// File: contracts/Issuer.sol



//SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

import "./CoreRef.sol";
import "./interfaces/IKeysManager.sol";
import "./interfaces/IDepositContract.sol";
import "./interfaces/IIssuer.sol";
import {ReentrancyGuard} from "@openzeppelin/contracts/security/ReentrancyGuard.sol";

/// @author PStake
/// @title Issuer
/// @notice contract for issuance of StkEth
contract Issuer is CoreRef, IIssuer, ReentrancyGuard {
    uint256 public constant VALIDATOR_DEPOSIT = 31e18;
    uint256 public constant VERIFICATION_DEPOSIT = 1e18;

    uint256 public constant BASIS_POINT = 10000;

    IDepositContract public immutable DEPOSIT_CONTRACT;
    uint256 public override pendingValidators;
    uint256 public pendingValidatorsLimit;
    uint256 public ethStaked = 0;


    /// @notice constructor for initializing core
    /// @param core address of the core 
    /// @param _pendingValidatorsLimit ...
    /// @param _depositContract Deposit Contract address for Eth2
    constructor(
        address core,
        uint256 _pendingValidatorsLimit,
        address _depositContract
    ) CoreRef(core) {
        require(_depositContract != address(0), "Issuer: Zero address");
        DEPOSIT_CONTRACT = IDepositContract(_depositContract);
        require(_pendingValidatorsLimit < BASIS_POINT, "Issuer: invalid limit");
        pendingValidatorsLimit = _pendingValidatorsLimit;
    }


    /// @notice function returns pending validator limit.
    /// @return pendingValidatorsLimit number of pending validators.
    function pendingValidatorLimit()
    public
    view
    returns (uint256)
    {
        return pendingValidatorsLimit;
    }

    function ethStakedIssuer()
    public override
    view
    returns (uint256)
    {
        return ethStaked;
    }


    /// @notice function for setting the count of pending validators limit.
    /// @param _pendingValidatorsLimit integer limit for number of pending validators.
    function setPendingValidatorsLimit(uint256 _pendingValidatorsLimit)
    external
    onlyGovernor
    {
        require(_pendingValidatorsLimit < BASIS_POINT, "Issuer: invalid limit");
        pendingValidatorsLimit = _pendingValidatorsLimit;
        emit SetPendingValidatorsLimit(_pendingValidatorsLimit);
    }


    /// @notice function for updating the count of pending validators with new activated validators.
    /// @param newActiveValidators the number of new activated validators.
    function updatePendingValidator(uint256 newActiveValidators)
    external
    override
    {
        require(
            core().oracle() == msg.sender,
            "Issuer: Only oracle can update"
        );

        pendingValidators = pendingValidators - newActiveValidators;
        emit UpdatePendingValidators(pendingValidators);
    }


    /// @notice function to mint Stk Eth for Eth.
    /// @param amount amount of ether.
    /// @param user address of user.
    function mintStkEthForEth(uint256 amount, address user) internal {
        uint256 stkEthToMint = (amount * 1e18) / stkEth().pricePerShare();

        stkEth().mint(user, stkEthToMint);
    }

    /// @notice function for issuer to stake
    function stake() public payable whenNotPaused {
        require(msg.value > 0, "Issuer: can't stake zero");
        emit Stake(msg.sender, msg.value, block.timestamp);
        ethStaked = ethStaked + msg.value;
        mintStkEthForEth(msg.value, msg.sender);
    }

    /// @notice function for deposit of 32 Ether.
    /// @param publicKey public key of the validator.
    function depositToEth2(bytes calldata publicKey) external {
        require(address(this).balance >= VALIDATOR_DEPOSIT + VERIFICATION_DEPOSIT, "Issuer: Not enough ether deposited");
        IKeysManager.Validator memory validator = IKeysManager(
            core().keysManager()
        ).validators(publicKey);

        withdrawalverificationDeposit(validator.nodeOperator);

        IKeysManager(core().keysManager()).depositValidator(publicKey);

        pendingValidators = pendingValidators + 1;
        DEPOSIT_CONTRACT.deposit{value : VALIDATOR_DEPOSIT}(
            publicKey, //
            abi.encodePacked(core().withdrawalCredential()),
            validator.signature,
            validator.deposit_root
        );
    }


    /// @notice function for sending 1 Ether to a node operator address.
    /// @param nodeOperator address of the node operator
    function withdrawalverificationDeposit(address nodeOperator) internal nonReentrant {

        (bool sent,) = nodeOperator.call{value : VERIFICATION_DEPOSIT}("");
        require(sent, "Issuer: Failed to send to Node Operator");
    }

