// File: contracts/SHIATokenCrowdsale.sol



// SPDX-License-Identifier: MIT
pragma solidity ^0.8.4;

import "@openzeppelin/contracts/access/Ownable.sol";
import "@openzeppelin/contracts/token/ERC20/ERC20.sol";
import "./Crowdsale.sol";
import "./AllowanceCrowdsale.sol";
import "./TimedCrowdsale.sol";

contract SHIATokenCrowdsale is AllowanceCrowdsale, TimedCrowdsale {
	constructor(
		uint256 _rate,
		address payable _wallet,
		ERC20 _token,
		ERC20 _usdt,
		address _tokenWallet,
		uint256 _openingTime,
		uint256 _closingTime,

		uint256 maxInvestment,
        uint256 minInvestment
	)
		Crowdsale(_rate, _wallet, _token, _usdt, maxInvestment, minInvestment)
		AllowanceCrowdsale(_tokenWallet)
		TimedCrowdsale(_openingTime, _closingTime)
	{}

	/**
	 * @dev Extend parent behavior requiring to be within contributing period.
	 * @param beneficiary Token purchaser
	 * @param weiAmount Amount of wei contributed
	 */
	function _preValidatePurchase(address beneficiary, uint256 weiAmount)
		internal
		view
		override(Crowdsale, TimedCrowdsale)
		onlyWhileOpen
	{
		super._preValidatePurchase(beneficiary, weiAmount);
	}

	/**
	 * @dev Overrides parent behavior by transferring tokens from wallet.
	 * @param beneficiary Token purchaser
	 * @param tokenAmount Amount of tokens purchased
	 */
	function _deliverTokens(address beneficiary, uint256 tokenAmount)
		internal
		override(Crowdsale, AllowanceCrowdsale)
	{
		super._deliverTokens(beneficiary, tokenAmount);
	}
}

// File: contracts/TimedCrowdsale.sol



// SPDX-License-Identifier: MIT
pragma solidity ^0.8.4;

import "./Crowdsale.sol";

/**
 * @title TimedCrowdsale
 * @dev Borrow from https://github.dev/OpenZeppelin/openzeppelin-contracts/blob/docs-v2.x/contracts/crowdsale/validation/TimedCrowdsale.sol
 * @dev Crowdsale accepting contributions only within a time frame.
 */
abstract contract TimedCrowdsale is Crowdsale {
	uint256 public _openingTime;
	uint256 public _closingTime;

	/**
	 * Event for crowdsale extending
	 * @param newClosingTime new closing time
	 * @param prevClosingTime old closing time
	 */
	event TimedCrowdsaleExtended(
		uint256 prevClosingTime,
		uint256 newClosingTime
	);

	/**
	 * @dev Reverts if not in crowdsale time range.
	 */
	modifier onlyWhileOpen() {
		require(isOpen(), "TimedCrowdsale: not open");
		_;
	}

	/**
	 * @dev Constructor, takes crowdsale opening and closing times.
	 * @param openingTime Crowdsale opening time
	 * @param closingTime Crowdsale closing time
	 */
	constructor(uint256 openingTime, uint256 closingTime) {
		// solhint-disable-next-line not-rely-on-time
		require(
			openingTime >= block.timestamp,
			"TimedCrowdsale: opening time is before current time"
		);
		// solhint-disable-next-line max-line-length
		require(
			closingTime > openingTime,
			"TimedCrowdsale: opening time is not before closing time"
		);

		_openingTime = openingTime;
		_closingTime = closingTime;
	}


	/**
	 * @return true if the crowdsale is open, false otherwise.
	 */
	function isOpen() public view returns (bool) {
		// solhint-disable-next-line not-rely-on-time
		return
			block.timestamp >= _openingTime && block.timestamp <= _closingTime;
	}

	/**
	 * @dev Checks whether the period in which the crowdsale is open has already elapsed.
	 * @return Whether crowdsale period has elapsed
	 */
	function hasClosed() public view returns (bool) {
		// solhint-disable-next-line not-rely-on-time
		return block.timestamp > _closingTime;
	}

	/**
	 * @dev Extend parent behavior requiring to be within contributing period.
	 * @param beneficiary Token purchaser
	 * @param weiAmount Amount of wei contributed
	 */
	function _preValidatePurchase(address beneficiary, uint256 weiAmount)
		internal
		view
		virtual
		override
		onlyWhileOpen
	{
		super._preValidatePurchase(beneficiary, weiAmount);
	}

