may be zero.
   */
  event Transfer(address indexed from, address indexed to, uint256 value);

  /**
   * @dev Emitted when the allowance of a `spender` for an `owner` is set by
   * a call to {approve}. `value` is the new allowance.
   */
  event Approval(address indexed owner, address indexed spender, uint256 value);
}

// File: contracts/dependencies/openzeppelin/contracts/Address.sol



// SPDX-License-Identifier: agpl-3.0
pragma solidity 0.6.12;

/**
 * @dev Collection of functions related to the address type
 */
library Address {
  /**
   * @dev Returns true if `account` is a contract.
   *
   * [IMPORTANT]
   * ====
   * It is unsafe to assume that an address for which this function returns
   * false is an externally-owned account (EOA) and not a contract.
   *
   * Among others, `isContract` will return false for the following
   * types of addresses:
   *
   *  - an externally-owned account
   *  - a contract in construction
   *  - an address where a contract will be created
   *  - an address where a contract lived, but was destroyed
   * ====
   */
  function isContract(address account) internal view returns (bool) {
    // According to EIP-1052, 0x0 is the value returned for not-yet created accounts
    // and 0xc5d2460186f7233c927e7db2dcc703c0e500b653ca82273b7bfad8045d85a470 is returned
    // for accounts without code, i.e. `keccak256('')`
    bytes32 codehash;
    bytes32 accountHash = 0xc5d2460186f7233c927e7db2dcc703c0e500b653ca82273b7bfad8045d85a470;
    // solhint-disable-next-line no-inline-assembly
    assembly {
      codehash := extcodehash(account)
    }
    return (codehash != accountHash && codehash != 0x0);
  }

  /**
   * @dev Replacement for Solidity's `transfer`: sends `amount` wei to
   * `recipient`, forwarding all available gas and reverting on errors.
   *
   * https://eips.ethereum.org/EIPS/eip-1884[EIP1884] increases the gas cost
   * of certain opcodes, possibly making contracts go over the 2300 gas limit
   * imposed by `transfer`, making them unable to receive funds via
   * `transfer`. {sendValue} removes this limitation.
   *
   * https://diligence.consensys.net/posts/2019/09/stop-using-soliditys-transfer-now/[Learn more].
   *
   * IMPORTANT: because control is transferred to `recipient`, care must be
   * taken to not create reentrancy vulnerabilities. Consider using
   * {ReentrancyGuard} or the
   * https://solidity.readthedocs.io/en/v0.5.11/security-considerations.html#use-the-checks-effects-interactions-pattern[checks-effects-interactions pattern].
   */
  function sendValue(address payable recipient, uint256 amount) internal {
    require(address(this).balance >= amount, 'Address: insufficient balance');

    // solhint-disable-next-line avoid-low-level-calls, avoid-call-value
    (bool success, ) = recipient.call{value: amount}('');
    require(success, 'Address: unable to send value, recipient may have reverted');
  }

  /**
   * @dev Performs a Solidity function call using a low level `call`. A
   * plain`call` is an unsafe replacement for a function call: use this
   * function instead.
   *
   * If `target` reverts with a revert reason, it is bubbled up by this
   * function (like regular Solidity function calls).
   *
   * Returns the raw returned data. To convert to the expected return value,
   * use https://solidity.readthedocs.io/en/latest/units-and-global-variables.html?highlight=abi.decode#abi-encoding-and-decoding-functions[`abi.decode`].
   *
   * Requirements:
   *
   * - `target` must be a contract.
   * - calling `target` with `data` must not revert.
   *
   * _Available since v3.1._
   */
  function functionCall(address target, bytes memory data) internal returns (bytes memory) {
    return functionCall(target, data, 'Address: low-level call failed');
  }

  /**
   * @dev Same as {xref-Address-functionCall-address-bytes-}[`functionCall`], but with
   * `errorMessage` as a fallback revert reason when `target` reverts.
   *
   * _Available since v3.1._
   */
  function functionCall(
    address target,
    bytes memory data,
    string memory errorMessage
  ) internal returns (bytes memory) {
    return functionCallWithValue(target, data, 0, errorMessage);
  }

  /**
   * @dev Same as {xref-Address-functionCall-address-bytes-}[`functionCall`],
   * but also transferring `value` wei to `target`.
   *
   * Requirements:
   *
   * - the calling contract must have an ETH balance of at least `value`.
   * - the called Solidity function must be `payable`.
   *
   * _Available since v3.1._
   */
  function functionCallWithValue(
    address target,
    bytes memory data,
    uint256 value
  ) internal returns (bytes memory) {
    return functionCallWithValue(target, data, value, 'Address: low-level call with value failed');
  }

  /**
   * @dev Same as {xref-Address-functionCallWithValue-address-bytes-uint256-}[`functionCallWithValue`], but
   * with `errorMessage` as a fallback revert reason when `target` reverts.
   *
   * _Available since v3.1._
   */
  function functionCallWithValue(
    address target,
    bytes memory data,
    uint256 value,
    string memory errorMessage
  ) internal returns (bytes memory) {
    require(address(this).balance >= value, 'Address: insufficient balance for call');
    require(isContract(target), 'Address: call to non-contract');

    // solhint-disable-next-line avoid-low-level-calls
    (bool success, bytes memory returndata) = target.call{value: value}(data);
    return _verifyCallResult(success, returndata, errorMessage);
  }

  function _verifyCallResult(
    bool success,
    bytes memory returndata,
    string memory errorMessage
  ) private pure returns (bytes memory) {
    if (success) {
      return returndata;
    } else {
      // Look for revert reason and bubble it up if present
      if (returndata.length > 0) {
        // The easiest way to bubble the revert reason is using memory via assembly

