// SPDX-License-Identifier: MIT
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) {
        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;
    }
}

/*
 * @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 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/utils/Address.sol
/**
 * @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) {
        // This method relies on extcodesize, which returns 0 for contracts in
        // construction, since the code is only stored at the end of the
        // constructor execution.

        uint256 size;
        // solhint-disable-next-line no-inline-assembly
        assembly { size := extcodesize(account) }
        return size > 0;
    }

    /**
     * @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");
        return _functionCallWithValue(target, data, value, errorMessage);
    }

    function _functionCallWithValue(address target, bytes memory data, uint256 weiValue, string memory errorMessage) private returns (bytes memory) {
        require(isContract(target), "Address: call to non-contract");

        // solhint-disable-next-line avoid-low-level-calls
        (bool success, bytes memory returndata) = target.call{ value: weiValue }(data);
        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/token/ERC20/ERC20.sol
/**
 * @dev Implementation of the {IERC20} interface.
 *
 * This implementation is agnostic to the way tokens are created. This means
 * that a supply mechanism has to be added in a derived contract using {_mint}.
 * For a generic mechanism see {ERC20PresetMinterPauser}.
 *
 * TIP: For a detailed writeup see our guide
 * https://forum.zeppelin.solutions/t/how-to-implement-erc20-supply-mechanisms/226[How
 * to implement supply mechanisms].
 *
 * We have followed general OpenZeppelin guidelines: functions revert instead
 * of returning `false` on failure. This behavior is nonetheless conventional
 * and does not conflict with the expectations of ERC20 applications.
 *
 * Additionally, an {Approval} event is emitted on calls to {transferFrom}.
 * This allows applications to reconstruct the allowance for all accounts just
 * by listening to said events. Other implementations of the EIP may not emit
 * these events, as it isn't required by the specification.
 *
 * Finally, the non-standard {decreaseAllowance} and {increaseAllowance}
 * functions have been added to mitigate the well-known issues around setting
 * allowances. See {IERC20-approve}.
 */
contract ERC20 is Context, IERC20 {
    using SafeMath for uint256;
    using Address for address;

    mapping (address => uint256) private _balances;

    mapping (address => mapping (address => uint256)) private _allowances;

    uint256 private _totalSupply;

    string private _name;
    string private _symbol;
    uint8 private _decimals;

    /**
     * @dev Sets the values for {name} and {symbol}, initializes {decimals} with
     * a default value of 18.
     *
     * To select a different value for {decimals}, use {_setupDecimals}.
     *
     * All three of these values are immutable: they can only be set once during
     * construction.
     */
    constructor (string memory name, string memory symbol) public {
        _name = name;
        _symbol = symbol;
        _decimals = 18;
    }

    /**
     * @dev Returns the name of the token.
     */
    function name() public view returns (string memory) {
        return _name;
    }

    /**
     * @dev Returns the symbol of the token, usually a shorter version of the
     * name.
     */
    function symbol() public view returns (string memory) {
        return _symbol;
    }

    /**
     * @dev Returns the number of decimals used to get its user representation.
     * For example, if `decimals` equals `2`, a balance of `505` tokens should
     * be displayed to a user as `5,05` (`505 / 10 ** 2`).
     *
     * Tokens usually opt for a value of 18, imitating the relationship between
     * Ether and Wei. This is the value {ERC20} uses, unless {_setupDecimals} is
     * called.
     *
     * NOTE: This information is only used for _display_ purposes: it in
     * no way affects any of the arithmetic of the contract, including
     * {IERC20-balanceOf} and {IERC20-transfer}.
     */
    function decimals() public view returns (uint8) {
        return _decimals;
    }

    /**
     * @dev See {IERC20-totalSupply}.
     */
    function totalSupply() public view override returns (uint256) {
        return _totalSupply;
    }

    /**
     * @dev See {IERC20-balanceOf}.
     */
    function balanceOf(address account) public view override returns (uint256) {
        return _balances[account];
    }