	/**
	 * @dev Extend crowdsale.
	 * @param newClosingTime Crowdsale closing time
	 */
	function _extendTime(uint256 newClosingTime) internal {
		require(!hasClosed(), "TimedCrowdsale: already closed");
		// solhint-disable-next-line max-line-length
		require(
			newClosingTime > _closingTime,
			"TimedCrowdsale: new closing time is before current closing time"
		);

		emit TimedCrowdsaleExtended(_closingTime, newClosingTime);
		_closingTime = newClosingTime;
	}
}

// File: contracts/AllowanceCrowdsale.sol



// SPDX-License-Identifier: MIT
pragma solidity ^0.8.4;

import "@openzeppelin/contracts/token/ERC20/IERC20.sol";
import "@openzeppelin/contracts/token/ERC20/utils/SafeERC20.sol";
import "@openzeppelin/contracts/utils/math/Math.sol";
import "./Crowdsale.sol";

/**
 * @title AllowanceCrowdsale
 * @dev Borrow from https://github.dev/OpenZeppelin/openzeppelin-contracts/blob/docs-v2.x/contracts/crowdsale/emission/AllowanceCrowdsale.sol
 * @dev Extension of Crowdsale where tokens are held by a wallet, which approves an allowance to the crowdsale.
 */
abstract contract AllowanceCrowdsale is Crowdsale {
	using SafeERC20 for IERC20;

	address public _tokenWallet;

	/**
	 * @dev Constructor, takes token wallet address.
	 * @param tokenWallet Address holding the tokens, which has approved allowance to the crowdsale.
	 */
	constructor(address tokenWallet) {
		require(
			tokenWallet != address(0),
			"AllowanceCrowdsale: token wallet is the zero address"
		);
		_tokenWallet = tokenWallet;
	}
	
	/**
	 * @dev Checks the amount of tokens left in the allowance.
	 * @return Amount of tokens left in the allowance
	 */
	function remainingTokens() public view returns (uint256) {
		return
			Math.min(
				_token.balanceOf(_tokenWallet),
				_token.allowance(_tokenWallet, address(this))
			);
	}

	/**
	 * @dev Overrides parent behavior by transferring tokens from wallet.
	 * @param beneficiary Token purchaser
	 * @param tokenAmount Amount of tokens purchased
	 */
	function _deliverTokens(address beneficiary, uint256 tokenAmount)
		internal
		virtual
		override
	{
		_token.safeTransferFrom(_tokenWallet, beneficiary, tokenAmount);
	}
}

// File: contracts/Crowdsale.sol



// SPDX-License-Identifier: MIT
pragma solidity ^0.8.4;

import "@openzeppelin/contracts/utils/Context.sol";
import "@openzeppelin/contracts/access/Ownable.sol";
import "@openzeppelin/contracts/security/ReentrancyGuard.sol";
import "@openzeppelin/contracts/token/ERC20/IERC20.sol";
import "@openzeppelin/contracts/token/ERC20/utils/SafeERC20.sol";

/**
 * @title Crowdsale
 * @dev Borrow from https://github.dev/OpenZeppelin/openzeppelin-contracts/blob/docs-v2.x/contracts/crowdsale/Crowdsale.ol and upgrade to solidity v0.8.0
 * @dev Crowdsale is a base contract for managing a token crowdsale,
 * allowing investors to purchase tokens with ether. This contract implements
 * such functionality in its most fundamental form and can be extended to provide additional
 * functionality and/or custom behavior.
 * The external interface represents the basic interface for purchasing tokens, and conforms
 * the base architecture for crowdsales. It is *not* intended to be modified / overridden.
 * The internal interface conforms the extensible and modifiable surface of crowdsales. Override
 * the methods to add functionality. Consider using 'super' where appropriate to concatenate
 * behavior.
 */
contract Crowdsale is Context, Ownable, ReentrancyGuard {
	using SafeERC20 for IERC20;

	// The token being sold
	IERC20 public _token;

	// The token used to pruchase
	IERC20 public _usdt;

	// Maximum investment per investor (in USDT)
    uint256 public _maxInvestment;

    // Minimum investment per investor (in USDT)
    uint256 public _minInvestment;

	// Address where funds are collected
	address payable public _wallet;

	// How many token units a buyer gets per wei.
	// The rate is the conversion between wei and the smallest and indivisible token unit.
	// So, if you are using a rate of 1 with a ERC20Detailed token with 3 decimals called TOK
	// 1 wei will give you 1 unit, or 0.001 TOK.
	uint256 public _rate;

	// Amount of wei raised
	uint256 public _weiRaised;

