// File: /contracts/AstaSwapBridge.sol



// SPDX-License-Identifier: Apache-2.0
pragma solidity 0.8.11;

import "./interfaces/IERC20Query.sol";
import "@openzeppelin/contracts/utils/Context.sol";
import "@openzeppelin/contracts/utils/math/SafeMath.sol";
import "@openzeppelin/contracts/token/ERC20/extensions/ERC20Burnable.sol";

contract AstaSwapBridge is Context {
    using SafeMath for uint256;
    ERC20Burnable _token;

    mapping(string => bool) private registeredBEP20;
    mapping(bytes32 => bool) public filledOtherChainsTx;

    address payable public owner;
    address public superAdmin;
    address payable public feeReceiver;
    uint256 public swapFee;
    uint256 public burntPercentage;
    uint256 public amountDivisonPercentage;
    uint256 public feePercentageInAsta;
    uint256 private hunderedPercent = 100000000000000000000;

    event OwnershipTransferred(
        address indexed previousOwner,
        address indexed newOwner
    );
    event SuperAdminChanged(
        address indexed previousSuperAdmin,
        address indexed newSuperAdmin
    );
    event FeeReceiverUpdated(
        address indexed prevFeeReceiver,
        address indexed newFeeReceiver
    );
    event SwapPairRegisterFor(
        address indexed sponsor,
        address indexed eth20Addr,
        string name,
        string symbol,
        uint8 decimals,
        string pair
    );
    event SwapStarted(
        address indexed eth20Addr,
        address indexed fromAddr,
        uint256 amount,
        uint256 feeAmount,
        uint256 swapAmount,
        uint256 feeInAsta,
        uint256 amountToBurn,
        string chain
    );
    event SwapFilled(
        address indexed eth20Addr,
        bytes32 indexed inputTxHash,
        address indexed toAddress,
        uint256 swapAmount,
        uint256 feeInAsta,
        uint256 amountToBurn
    );

    constructor(
        uint256 fee_Native,
        uint256 amount_Division,
        uint256 fee_PerAsta,
        address payable fee_Receiver,
        address super_Admin
    ) {
        swapFee = fee_Native;
        require(
            amount_Division <= hunderedPercent,
            "amountDivisonPercentage: SHould be less then 100"
        );
        amountDivisonPercentage = amount_Division;
        require(
            fee_PerAsta <= hunderedPercent,
            "feePercentageInAsta: SHould be less then 100"
        );
        feePercentageInAsta = fee_PerAsta;
        owner = payable(msg.sender);
        feeReceiver = fee_Receiver;
        superAdmin = super_Admin;
        burntPercentage = hunderedPercent.sub(fee_PerAsta);
    }

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

    /**
     * Throws if called transferOwnership by any account other than the super admin.
     */
    modifier onlySuperAdmin() {
        require(
            superAdmin == _msgSender(),
            "Super Admin: caller is not the super admin"
        );
        _;
    }

    modifier notContract() {
        require(!isContract(msg.sender), "contract is not allowed to swap");
        _;
    }

    modifier noProxy() {
        require(msg.sender == tx.origin, "no proxy is allowed");
        _;
    }

    function isContract(address addr) internal view returns (bool) {
        uint256 size;
        assembly {
            size := extcodesize(addr)
        }
        return size > 0;
    }

    /**
     * Leaves the contract without owner. It will not be possible to call
     * `onlySuperAdmin` 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 onlySuperAdmin {
        emit OwnershipTransferred(owner, address(0));
        owner = payable(0);
    }

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

    /**
     * Change Super Admin of the contract to a new account (`newSuperAdmin`).
     * Can only be called by the current super admin.
     */
    function changeSuperAdmin(address newSuperAdmin) public onlySuperAdmin {
        require(
            newSuperAdmin != address(0),
            "Super Admin: new super admin is the zero address"
        );
        emit SuperAdminChanged(superAdmin, newSuperAdmin);
        superAdmin = newSuperAdmin;
    }

    /**
     * Transfers fee receiver to a new account (`newFeeReceiver`).
     * Can only be called by the current owner.
     */
    function changeFeeReceiver(address payable newFeeReceiver)
        public
        onlySuperAdmin
    {
        require(
            newFeeReceiver != address(0),
            "Fee Receiver: new fee receiver address is zero "
        );
        emit FeeReceiverUpdated(feeReceiver, newFeeReceiver);
        feeReceiver = newFeeReceiver;
    }

    /**
     * Returns set minimum swap fee from BEP20 to other chains
     */
    function setSwapFee(uint256 fee) external onlyOwner {
        swapFee = fee;
    }