        // solhint-disable-next-line no-inline-assembly
        assembly {
          let returndata_size := mload(returndata)
          revert(add(32, returndata), returndata_size)
        }
      } else {
        revert(errorMessage);
      }
    }
  }
}

// File: contracts/dependencies/openzeppelin/contracts/SafeMath.sol



// SPDX-License-Identifier: agpl-3.0
pragma solidity 0.6.12;

/**
 * @dev Wrappers over Solidity's arithmetic operations with added overflow
 * checks.
 *
 * Arithmetic operations in Solidity wrap on overflow. This can easily result
 * in bugs, because programmers usually assume that an overflow raises an
 * error, which is the standard behavior in high level programming languages.
 * `SafeMath` restores this intuition by reverting the transaction when an
 * operation overflows.
 *
 * Using this library instead of the unchecked operations eliminates an entire
 * class of bugs, so it's recommended to use it always.
 */
library SafeMath {
  /**
   * @dev Returns the addition of two unsigned integers, reverting on
   * overflow.
   *
   * Counterpart to Solidity's `+` operator.
   *
   * Requirements:
   * - Addition cannot overflow.
   */
  function add(uint256 a, uint256 b) internal pure returns (uint256) {
    uint256 c = a + b;
    require(c >= a, 'SafeMath: addition overflow');

    return c;
  }

  /**
   * @dev Returns the subtraction of two unsigned integers, reverting on
   * overflow (when the result is negative).
   *
   * Counterpart to Solidity's `-` operator.
   *
   * Requirements:
   * - Subtraction cannot overflow.
   */
  function sub(uint256 a, uint256 b) internal pure returns (uint256) {
    return sub(a, b, 'SafeMath: subtraction overflow');
  }

  /**
   * @dev Returns the subtraction of two unsigned integers, reverting with custom message on
   * overflow (when the result is negative).
   *
   * Counterpart to Solidity's `-` operator.
   *
   * Requirements:
   * - Subtraction cannot overflow.
   */
  function sub(
    uint256 a,
    uint256 b,
    string memory errorMessage
  ) internal pure returns (uint256) {
    require(b <= a, errorMessage);
    uint256 c = a - b;

    return c;
  }

  /**
   * @dev Returns the multiplication of two unsigned integers, reverting on
   * overflow.
   *
   * Counterpart to Solidity's `*` operator.
   *
   * Requirements:
   * - Multiplication cannot overflow.
   */
  function mul(uint256 a, uint256 b) internal pure returns (uint256) {
    // Gas optimization: this is cheaper than requiring 'a' not being zero, but the
    // benefit is lost if 'b' is also tested.
    // See: https://github.com/OpenZeppelin/openzeppelin-contracts/pull/522
    if (a == 0) {
      return 0;
    }

    uint256 c = a * b;
    require(c / a == b, 'SafeMath: multiplication overflow');

    return c;
  }

  /**
   * @dev Returns the integer division of two unsigned integers. Reverts on
   * division by zero. The result is rounded towards zero.
   *
   * Counterpart to Solidity's `/` operator. Note: this function uses a
   * `revert` opcode (which leaves remaining gas untouched) while Solidity
   * uses an invalid opcode to revert (consuming all remaining gas).
   *
   * Requirements:
   * - The divisor cannot be zero.
   */
  function div(uint256 a, uint256 b) internal pure returns (uint256) {
    return div(a, b, 'SafeMath: division by zero');
  }

  /**
   * @dev Returns the integer division of two unsigned integers. Reverts with custom message on
   * division by zero. The result is rounded towards zero.
   *
   * Counterpart to Solidity's `/` operator. Note: this function uses a
   * `revert` opcode (which leaves remaining gas untouched) while Solidity
   * uses an invalid opcode to revert (consuming all remaining gas).
   *
   * Requirements:
   * - The divisor cannot be zero.
   */
  function div(
    uint256 a,
    uint256 b,
    string memory errorMessage
  ) internal pure returns (uint256) {
    // Solidity only automatically asserts when dividing by 0
    require(b > 0, errorMessage);
    uint256 c = a / b;
    // assert(a == b * c + a % b); // There is no case in which this doesn't hold

    return c;
  }

  /**
   * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
   * Reverts when dividing by zero.
   *
   * Counterpart to Solidity's `%` operator. This function uses a `revert`
   * opcode (which leaves remaining gas untouched) while Solidity uses an
   * invalid opcode to revert (consuming all remaining gas).
   *
   * Requirements:
   * - The divisor cannot be zero.
   */
  function mod(uint256 a, uint256 b) internal pure returns (uint256) {
    return mod(a, b, 'SafeMath: modulo by zero');
  }

  /**
   * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
   * Reverts with custom message when dividing by zero.
   *
   * Counterpart to Solidity's `%` operator. This function uses a `revert`
   * opcode (which leaves remaining gas untouched) while Solidity uses an
   * invalid opcode to revert (consuming all remaining gas).
   *
   * Requirements:
   * - The divisor cannot be zero.
   */
  function mod(
    uint256 a,
    uint256 b,
    string memory errorMessage
  ) internal pure returns (uint256) {
    require(b != 0, errorMessage);
    return a % b;
  }
}