    /**
     * @dev See {IERC20-transfer}.
     *
     * Requirements:
     *
     * - `recipient` cannot be the zero address.
     * - the caller must have a balance of at least `amount`.
     */
    function transfer(address recipient, uint256 amount) public virtual override returns (bool) {
        _transfer(_msgSender(), recipient, amount);
        return true;
    }

    /**
     * @dev See {IERC20-allowance}.
     */
    function allowance(address owner, address spender) public view virtual override returns (uint256) {
        return _allowances[owner][spender];
    }

    /**
     * @dev See {IERC20-approve}.
     *
     * Requirements:
     *
     * - `spender` cannot be the zero address.
     */
    function approve(address spender, uint256 amount) public virtual override returns (bool) {
        _approve(_msgSender(), spender, amount);
        return true;
    }

    /**
     * @dev See {IERC20-transferFrom}.
     *
     * Emits an {Approval} event indicating the updated allowance. This is not
     * required by the EIP. See the note at the beginning of {ERC20};
     *
     * Requirements:
     * - `sender` and `recipient` cannot be the zero address.
     * - `sender` must have a balance of at least `amount`.
     * - the caller must have allowance for ``sender``'s tokens of at least
     * `amount`.
     */
    function transferFrom(address sender, address recipient, uint256 amount) public virtual override returns (bool) {
        _transfer(sender, recipient, amount);
        _approve(sender, _msgSender(), _allowances[sender][_msgSender()].sub(amount, "ERC20: transfer amount exceeds allowance"));
        return true;
    }

    /**
     * @dev Atomically increases the allowance granted to `spender` by the caller.
     *
     * This is an alternative to {approve} that can be used as a mitigation for
     * problems described in {IERC20-approve}.
     *
     * Emits an {Approval} event indicating the updated allowance.
     *
     * Requirements:
     *
     * - `spender` cannot be the zero address.
     */
    function increaseAllowance(address spender, uint256 addedValue) public virtual returns (bool) {
        _approve(_msgSender(), spender, _allowances[_msgSender()][spender].add(addedValue));
        return true;
    }

    /**
     * @dev Atomically decreases the allowance granted to `spender` by the caller.
     *
     * This is an alternative to {approve} that can be used as a mitigation for
     * problems described in {IERC20-approve}.
     *
     * Emits an {Approval} event indicating the updated allowance.
     *
     * Requirements:
     *
     * - `spender` cannot be the zero address.
     * - `spender` must have allowance for the caller of at least
     * `subtractedValue`.
     */
    function decreaseAllowance(address spender, uint256 subtractedValue) public virtual returns (bool) {
        _approve(_msgSender(), spender, _allowances[_msgSender()][spender].sub(subtractedValue, "ERC20: decreased allowance below zero"));
        return true;
    }

    /**
     * @dev Moves tokens `amount` from `sender` to `recipient`.
     *
     * This is internal function is equivalent to {transfer}, and can be used to
     * e.g. implement automatic token fees, slashing mechanisms, etc.
     *
     * Emits a {Transfer} event.
     *
     * Requirements:
     *
     * - `sender` cannot be the zero address.
     * - `recipient` cannot be the zero address.
     * - `sender` must have a balance of at least `amount`.
     */
    function _transfer(address sender, address recipient, uint256 amount) internal virtual {
        require(sender != address(0), "ERC20: transfer from the zero address");
        require(recipient != address(0), "ERC20: transfer to the zero address");

        _beforeTokenTransfer(sender, recipient, amount);

        _balances[sender] = _balances[sender].sub(amount, "ERC20: transfer amount exceeds balance");
        _balances[recipient] = _balances[recipient].add(amount);
        emit Transfer(sender, recipient, amount);
    }

    /** @dev Creates `amount` tokens and assigns them to `account`, increasing
     * the total supply.
     *
     * Emits a {Transfer} event with `from` set to the zero address.
     *
     * Requirements
     *
     * - `to` cannot be the zero address.
     */
    function _mint(address account, uint256 amount) internal virtual {
        require(account != address(0), "ERC20: mint to the zero address");

        _beforeTokenTransfer(address(0), account, amount);

        _totalSupply = _totalSupply.add(amount);
        _balances[account] = _balances[account].add(amount);
        emit Transfer(address(0), account, amount);
    }