}




// File: contracts/CoreRef.sol



//SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

import "./interfaces/ICoreRef.sol";
import "./interfaces/ICore.sol";
import "./interfaces/IOracle.sol";
import "@openzeppelin/contracts/security/Pausable.sol";
import "@openzeppelin/contracts/utils/Address.sol";

abstract contract CoreRef is ICoreRef, Pausable {

    ICore private _core;

    constructor(address core){
        require(core != address(0), "CoreRef: Zero address");
        _core = ICore(core);
        emit SetCore(core);
    }

    modifier ifMinterSelf() {
        if (_core.isMinter(address(this))) {
            _;
        }
    }

    modifier onlyMinter() {
        require(_core.isMinter(msg.sender), "CoreRef: Caller is not a minter");
        _;
    }

    modifier onlyBurner() {
        require(_core.isBurner(msg.sender), "CoreRef: Caller is not a burner");
        _;
    }

    modifier onlyKeyAdmin() {
        require(_core.isKeyAdmin(msg.sender), "Permissions: Caller is not a key admin");
        _;
    }

    modifier onlyNodeOperator() {
        require(_core.isNodeOperator(msg.sender), "Permissions: Caller is not a Node Operator");
        _;
    }

    modifier onlyGovernor() {
        require(
            _core.isGovernor(msg.sender),
            "CoreRef: Caller is not a governor"
        );
        _;
    }

    /// @notice set pausable methods to paused
    function pause() public override onlyGovernor {
        _pause();
    }

    /// @notice set pausable methods to unpaused
    function unpause() public override onlyGovernor {
        _unpause();
    }

    /// @notice set new Core reference address
    /// @param core the new core address
    function setCore(address core) external override onlyGovernor {
        _core = ICore(core);
        emit SetCore(core);
    }

    function stkEth() public view override returns (IStkEth) {
        return _core.stkEth();
    }

    function core() public view override returns (ICore) {
        return _core;
    }

    function oracle() public view override returns (IOracle) {
        return IOracle(_core.oracle());
    }
}
// File: contracts/interfaces/IKeysManager.sol



//SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

/// @title KeysManager interface
/// @author Ankit Parashar
interface IKeysManager {

    enum State { INVALID, VALID, ACTIVATED, DEPOSITED }

    struct Validator {
        State state;
        bytes signature;
        address nodeOperator;
        bytes32 deposit_root;
    }

    function validators(bytes calldata publicKey) external view returns (Validator memory);

    function addValidator(bytes calldata publicKey, bytes calldata signature,  address nodeOperator) external;

    function activateValidator(bytes[] memory publicKey) external;

    function depositValidator(bytes memory publicKey) external;

    function nodeOperatorValidatorCount(address usr) external returns (uint256);
}
// File: contracts/interfaces/IDepositContract.sol



//SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

interface IDepositContract {
    /// @notice A processed deposit event.
    event DepositEvent(
        bytes pubkey,
        bytes withdrawal_credentials,
        bytes amount,
        bytes signature,
        bytes index
    );

    /// @notice Submit a Phase 0 DepositData object.
    /// @param pubkey A BLS12-381 public key.
    /// @param withdrawal_credentials Commitment to a public key for withdrawals.
    /// @param signature A BLS12-381 signature.
    /// @param deposit_data_root The SHA-256 hash of the SSZ-encoded DepositData object.
    /// Used as a protection against malformed input.
    function deposit(
        bytes calldata pubkey,
        bytes calldata withdrawal_credentials,
        bytes calldata signature,
        bytes32 deposit_data_root
    ) external payable;

    /// @notice Query the current deposit root hash.
    /// @return The deposit root hash.
    function get_deposit_root() external view returns (bytes32);

    /// @notice Query the current deposit count.
    /// @return The deposit count encoded as a little endian 64-bit number.
    function get_deposit_count() external view returns (bytes memory);
}

// File: contracts/interfaces/IIssuer.sol



//SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

/// @title Issuer interface
/// @author Ankit Parashar
interface IIssuer {

    event SetMinActivationDeposit(uint256 _minActivatingDeposit);
    event SetPendingValidatorsLimit(uint256 _pendingValidatorsLimit);
    event UpdatePendingValidators(uint256 _pendingValidators);
    event Stake(address indexed_user,uint256 amount,uint256 block_time);
    function updatePendingValidator(uint256 newActiveValidators) external;

    function pendingValidators() external view returns (uint256);
    function ethStakedIssuer() external view returns (uint256);

}
// 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: contracts/interfaces/ICoreRef.sol



//SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

import "./ICore.sol";
import "./IStkEth.sol";
import "./IOracle.sol";

/// @title CoreRef interface
/// @author Ankit Parashar
interface ICoreRef {

    event SetCore(address _core);

    function setCore(address core) external;

    function pause() external;

    function unpause() external;

    // ----------- Getters -----------

    function core() external view returns (ICore);

    function stkEth() external view returns (IStkEth);

    function oracle() external view returns (IOracle);
}
// File: contracts/interfaces/ICore.sol



//SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

import "./IPermissions.sol";
import "./IStkEth.sol";

/// @title Core interface
/// @author Ankit Parashar
interface ICore is IPermissions {

    event SetCoreContract(bytes32 _key, address indexed _address);

    event SetWithdrawalCredential(bytes32 _withdrawalCreds);

    function stkEth() external view returns(IStkEth);

    function oracle() external view returns(address);

    function withdrawalCredential() external view returns(bytes32);

    function keysManager() external view returns(address);

    function pstakeTreasury() external view returns(address);

    function validatorPool() external view returns(address);

    function issuer() external view returns(address);

    function set(bytes32 _key, address _address) external;

    function coreContract(bytes32 key) external view returns (address);