    /**
     * Set the percentage to divide for swap and (fee, burn)
     */
    function setSwapAmountToDivASTA(uint256 _amountDiv) external onlyOwner {
        require(
            _amountDiv <= hunderedPercent,
            "amountDivisonPercentage: Greater than 100 %"
        );
        amountDivisonPercentage = _amountDiv;
    }

    /**
     * Returns set minimum swap fee in ASTA from BEP20 to other chains
     */
    function setSwapFeePercentageOfASTA(uint256 _feePerAsta)
        external
        onlyOwner
    {
        require(
            _feePerAsta <= hunderedPercent,
            "amountDivisonPercentage: Greater than 100 %"
        );
        feePercentageInAsta = _feePerAsta;
        burntPercentage = hunderedPercent.sub(_feePerAsta);
    }

    /**
     * Register swap pair for chain
     */
    function registerSwapPair(address eth20Addr, string calldata chain)
        external
        onlyOwner
        returns (bool)
    {
        require(
            !registeredBEP20[string(abi.encode(eth20Addr, chain))],
            "already registered"
        );

        string memory name = IERC20Query(eth20Addr).name();
        string memory symbol = IERC20Query(eth20Addr).symbol();
        uint8 decimals = IERC20Query(eth20Addr).decimals();

        require(bytes(name).length > 0, "empty name");
        require(bytes(symbol).length > 0, "empty symbol");

        registeredBEP20[string(abi.encode(eth20Addr, chain))] = true;

        emit SwapPairRegisterFor(
            msg.sender,
            eth20Addr,
            name,
            symbol,
            decimals,
            chain
        );
        return true;
    }

    // function used to calculate the fee and burn amount
    function deductFeeAndBurntAmount(uint256 _amount)
        internal
        view
        returns (
            uint256,
            uint256,
            uint256
        )
    {
        require(_amount > 0, "Amount: Amount must be greater then 0");
        uint256 amountToSwap;
        uint256 feeInAsta;
        uint256 amountToBurn;

        amountToSwap = _amount.mul(amountDivisonPercentage);
        amountToSwap = amountToSwap.div(hunderedPercent);

        uint256 remainingAmount = _amount.sub(amountToSwap);

        feeInAsta = remainingAmount.mul(feePercentageInAsta);
        feeInAsta = feeInAsta.div(hunderedPercent);

        amountToBurn = remainingAmount.sub(feeInAsta);
        return (amountToSwap, feeInAsta, amountToBurn);
    }

    /**
     * Fill swap by BEP20
     */
    function fillSwap(
        bytes32 crossChainTxHash,
        address eth20Addr,
        address toAddress,
        uint256 amount,
        string calldata chain
    ) external onlyOwner returns (bool) {
        require(!filledOtherChainsTx[crossChainTxHash], "tx filled already");
        require(
            registeredBEP20[string(abi.encode(eth20Addr, chain))],
            "not registered token"
        );
        require(amount > 0, "Amount should be greater than 0");
        require(
            amountDivisonPercentage <= hunderedPercent,
            "amountDivisonPercentage: Greater than 100 %"
        );

        uint256 swapAmount = amount;
        uint256 feeInAsta = 0;
        uint256 amountToBurn = 0;

        _token = ERC20Burnable(eth20Addr);
        if (amountDivisonPercentage == hunderedPercent) {
            _token.transfer(toAddress, swapAmount);
        } else {
            (swapAmount, feeInAsta, amountToBurn) = deductFeeAndBurntAmount(
                amount
            );
            _token.transfer(toAddress, swapAmount);
            _token.transfer(feeReceiver, feeInAsta);
            _token.burn(amountToBurn);
        }
        filledOtherChainsTx[crossChainTxHash] = true;

        emit SwapFilled(
            eth20Addr,
            crossChainTxHash,
            toAddress,
            swapAmount,
            feeInAsta,
            amountToBurn
        );
        return true;
    }