	/**
	 * Event for token purchase logging
	 * @param purchaser who paid for the tokens
	 * @param beneficiary who got the tokens
	 * @param value weis paid for purchase
	 * @param amount amount of tokens purchased
	 */
	event TokensPurchased(
		address indexed purchaser,
		address indexed beneficiary,
		uint256 value,
		uint256 amount
	);

	/**
	 * @param rate Number of token units a buyer gets per wei
	 * @dev The rate is the conversion between wei and the smallest and indivisible
	 * token unit. So, if you are using a rate of 1 with a ERC20Detailed token
	 * with 3 decimals called TOK, 1 wei will give you 1 unit, or 0.001 TOK.
	 * @param wallet Address where collected funds will be forwarded to
	 * @param token Address of the token being sold
	 */
	constructor(
		uint256 rate,
		address payable wallet,
		IERC20 token,
		IERC20 usdt,

		uint256 maxInvestment,
        uint256 minInvestment
	) {
		require(rate > 0, "Crowdsale: rate is 0");
		require(wallet != address(0), "Crowdsale: wallet is the zero address");
		require(
			address(token) != address(0),
			"Crowdsale: token is the zero address"
		);
		require(maxInvestment > 0, "SHIACrowdsale: max investment is 0");
        require(minInvestment > 0, "SHIACrowdsale: min investment is 0");

		_rate = rate;
		_wallet = wallet;
		_token = token;
		_usdt = usdt;
		_maxInvestment = maxInvestment;
        _minInvestment = minInvestment;
	}


	/**
	 * @dev fallback function ***DO NOT OVERRIDE***
	 * Note that other contracts will transfer funds with a base gas stipend
	 * of 2300, which is not enough to call buyTokens. Consider calling
	 * buyTokens directly when purchasing tokens from a contract.
	 */
	// receive() external payable {
	// 	buyTokens(_msgSender());
	// }

	/**
	 * @dev This function updates the minimum investment
	 * This function has a non-reentrancy guard, so it shouldn't be called by
	 * another `nonReentrant` function.
	 * @param newMaxInvestment new max investment required
	 */
	function updateMaximumInvestment(uint256 newMaxInvestment) public onlyOwner nonReentrant  {
		// update rate
		_maxInvestment = newMaxInvestment;
	}

	/**
	 * @dev This function updates the minimum investment
	 * This function has a non-reentrancy guard, so it shouldn't be called by
	 * another `nonReentrant` function.
	 * @param newMinInvesetment new minimum investment required
	 */
	function updateMinimumInvestment(uint256 newMinInvesetment) public onlyOwner nonReentrant  {
		// update rate
		_minInvestment = newMinInvesetment;
	}

	/**
	 * @dev This function chnages the rate
	 * This function has a non-reentrancy guard, so it shouldn't be called by
	 * another `nonReentrant` function.
	 * @param newRate Value in usdt involved in the purchase
	 */
	function updateRate(uint256 newRate) public onlyOwner nonReentrant  {
		// update rate
		_rate = newRate;
	}


	/**
	 * @dev low level token purchase ***DO NOT OVERRIDE***
	 * This function has a non-reentrancy guard, so it shouldn't be called by
	 * another `nonReentrant` function.
	 * @param beneficiary Recipient of the token purchase
	 * @param usdtAmount Value in usdt involved in the purchase
	 */
	function buyTokens(address beneficiary, uint256 usdtAmount) public nonReentrant {
		_preValidatePurchase(beneficiary, usdtAmount);

		// calculate token amount to be created
		uint256 tokenAmount = _getTokenAmount(usdtAmount);

		// update state
		_weiRaised = _weiRaised + usdtAmount;

		_processPurchase(beneficiary, tokenAmount);
		emit TokensPurchased(_msgSender(), beneficiary, usdtAmount, tokenAmount);

		_updatePurchasingState(beneficiary, usdtAmount);

		_forwardFunds(usdtAmount);
		_postValidatePurchase(beneficiary, usdtAmount);
	}

	/**
	 * @dev Validation of an incoming purchase. Use require statements to revert state when conditions are not met.
	 * Use `super` in contracts that inherit from Crowdsale to extend their validations.
	 * Example from CappedCrowdsale.sol's _preValidatePurchase method:
	 *     super._preValidatePurchase(beneficiary, usdtAmount);
	 *     require(weiRaised().add(usdtAmount) <= cap);
	 * @param beneficiary Address performing the token purchase
	 * @param usdtAmount Value in wei involved in the purchase
	 */
	function _preValidatePurchase(address beneficiary, uint256 usdtAmount)
		internal
		view
		virtual
	{
		require(
			beneficiary != address(0),
			"Crowdsale: beneficiary is the zero address"
		);
		require(usdtAmount != 0, "Crowdsale: usdtAmount is 0");
		require(usdtAmount >= _minInvestment, "SHIACrowdsale: investment amount is less than minimum");
        require(usdtAmount <= _maxInvestment, "SHIACrowdsale: investment amount is greater than maximum");

		this; // silence state mutability warning without generating bytecode - see https://github.com/ethereum/solidity/issues/2691
	}