}
// File: contracts/interfaces/IOracle.sol



//SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

/// @title Oracle interface
/// @author Ankit Parashar
interface IOracle {

    event Distribute(uint256 amount,uint256 pricePerShare,uint256 timestamp);
    event Slash(uint256 amount,uint256 pricePerShare,uint256 timestamp);

    function pricePerShare() external view returns (uint256);

    function activatedValidators() external view returns (uint256);

    function addOracleMember(address newOracleMember) external;

    function removeOracleMember(address oracleMeberToDelete) external;

    function pushData(
        uint256 latestEthBalance,
        uint256 latestNonce,
        uint32 numberOfValidators
    ) external;

    function activateValidator(bytes[] calldata _publicKeys) external;
}

// File: @openzeppelin/contracts/security/Pausable.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;

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

/**
 * @dev Contract module which allows children to implement an emergency stop
 * mechanism that can be triggered by an authorized account.
 *
 * This module is used through inheritance. It will make available the
 * modifiers `whenNotPaused` and `whenPaused`, which can be applied to
 * the functions of your contract. Note that they will not be pausable by
 * simply including this module, only once the modifiers are put in place.
 */
abstract contract Pausable is Context {
    /**
     * @dev Emitted when the pause is triggered by `account`.
     */
    event Paused(address account);

    /**
     * @dev Emitted when the pause is lifted by `account`.
     */
    event Unpaused(address account);

    bool private _paused;

    /**
     * @dev Initializes the contract in unpaused state.
     */
    constructor() {
        _paused = false;
    }

    /**
     * @dev Returns true if the contract is paused, and false otherwise.
     */
    function paused() public view virtual returns (bool) {
        return _paused;
    }

    /**
     * @dev Modifier to make a function callable only when the contract is not paused.
     *
     * Requirements:
     *
     * - The contract must not be paused.
     */
    modifier whenNotPaused() {
        require(!paused(), "Pausable: paused");
        _;
    }

    /**
     * @dev Modifier to make a function callable only when the contract is paused.
     *
     * Requirements:
     *
     * - The contract must be paused.
     */
    modifier whenPaused() {
        require(paused(), "Pausable: not paused");
        _;
    }

    /**
     * @dev Triggers stopped state.
     *
     * Requirements:
     *
     * - The contract must not be paused.
     */
    function _pause() internal virtual whenNotPaused {
        _paused = true;
        emit Paused(_msgSender());
    }

    /**
     * @dev Returns to normal state.
     *
     * Requirements:
     *
     * - The contract must be paused.
     */
    function _unpause() internal virtual whenPaused {
        _paused = false;
        emit Unpaused(_msgSender());
    }
}

// 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;
        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");

        (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");

        (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");

        (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");

        (bool success, bytes memory returndata) = target.delegatecall(data);
        return verifyCallResult(success, returndata, errorMessage);
    }

    /**
     * @dev Tool to verifies that a low level call was successful, and revert if it wasn't, either by bubbling the
     * revert reason using the provided one.
     *
     * _Available since v4.3._
     */
    function verifyCallResult(
        bool success,
        bytes memory returndata,
        string memory errorMessage
    ) internal 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

                assembly {
                    let returndata_size := mload(returndata)
                    revert(add(32, returndata), returndata_size)
                }
            } else {
                revert(errorMessage);
            }
        }
    }
}

// File: contracts/interfaces/IStkEth.sol



//SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

import "@openzeppelin/contracts/token/ERC20/IERC20.sol";
import "./ICoreRef.sol";

/// @title Oracle interface
/// @author Ankit Parashar
interface IStkEth is IERC20{

    function pricePerShare() external view returns (uint256 amount);

    function mint(address user, uint256 amount) external;

    function burn(address user, uint256 amount) external;
}

// File: contracts/interfaces/IPermissions.sol



//SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

/// @title Permissions interface
/// @author Ankit Parashar
interface IPermissions {

    // ----------- Governor only state changing functions -----------

    function createRole(bytes32 role, bytes32 adminRole) external;

    function grantGovernor(address governor) external;

    function grantMinter(address minter) external;

    function grantBurner(address burner) external;

    function grantNodeOperator(address nodeOperator) external;

    function grantKeyAdmin(address keyAdmin) external;

    function revokeGovernor(address governor) external;

    function revokeMinter(address minter) external;

    function revokeBurner(address burner) external;

    function revokeNodeOperator(address nodeOperator) external;

    function revokeKeyAdmin(address keyAdmin) external;

    // ----------- Getters -----------

    function isMinter(address _address) external view returns (bool);

    function isGovernor(address _address) external view returns (bool);

    function isBurner(address _address) external view returns (bool);

    function isNodeOperator(address _address) external view returns (bool);

    function isKeyAdmin(address _address) external view returns (bool);
}
// 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/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) {
        return msg.data;
    }
}