    /**
     * @dev Destroys `amount` tokens from `account`, reducing the
     * total supply.
     *
     * Emits a {Transfer} event with `to` set to the zero address.
     *
     * Requirements
     *
     * - `account` cannot be the zero address.
     * - `account` must have at least `amount` tokens.
     */
    function _burn(address account, uint256 amount) internal virtual {
        require(account != address(0), "ERC20: burn from the zero address");

        _beforeTokenTransfer(account, address(0), amount);

        _balances[account] = _balances[account].sub(amount, "ERC20: burn amount exceeds balance");
        _totalSupply = _totalSupply.sub(amount);
        emit Transfer(account, address(0), amount);
    }

    /**
     * @dev Sets `amount` as the allowance of `spender` over the `owner` s tokens.
     *
     * This internal function is equivalent to `approve`, and can be used to
     * e.g. set automatic allowances for certain subsystems, etc.
     *
     * Emits an {Approval} event.
     *
     * Requirements:
     *
     * - `owner` cannot be the zero address.
     * - `spender` cannot be the zero address.
     */
    function _approve(address owner, address spender, uint256 amount) internal virtual {
        require(owner != address(0), "ERC20: approve from the zero address");
        require(spender != address(0), "ERC20: approve to the zero address");

        _allowances[owner][spender] = amount;
        emit Approval(owner, spender, amount);
    }

    /**
     * @dev Sets {decimals} to a value other than the default one of 18.
     *
     * WARNING: This function should only be called from the constructor. Most
     * applications that interact with token contracts will not expect
     * {decimals} to ever change, and may work incorrectly if it does.
     */
    function _setupDecimals(uint8 decimals_) internal {
        _decimals = decimals_;
    }

    /**
     * @dev Hook that is called before any transfer of tokens. This includes
     * minting and burning.
     *
     * Calling conditions:
     *
     * - when `from` and `to` are both non-zero, `amount` of ``from``'s tokens
     * will be to transferred to `to`.
     * - when `from` is zero, `amount` tokens will be minted for `to`.
     * - when `to` is zero, `amount` of ``from``'s tokens will be burned.
     * - `from` and `to` are never both zero.
     *
     * To learn more about hooks, head to xref:ROOT:extending-contracts.adoc#using-hooks[Using Hooks].
     */
    function _beforeTokenTransfer(address from, address to, uint256 amount) internal virtual { }
}

/**
 * @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));
    }

    /**
     * @dev Deprecated. This function has issues similar to the ones found in
     * {IERC20-approve}, and its usage is discouraged.
     *
     * Whenever possible, use {safeIncreaseAllowance} and
     * {safeDecreaseAllowance} instead.
     */
    function safeApprove(IERC20 token, address spender, uint256 value) internal {
        // safeApprove should only be called when setting an initial allowance,
        // or when resetting it to zero. To increase and decrease it, use
        // 'safeIncreaseAllowance' and 'safeDecreaseAllowance'
        // solhint-disable-next-line max-line-length
        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 safeIncreaseAllowance(IERC20 token, address spender, uint256 value) internal {
        uint256 newAllowance = token.allowance(address(this), spender).add(value);
        _callOptionalReturn(token, abi.encodeWithSelector(token.approve.selector, spender, newAllowance));
    }

    function safeDecreaseAllowance(IERC20 token, address spender, uint256 value) internal {
        uint256 newAllowance = token.allowance(address(this), spender).sub(value, "SafeERC20: decreased allowance below zero");
        _callOptionalReturn(token, abi.encodeWithSelector(token.approve.selector, spender, newAllowance));
    }

    /**
     * @dev Imitates a Solidity high-level call (i.e. a regular function call to a contract), relaxing the requirement
     * on the return value: the return value is optional (but if data is returned, it must not be false).
     * @param token The token targeted by the call.
     * @param data The call data (encoded using abi.encode or one of its variants).
     */
    function _callOptionalReturn(IERC20 token, bytes memory data) private {
        // We need to perform a low level call here, to bypass Solidity's return data size checking mechanism, since
        // we're implementing it ourselves. We use {Address.functionCall} to perform this call, which verifies that
        // the target address contains contract code and also asserts for success in the low-level call.