	/**
	 * @dev Validation of an executed purchase. Observe state and use revert statements to undo rollback when valid
	 * conditions are not met.
	 * @param beneficiary Address performing the token purchase
	 * @param usdtAmount Value in wei involved in the purchase
	 */
	function _postValidatePurchase(address beneficiary, uint256 usdtAmount)
		internal
		view
	{
		// solhint-disable-previous-line no-empty-blocks
	}

	/**
	 * @dev Source of tokens. Override this method to modify the way in which the crowdsale ultimately gets and sends
	 * its tokens.
	 * @param beneficiary Address performing the token purchase
	 * @param tokenAmount Number of tokens to be emitted
	 */
	function _deliverTokens(address beneficiary, uint256 tokenAmount)
		internal
		virtual
	{
		_token.safeTransfer(beneficiary, tokenAmount);
	}

	/**
	 * @dev Executed when a purchase has been validated and is ready to be executed. Doesn't necessarily emit/send
	 * tokens.
	 * @param beneficiary Address receiving the tokens
	 * @param tokenAmount Number of tokens to be purchased
	 */
	function _processPurchase(address beneficiary, uint256 tokenAmount)
		internal
	{
		_deliverTokens(beneficiary, tokenAmount);
	}

	/**
	 * @dev Override for extensions that require an internal state to check for validity (current user contributions,
	 * etc.)
	 * @param beneficiary Address receiving the tokens
	 * @param usdtAmount Value in wei involved in the purchase
	 */
	function _updatePurchasingState(address beneficiary, uint256 usdtAmount)
		internal
	{
		// solhint-disable-previous-line no-empty-blocks
	}

	/**
	 * @dev Override to extend the way in which ether is converted to tokens.
	 * @param usdtAmount Value in usdt to be converted into tokens
	 * @return Number of tokens that can be purchased with the specified _weiAmount
	 */
	function _getTokenAmount(uint256 usdtAmount)
		internal
		view
		returns (uint256)
	{
		return usdtAmount * _rate;
	}


	/**
	 * @dev Determines how ETH is stored/forwarded on purchases.
	 */
	function _forwardFunds(uint256 usdtAmount) internal {
		// conver usdtAmount to kwei because usdt is 6 decimals
		uint256 kweiAmount = usdtAmount / 10 ** 12;
		
		require(_wallet != address(0), "SHIACrowdsale: wallet is the zero address");
        require(_usdt.balanceOf(msg.sender) >= kweiAmount, "SHIACrowdsale: insufficient USDT balance");
        SafeERC20.safeTransferFrom(_usdt, msg.sender, _wallet, kweiAmount);
	}
}

// File: @openzeppelin/contracts/token/ERC20/ERC20.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;

import "./IERC20.sol";
import "./extensions/IERC20Metadata.sol";
import "../../utils/Context.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, IERC20Metadata {
    mapping (address => uint256) private _balances;

    mapping (address => mapping (address => uint256)) private _allowances;

    uint256 private _totalSupply;

    string private _name;
    string private _symbol;

    /**
     * @dev Sets the values for {name} and {symbol}.
     *
     * The defaut value of {decimals} is 18. To select a different value for
     * {decimals} you should overload it.
     *
     * All two of these values are immutable: they can only be set once during
     * construction.
     */
    constructor (string memory name_, string memory symbol_) {
        _name = name_;
        _symbol = symbol_;
    }

    /**
     * @dev Returns the name of the token.
     */
    function name() public view virtual override returns (string memory) {
        return _name;
    }

    /**
     * @dev Returns the symbol of the token, usually a shorter version of the
     * name.
     */
    function symbol() public view virtual override 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 this function is
     * overridden;
     *
     * 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 virtual override returns (uint8) {
        return 18;
    }

    /**
     * @dev See {IERC20-totalSupply}.
     */
    function totalSupply() public view virtual override returns (uint256) {
        return _totalSupply;
    }

    /**
     * @dev See {IERC20-balanceOf}.
     */
    function balanceOf(address account) public view virtual 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);

        uint256 currentAllowance = _allowances[sender][_msgSender()];
        require(currentAllowance >= amount, "ERC20: transfer amount exceeds allowance");
        _approve(sender, _msgSender(), currentAllowance - amount);