    /**
     * Swap BEP20 on other chain
     */
    function swapToken(
        address eth20Addr,
        uint256 amount,
        string calldata chain
    ) external payable notContract noProxy returns (bool) {
        require(
            registeredBEP20[string(abi.encode(eth20Addr, chain))],
            "not registered token"
        );
        require(msg.value >= swapFee, "swap fee is not enough");
        require(amount > 0, "Amount should be greater than 0");
        require(
            amountDivisonPercentage <= hunderedPercent,
            "amountDivisonPercentage: Greater than 100 %"
        );

        if (msg.value != 0) {
            owner.transfer(msg.value);
        }

        _token = ERC20Burnable(eth20Addr);

        uint256 swapAmount = amount;
        uint256 feeInAsta = 0;
        uint256 amountToBurn = 0;
        if (amountDivisonPercentage == hunderedPercent) {
            _token.transferFrom(msg.sender, address(this), swapAmount);
        } else {
            (swapAmount, feeInAsta, amountToBurn) = deductFeeAndBurntAmount(
                amount
            );
            _token.transferFrom(msg.sender, address(this), swapAmount);
            _token.transferFrom(msg.sender, feeReceiver, feeInAsta);
            _token.burnFrom(msg.sender, amountToBurn);
        }

        emit SwapStarted(
            eth20Addr,
            msg.sender,
            amount,
            msg.value,
            swapAmount,
            feeInAsta,
            amountToBurn,
            chain
        );
        return true;
    }

    /**
     * Calculate the values for swap, mint and burn
     */
    function calculateDivisonAmount(uint256 amount)
        external
        view
        returns (
            uint256,
            uint256,
            uint256
        )
    {
        require(amount > 0, "Amount should be greater than 0");
        require(
            amountDivisonPercentage <= hunderedPercent,
            "amountDivisonPercentage: Greater than 100 %"
        );

        (
            uint256 swapAmount,
            uint256 feeInAsta,
            uint256 amountToBurn
        ) = deductFeeAndBurntAmount(amount);

        return (swapAmount, feeInAsta, amountToBurn);
    }

    /**
     * Check the token pair
     */
    function getRegisteredPairs(address eth20Addr, string calldata chain)
        external
        view
        returns (bool)
    {
        return registeredBEP20[string(abi.encode(eth20Addr, chain))];
    }
}
// File: /contracts/interfaces/IERC20Query.sol



// SPDX-License-Identifier: Apache-2.0
pragma solidity 0.8.11;

interface IERC20Query {
    /**
     * Returns the amount of tokens in existence.
     */
    function totalSupply() external view returns (uint256);

    /**
     * Returns the token decimals.
     */
    function decimals() external view returns (uint8);

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

    /**
     * Returns the token name.
     */
    function name() external view returns (string memory);

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

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



// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts v4.4.1 (utils/math/SafeMath.sol)

pragma solidity ^0.8.0;

// CAUTION
// This version of SafeMath should only be used with Solidity 0.8 or later,
// because it relies on the compiler's built in overflow checks.

/**
 * @dev Wrappers over Solidity's arithmetic operations.
 *
 * NOTE: `SafeMath` is generally not needed starting with Solidity 0.8, since the compiler
 * now has built in overflow checking.
 */
library SafeMath {
    /**
     * @dev Returns the addition of two unsigned integers, with an overflow flag.
     *
     * _Available since v3.4._
     */
    function tryAdd(uint256 a, uint256 b) internal pure returns (bool, uint256) {
        unchecked {
            uint256 c = a + b;
            if (c < a) return (false, 0);
            return (true, c);
        }
    }

    /**
     * @dev Returns the substraction of two unsigned integers, with an overflow flag.
     *
     * _Available since v3.4._
     */
    function trySub(uint256 a, uint256 b) internal pure returns (bool, uint256) {
        unchecked {
            if (b > a) return (false, 0);
            return (true, a - b);
        }
    }

    /**
     * @dev Returns the multiplication of two unsigned integers, with an overflow flag.
     *
     * _Available since v3.4._
     */
    function tryMul(uint256 a, uint256 b) internal pure returns (bool, uint256) {
        unchecked {
            // 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 (true, 0);
            uint256 c = a * b;
            if (c / a != b) return (false, 0);
            return (true, c);
        }
    }

    /**
     * @dev Returns the division of two unsigned integers, with a division by zero flag.
     *
     * _Available since v3.4._
     */
    function tryDiv(uint256 a, uint256 b) internal pure returns (bool, uint256) {
        unchecked {
            if (b == 0) return (false, 0);
            return (true, a / b);
        }
    }

    /**
     * @dev Returns the remainder of dividing two unsigned integers, with a division by zero flag.
     *
     * _Available since v3.4._
     */
    function tryMod(uint256 a, uint256 b) internal pure returns (bool, uint256) {
        unchecked {
            if (b == 0) return (false, 0);
            return (true, a % b);
        }
    }

    /**
     * @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) {
        return a + b;
    }

    /**
     * @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 a - b;
    }

    /**
     * @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) {
        return a * b;
    }