        bytes memory returndata = address(token).functionCall(data, "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");
        }
    }
}

/**
 * @dev Contract module that helps prevent reentrant calls to a function.
 *
 * Inheriting from `ReentrancyGuard` will make the {nonReentrant} modifier
 * available, which can be applied to functions to make sure there are no nested
 * (reentrant) calls to them.
 *
 * Note that because there is a single `nonReentrant` guard, functions marked as
 * `nonReentrant` may not call one another. This can be worked around by making
 * those functions `private`, and then adding `external` `nonReentrant` entry
 * points to them.
 *
 * TIP: If you would like to learn more about reentrancy and alternative ways
 * to protect against it, check out our blog post
 * https://blog.openzeppelin.com/reentrancy-after-istanbul/[Reentrancy After Istanbul].
 */
contract ReentrancyGuard {
    // Booleans are more expensive than uint256 or any type that takes up a full
    // word because each write operation emits an extra SLOAD to first read the
    // slot's contents, replace the bits taken up by the boolean, and then write
    // back. This is the compiler's defense against contract upgrades and
    // pointer aliasing, and it cannot be disabled.

    // The values being non-zero value makes deployment a bit more expensive,
    // but in exchange the refund on every call to nonReentrant will be lower in
    // amount. Since refunds are capped to a percentage of the total
    // transaction's gas, it is best to keep them low in cases like this one, to
    // increase the likelihood of the full refund coming into effect.
    uint256 private constant _NOT_ENTERED = 1;
    uint256 private constant _ENTERED = 2;

    uint256 private _status;

    constructor () internal {
        _status = _NOT_ENTERED;
    }

    /**
     * @dev Prevents a contract from calling itself, directly or indirectly.
     * Calling a `nonReentrant` function from another `nonReentrant`
     * function is not supported. It is possible to prevent this from happening
     * by making the `nonReentrant` function external, and make it call a
     * `private` function that does the actual work.
     */
    modifier nonReentrant() {
        // On the first call to nonReentrant, _notEntered will be true
        require(_status != _ENTERED, "ReentrancyGuard: reentrant call");

        // Any calls to nonReentrant after this point will fail
        _status = _ENTERED;

        _;

        // By storing the original value once again, a refund is triggered (see
        // https://eips.ethereum.org/EIPS/eip-2200)
        _status = _NOT_ENTERED;
    }
}

interface Burnable {
    function burn(uint256 amount) external returns (bool);
}

interface IStrategy {
    function rewards() external view returns (address);

    function gauge() external view returns (address);

    function underlying() external view returns (address);

    function timelock() external view returns (address);

    function vault() external view returns (address);

    function deposit() external;

    function withdraw(uint256) external;

    function withdrawAll() external returns (uint256);

    function balanceOf() external view returns (uint256);

    function harvest() external;

    function salvage(address) external;

    function setTimelock(address _timelock) external;

    function setGovernance(address _governance) external;

    function setTreasury(address _treasury) external;
}

contract YvsVault is ERC20, ReentrancyGuard {
    using SafeERC20 for IERC20;
    using Address for address;
    using SafeMath for uint256;

    IERC20 internal token;
    IERC20 internal yvs;

    // Underlying token address
    address public underlying;

    // Address of controller
    address public controller;

    // Minimum/maximum allowed to be invested
    uint256 public min = 9500;
    uint256 public constant max = 10000;

    // Burn fee on purchases
    uint256 public burnFee = 5000;
    uint256 public constant burnFeeMax = 7500;
    uint256 public constant burnFeeMin = 2500;
    uint256 public constant burnFeeBase = 10000;

    // Withdrawal fee
    uint256 public withdrawalFee = 25;
    uint256 public constant withdrawalFeeMax = 25;
    uint256 public constant withdrawalFeeBase = 10000;

    // Minimum deposit period
    uint256 public minDepositPeriod = 7 days;

    // Is the strategy active (inactive on deploy)
    bool public isActive = false;