        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] + 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) {
        uint256 currentAllowance = _allowances[_msgSender()][spender];
        require(currentAllowance >= subtractedValue, "ERC20: decreased allowance below zero");
        _approve(_msgSender(), spender, currentAllowance - subtractedValue);

        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);

        uint256 senderBalance = _balances[sender];
        require(senderBalance >= amount, "ERC20: transfer amount exceeds balance");
        _balances[sender] = senderBalance - amount;
        _balances[recipient] += 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 += amount;
        _balances[account] += 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);

        uint256 accountBalance = _balances[account];
        require(accountBalance >= amount, "ERC20: burn amount exceeds balance");
        _balances[account] = accountBalance - amount;
        _totalSupply -= 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 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 { }
}

// File: @openzeppelin/contracts/access/Ownable.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;

import "../utils/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.
 */
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 () {
        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;
    }
}

// File: @openzeppelin/contracts/token/ERC20/utils/SafeERC20.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;

import "../IERC20.sol";
import "../../../utils/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 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) + value;
        _callOptionalReturn(token, abi.encodeWithSelector(token.approve.selector, spender, newAllowance));
    }

    function safeDecreaseAllowance(IERC20 token, address spender, uint256 value) internal {
        unchecked {
            uint256 oldAllowance = token.allowance(address(this), spender);
            require(oldAllowance >= value, "SafeERC20: decreased allowance below zero");
            uint256 newAllowance = oldAllowance - value;
            _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");
        }
    }
}

// File: @openzeppelin/contracts/token/ERC20/IERC20.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;

/**
 * @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: @openzeppelin/contracts/security/ReentrancyGuard.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;

/**
 * @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].
 */
abstract 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 () {
        _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;
    }
}

// File: @openzeppelin/contracts/utils/Context.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;

/*
 * @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 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) {
        return msg.sender;
    }

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

// File: @openzeppelin/contracts/utils/math/Math.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;

/**
 * @dev Standard math utilities missing in the Solidity language.
 */
library Math {
    /**
     * @dev Returns the largest of two numbers.
     */
    function max(uint256 a, uint256 b) internal pure returns (uint256) {
        return a >= b ? a : b;
    }

    /**
     * @dev Returns the smallest of two numbers.
     */
    function min(uint256 a, uint256 b) internal pure returns (uint256) {
        return a < b ? a : b;
    }

    /**
     * @dev Returns the average of two numbers. The result is rounded towards
     * zero.
     */
    function average(uint256 a, uint256 b) internal pure returns (uint256) {
        // (a + b) / 2 can overflow, so we distribute
        return (a / 2) + (b / 2) + ((a % 2 + b % 2) / 2);
    }
}

// File: @openzeppelin/contracts/token/ERC20/extensions/IERC20Metadata.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;

import "../IERC20.sol";

/**
 * @dev Interface for the optional metadata functions from the ERC20 standard.
 *
 * _Available since v4.1._
 */
interface IERC20Metadata is IERC20 {
    /**
     * @dev Returns the name of the token.
     */
    function name() external view returns (string memory);

    /**
     * @dev Returns the symbol of the token.
     */
    function symbol() external view returns (string memory);

    /**
     * @dev Returns the decimals places of the token.
     */
    function decimals() external view returns (uint8);
}

// File: @openzeppelin/contracts/utils/Address.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;

/**
 * @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");
        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);
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-}[`functionCall`],
     * but performing a static call.
     *
     * _Available since v3.3._
     */
    function functionStaticCall(address target, bytes memory data) internal view returns (bytes memory) {
        return functionStaticCall(target, data, "Address: low-level static call failed");
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-string-}[`functionCall`],
     * but performing a static call.
     *
     * _Available since v3.3._
     */
    function functionStaticCall(address target, bytes memory data, string memory errorMessage) internal view returns (bytes memory) {
        require(isContract(target), "Address: static call to non-contract");

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

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-}[`functionCall`],
     * but performing a delegate call.
     *
     * _Available since v3.4._
     */
    function functionDelegateCall(address target, bytes memory data) internal returns (bytes memory) {
        return functionDelegateCall(target, data, "Address: low-level delegate call failed");
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-string-}[`functionCall`],
     * but performing a delegate call.
     *
     * _Available since v3.4._
     */
    function functionDelegateCall(address target, bytes memory data, string memory errorMessage) internal returns (bytes memory) {
        require(isContract(target), "Address: delegate call to non-contract");

        // solhint-disable-next-line avoid-low-level-calls
        (bool success, bytes memory returndata) = target.delegatecall(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);
            }
        }
    }
}