    /**
     * @dev Returns the integer division of two unsigned integers, reverting on
     * division by zero. The result is rounded towards zero.
     *
     * Counterpart to Solidity's `/` operator.
     *
     * Requirements:
     *
     * - The divisor cannot be zero.
     */
    function div(uint256 a, uint256 b) internal pure returns (uint256) {
        return a / b;
    }

    /**
     * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
     * reverting 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 a % b;
    }

    /**
     * @dev Returns the subtraction of two unsigned integers, reverting with custom message on
     * overflow (when the result is negative).
     *
     * CAUTION: This function is deprecated because it requires allocating memory for the error
     * message unnecessarily. For custom revert reasons use {trySub}.
     *
     * Counterpart to Solidity's `-` operator.
     *
     * Requirements:
     *
     * - Subtraction cannot overflow.
     */
    function sub(
        uint256 a,
        uint256 b,
        string memory errorMessage
    ) internal pure returns (uint256) {
        unchecked {
            require(b <= a, errorMessage);
            return a - b;
        }
    }

    /**
     * @dev Returns the integer division of two unsigned integers, reverting 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) {
        unchecked {
            require(b > 0, errorMessage);
            return a / b;
        }
    }

    /**
     * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
     * reverting with custom message when dividing by zero.
     *
     * CAUTION: This function is deprecated because it requires allocating memory for the error
     * message unnecessarily. For custom revert reasons use {tryMod}.
     *
     * 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) {
        unchecked {
            require(b > 0, errorMessage);
            return a % b;
        }
    }
}

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



// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts v4.4.1 (utils/Context.sol)

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) {
        return msg.data;
    }
}

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



// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts v4.4.1 (token/ERC20/extensions/IERC20Metadata.sol)

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/token/ERC20/extensions/ERC20Burnable.sol



// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts v4.4.1 (token/ERC20/extensions/ERC20Burnable.sol)

pragma solidity ^0.8.0;

import "../ERC20.sol";
import "../../../utils/Context.sol";

/**
 * @dev Extension of {ERC20} that allows token holders to destroy both their own
 * tokens and those that they have an allowance for, in a way that can be
 * recognized off-chain (via event analysis).
 */
abstract contract ERC20Burnable is Context, ERC20 {
    /**
     * @dev Destroys `amount` tokens from the caller.
     *
     * See {ERC20-_burn}.
     */
    function burn(uint256 amount) public virtual {
        _burn(_msgSender(), amount);
    }

    /**
     * @dev Destroys `amount` tokens from `account`, deducting from the caller's
     * allowance.
     *
     * See {ERC20-_burn} and {ERC20-allowance}.
     *
     * Requirements:
     *
     * - the caller must have allowance for ``accounts``'s tokens of at least
     * `amount`.
     */
    function burnFrom(address account, uint256 amount) public virtual {
        uint256 currentAllowance = allowance(account, _msgSender());
        require(currentAllowance >= amount, "ERC20: burn amount exceeds allowance");
        unchecked {
            _approve(account, _msgSender(), currentAllowance - amount);
        }
        _burn(account, amount);
    }
}

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



// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts v4.4.1 (token/ERC20/IERC20.sol)

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/token/ERC20/ERC20.sol



// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts v4.4.1 (token/ERC20/ERC20.sol)

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 Contracts guidelines: functions revert
 * instead 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 default 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");
        unchecked {
            _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");
        unchecked {
            _approve(_msgSender(), spender, currentAllowance - subtractedValue);
        }

        return true;
    }

    /**
     * @dev Moves `amount` of tokens from `sender` to `recipient`.
     *
     * This 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");
        unchecked {
            _balances[sender] = senderBalance - amount;
        }
        _balances[recipient] += amount;

        emit Transfer(sender, recipient, amount);

        _afterTokenTransfer(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:
     *
     * - `account` 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);

        _afterTokenTransfer(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");
        unchecked {
            _balances[account] = accountBalance - amount;
        }
        _totalSupply -= amount;

        emit Transfer(account, address(0), amount);

        _afterTokenTransfer(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 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 {}

    /**
     * @dev Hook that is called after any transfer of tokens. This includes
     * minting and burning.
     *
     * Calling conditions:
     *
     * - when `from` and `to` are both non-zero, `amount` of ``from``'s tokens
     * has been transferred to `to`.
     * - when `from` is zero, `amount` tokens have been minted for `to`.
     * - when `to` is zero, `amount` of ``from``'s tokens have been 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 _afterTokenTransfer(
        address from,
        address to,
        uint256 amount
    ) internal virtual {}
}