    // Addresses
    address public governance;
    address public treasury;
    address public timelock;
    address public strategy;

    mapping(address => uint256) public depositBlocks;
    mapping(address => uint256) public deposits;
    mapping(address => uint256) public issued;
    mapping(address => uint256) public tiers;
    uint256[] public multiplierCosts;
    uint256 internal constant tierMultiplier = 5;
    uint256 internal constant tierBase = 100;
    uint256 public totalDeposited = 0;

    // EVENTS
    event Deposit(address indexed user, uint256 amount);
    event Withdraw(address indexed user, uint256 amount);
    event SharesIssued(address indexed user, uint256 amount);
    event SharesPurged(address indexed user, uint256 amount);
    event ClaimRewards(address indexed user, uint256 amount);
    event MultiplierPurchased(address indexed user, uint256 tiers, uint256 totalCost);

    constructor(address _underlying, address _yvs, address _governance, address _treasury, address _timelock)
        public
        ERC20(
            string(abi.encodePacked("yvsie ", ERC20(_underlying).name())),
            string(abi.encodePacked("yvs", ERC20(_underlying).symbol()))
        )
    {
        require(address(_underlying) != address(_yvs), "!underlying");

        _setupDecimals(ERC20(_underlying).decimals());
        token = IERC20(_underlying);
        yvs = IERC20(_yvs);
        underlying = _underlying;
        governance = _governance;
        treasury = _treasury;
        timelock = _timelock;

        // multiplier costs from tier 1 to 5
        multiplierCosts.push(5000000000000000000); // 5 $yvs
        multiplierCosts.push(10000000000000000000); // 10 $yvs
        multiplierCosts.push(20000000000000000000); // 20 $yvs
        multiplierCosts.push(40000000000000000000); // 40 $yvs
        multiplierCosts.push(80000000000000000000); // 80 $yvs
    }

    // Check the total underyling token balance to see if we should earn();
    function balance() public view returns (uint256) {
        return
            token.balanceOf(address(this)).add(
                IStrategy(strategy).balanceOf()
            );
    }

    // Sets whether deposits are accepted by the vault
    function setActive(bool _isActive) external isGovernance {
        isActive = _isActive;
    }

    // Set the minimum percentage of tokens that can be deposited to earn 
    function setMin(uint256 _min) external isGovernance {
        require(_min <= max, "min>max");
        min = _min;
    }

    // Set a new governance address, can only be triggered by the old address
    function setGovernance(address _governance) external isGovernance {
        governance = _governance;
    }

    // Set a new treasury address, can only be triggered by the governance
    function setTreasury(address _treasury) external isGovernance {
        treasury = _treasury;
    }

    // Set the timelock address, can only be triggered by the old address
    function setTimelock(address _timelock) external isTimelock {
        timelock = _timelock;
    }

    // Set a new strategy address, can only be triggered by the timelock
    function setStrategy(address _strategy) external isTimelock {
        require(IStrategy(_strategy).underlying() == address(token), '!underlying');
        strategy = _strategy;
    }

    // Set the controller address, can only be set once after deployment
    function setController(address _controller) external isGovernance {
        require(controller == address(0), "!controller");
        controller = _controller;
    }

    // Set the burn fee for multipliers
    function setBurnFee(uint256 _burnFee) public isTimelock {
        require(_burnFee <= burnFeeMax, 'max');
        require(_burnFee >= burnFeeMin, 'min');
        burnFee = _burnFee;
    }

    // Set withdrawal fee for the vault
    function setWithdrawalFee(uint256 _withdrawalFee) external isTimelock {
        require(_withdrawalFee <= withdrawalFeeMax, "!max");
        withdrawalFee = _withdrawalFee;
    }

    // Add a new multplier with the selected cost
    function addMultiplier(uint256 _cost) public isTimelock returns (uint256 index) {
        multiplierCosts.push(_cost);
        index = multiplierCosts.length - 1;
    }

    // Set new cost for multiplier, can only be triggered by the timelock
    function setMultiplier(uint256 index, uint256 _cost) public isTimelock {
        multiplierCosts[index] = _cost;
    }

    // Custom logic in here for how much of the underlying asset can be deposited
    // Sets the minimum required on-hand to keep small withdrawals cheap
    function available() public view returns (uint256) {
        return token.balanceOf(address(this)).mul(min).div(max);
    }

    // Deposits collected underlying assets into the strategy and starts earning
    function earn() public {
        require(isActive, 'earn: !active');
        require(strategy != address(0), 'earn: !strategy');
        uint256 _bal = available();
        token.safeTransfer(strategy, _bal);
        IStrategy(strategy).deposit();
    }

    // Deposits underlying assets from the user into the vault contract
    function deposit(uint256 _amount) public nonReentrant {
        require(!address(msg.sender).isContract() && msg.sender == tx.origin, "deposit: !contract");
        require(isActive, 'deposit: !vault');
        require(strategy != address(0), 'deposit: !strategy');
        
        uint256 _pool = balance();
        uint256 _before = token.balanceOf(address(this));
        token.safeTransferFrom(msg.sender, address(this), _amount);
        uint256 _after = token.balanceOf(address(this));
        _amount = _after.sub(_before); // Additional check for deflationary tokens
        deposits[msg.sender] = deposits[msg.sender].add(_amount);
        totalDeposited = totalDeposited.add(_amount);
        uint256 shares = 0;
        if (totalSupply() == 0) {
            uint256 userMultiplier = tiers[msg.sender].mul(tierMultiplier).add(tierBase); // 5 %, 10 %, 15 %, 20 %, 25 %
            shares = _amount.mul(userMultiplier).div(tierBase);
        } else {
            uint256 userMultiplier = tiers[msg.sender].mul(tierMultiplier).add(tierBase); // 5 %, 10 %, 15 %, 20 %, 25 %
            shares = (_amount.mul(userMultiplier).div(tierBase).mul(totalSupply())).div(_pool);
        }

        _mint(msg.sender, shares);
        issued[msg.sender] = issued[msg.sender].add(shares);
        depositBlocks[msg.sender] = block.number;
        emit Deposit(msg.sender, _amount);
        emit SharesIssued(msg.sender, shares);
    }

    // Deposits all the funds of the user
    function depositAll() external {
        deposit(token.balanceOf(msg.sender));
    }

    // No rebalance implementation for lower fees and faster swaps
    function withdraw(uint256 _amount) public nonReentrant {
        require(!address(msg.sender).isContract() && msg.sender == tx.origin, "withdraw: !no contract");
        require(block.number >= depositBlocks[msg.sender].add(minDepositPeriod), 'withdraw: !minDepositPeriod');
        require(_amount > 0, '!positive');
        require(_amount <= deposits[msg.sender], '>deposit');
        require(issued[msg.sender] > 0, '!deposit');

        // Get the amount of user shares
        uint256 shares = issued[msg.sender];
        // Calculate percentage of principal being withdrawn
        uint256 p = (_amount.mul(1e18).div(deposits[msg.sender]));
        // Calculate amount of shares to be burned
        uint256 r = shares.mul(p).div(1e18);

        // Make sure the user has the required amount in his balance
        require(balanceOf(msg.sender) >= r, "!shares");
        // Burn the proportion of shares that are being withdrawn
        _burn(msg.sender, r);
        // Reduce the amount from user's issued amount
        issued[msg.sender] = issued[msg.sender].sub(r);

        // Calculate amount of rewards the user has gained
        uint256 rewards = balance().sub(totalDeposited);
        uint256 userRewards = 0;
        if (rewards > 0) {
            userRewards = (rewards.mul(shares)).div(totalSupply());
        }

        // Receive the correct proportion of the rewards
        if (userRewards > 0) {
            userRewards = userRewards.mul(p).div(1e18);
        }

        // Calculate the withdrawal amount as _amount + user rewards
        uint256 withdrawAmount = _amount.add(userRewards);

        // Check balance
        uint256 b = token.balanceOf(address(this));
        if (b < withdrawAmount) {
            uint256 _withdraw = withdrawAmount.sub(b);
            IStrategy(strategy).withdraw(_withdraw);
            uint256 _after = token.balanceOf(address(this));
            uint256 _diff = _after.sub(b);
            if (_diff < _withdraw) {
                withdrawAmount = b.add(_diff);
            }
        }

        // Remove the withdrawn principal from total and user deposits
        deposits[msg.sender] = deposits[msg.sender].sub(_amount);
        totalDeposited = totalDeposited.sub(_amount);

        // Calculate withdrawal fee and deduct from amount
        uint256 _withdrawalFee = _amount.mul(withdrawalFee).div(withdrawalFeeBase);
        token.safeTransfer(treasury, _withdrawalFee);
        token.safeTransfer(msg.sender, withdrawAmount.sub(_withdrawalFee));

        // Emit events
        emit Withdraw(msg.sender, _amount);
        emit SharesPurged(msg.sender, r);
        emit ClaimRewards(msg.sender, userRewards);
    }

    // Withdraws all underlying assets belonging to the user
    function withdrawAll() external {
        withdraw(deposits[msg.sender]);
    }

    function pendingRewards(address account) external view returns (uint256 pending) {
        // Calculate amount of rewards the user has gained
        uint256 rewards = balance().sub(totalDeposited);
        uint256 shares = issued[account];
        if (rewards > 0) {
            pending = (rewards.mul(shares)).div(totalSupply());
        }
    }

    // Purchase a multiplier tier for the user
    function purchaseMultiplier(uint256 _tiers) external returns (uint256 newTier) {
        require(isActive, '!active');
        require(strategy != address(0), '!strategy');
        require(_tiers > 0, '!tiers');
        uint256 multipliersLength = multiplierCosts.length;
        require(tiers[msg.sender].add(_tiers) <= multipliersLength, '!max');

        uint256 totalCost = 0;
        uint256 lastMultiplier = tiers[msg.sender].add(_tiers);
        for (uint256 i = tiers[msg.sender]; i < multipliersLength; i++) {
            if (i == lastMultiplier) {
                break;
            }
            totalCost = totalCost.add(multiplierCosts[i]);
        }

        require(IERC20(yvs).balanceOf(msg.sender) >= totalCost, '!yvs');
        yvs.safeTransferFrom(msg.sender, address(this), totalCost);
        newTier = tiers[msg.sender].add(_tiers);
        tiers[msg.sender] = newTier;
        emit MultiplierPurchased(msg.sender, _tiers, totalCost);
    }

    // Distribute the YVS tokens collected by the multiplier purchases
    function distribute() external restricted {
        uint256 b = yvs.balanceOf(address(this));
        if (b > 0) {
            uint256 toBurn = b.mul(burnFee).div(burnFeeBase);
            uint256 leftover = b.sub(toBurn);
            Burnable(address(yvs)).burn(toBurn);
            yvs.safeTransfer(treasury, leftover);
        }
    }

    // Used to salvage any non-underlying assets to governance
    function salvage(address reserve, uint256 amount) external isGovernance {
        require(reserve != address(token), "!token");
        require(reserve != address(yvs), "!yvs");
        IERC20(reserve).safeTransfer(treasury, amount);
    }

    // Returns the current multiplier tier for the user
    function getMultiplier() external view returns (uint256) {
        return tiers[msg.sender];
    }

    // Returns the next multiplier tier cost for the user
    function getNextMultiplierCost() external view returns (uint256) {
        require(tiers[msg.sender] < multiplierCosts.length, '!all');
        return multiplierCosts[tiers[msg.sender]];
    }

    // Returns the total number of multipliers
    function getCountOfMultipliers() external view returns (uint256) {
        return multiplierCosts.length;
    }

    // Returns the current ratio between earned assets and deposited assets
    function getRatio() public view returns (uint256) {
        return (balance().sub(totalDeposited)).mul(1e18).div(totalSupply());
    }

    // **** Modifiers **** //

    modifier restricted {
        require(
            (msg.sender == tx.origin && !address(msg.sender).isContract()) ||
                msg.sender == governance ||
                msg.sender == controller
        );

        _;
    }

    modifier isTimelock {
        require(
            msg.sender == timelock, 
            "!timelock"
        );

        _;
    }

    modifier isGovernance {
        require(
            msg.sender == governance, 
            "!governance"
        );

        _;
    }
}