Contract Source Code:

// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts (last updated v4.9.0) (access/Ownable.sol)

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() {
        _transferOwnership(_msgSender());
    }

    /**
     * @dev Throws if called by any account other than the owner.
     */
    modifier onlyOwner() {
        _checkOwner();
        _;
    }

    /**
     * @dev Returns the address of the current owner.
     */
    function owner() public view virtual returns (address) {
        return _owner;
    }

    /**
     * @dev Throws if the sender is not the owner.
     */
    function _checkOwner() internal view virtual {
        require(owner() == _msgSender(), "Ownable: caller is not the owner");
    }

    /**
     * @dev Leaves the contract without owner. It will not be possible to call
     * `onlyOwner` functions. Can only be called by the current owner.
     *
     * NOTE: Renouncing ownership will leave the contract without an owner,
     * thereby disabling any functionality that is only available to the owner.
     */
    function renounceOwnership() public virtual onlyOwner {
        _transferOwnership(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");
        _transferOwnership(newOwner);
    }

    /**
     * @dev Transfers ownership of the contract to a new account (`newOwner`).
     * Internal function without access restriction.
     */
    function _transferOwnership(address newOwner) internal virtual {
        address oldOwner = _owner;
        _owner = newOwner;
        emit OwnershipTransferred(oldOwner, newOwner);
    }
}

// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts (last updated v4.7.0) (security/Pausable.sol)

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 Modifier to make a function callable only when the contract is not paused.
     *
     * Requirements:
     *
     * - The contract must not be paused.
     */
    modifier whenNotPaused() {
        _requireNotPaused();
        _;
    }

    /**
     * @dev Modifier to make a function callable only when the contract is paused.
     *
     * Requirements:
     *
     * - The contract must be paused.
     */
    modifier whenPaused() {
        _requirePaused();
        _;
    }

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

    /**
     * @dev Throws if the contract is paused.
     */
    function _requireNotPaused() internal view virtual {
        require(!paused(), "Pausable: paused");
    }

    /**
     * @dev Throws if the contract is not paused.
     */
    function _requirePaused() internal view virtual {
        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());
    }
}

// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts (last updated v4.9.0) (security/ReentrancyGuard.sol)

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 making it call a
     * `private` function that does the actual work.
     */
    modifier nonReentrant() {
        _nonReentrantBefore();
        _;
        _nonReentrantAfter();
    }

    function _nonReentrantBefore() private {
        // On the first call to nonReentrant, _status will be _NOT_ENTERED
        require(_status != _ENTERED, "ReentrancyGuard: reentrant call");

        // Any calls to nonReentrant after this point will fail
        _status = _ENTERED;
    }

    function _nonReentrantAfter() private {
        // By storing the original value once again, a refund is triggered (see
        // https://eips.ethereum.org/EIPS/eip-2200)
        _status = _NOT_ENTERED;
    }

    /**
     * @dev Returns true if the reentrancy guard is currently set to "entered", which indicates there is a
     * `nonReentrant` function in the call stack.
     */
    function _reentrancyGuardEntered() internal view returns (bool) {
        return _status == _ENTERED;
    }
}

// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts (last updated v4.9.0) (token/ERC20/IERC20.sol)

pragma solidity ^0.8.0;

/**
 * @dev Interface of the ERC20 standard as defined in the EIP.
 */
interface IERC20 {
    /**
     * @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);

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

// 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;
    }
}

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

import "@openzeppelin/contracts/token/ERC20/IERC20.sol";
import "@openzeppelin/contracts/security/ReentrancyGuard.sol";
import "@openzeppelin/contracts/security/Pausable.sol";
import "@openzeppelin/contracts/access/Ownable.sol";
import "./interfaces/IClayToken.sol";

/**
    User can stake Sumero LP Tokens (received by providing liquidity to a Liquidity Pool on Sumero) to earn CLAY rewards.
    User can unstake the Sumero LP tokens and claim rewards at any point in time.
    Rewards would depend on your
    - time period of stake  
    - percentage of your staked tokens with respect to total staked tokens

    Owner of this contract can perform following actions:
    - pause / unpause this contract in case of closure of Staking Rewards scheme or other unforseen circumstances
    - change reward rate
 */
contract ClayStakingRewards is Ownable, ReentrancyGuard, Pausable {
    IClayToken public immutable clayToken;
    // Staking token would be Sumero LP tokens
    IERC20 public immutable stakingToken;

    // reward rate i.e. reward in wei rewarded per second for staking a whole token
    uint256 public rewardRate;
    uint256 public lastUpdateTime;
    uint256 public rewardPerTokenStored;
    uint256 public periodFinish; // Contract lifetime.
    uint256 public maxReward; // Max reward that this contract will emit during it's lifetime.

    mapping(address => uint256) public userRewardPerTokenPaid;
    mapping(address => uint256) public rewards;

    uint256 private _totalSupply;
    mapping(address => uint256) private _balances;

    constructor(
        address _stakedToken,
        address _clayToken,
        uint256 _periodFinish,
        uint256 _maxReward
    ) {
        require(
            _stakedToken != address(0) &&
            _clayToken != address(0),
            "ClayStakingRewards: ZERO_ADDRESS"
        );
        stakingToken = IERC20(_stakedToken);
        clayToken = IClayToken(_clayToken);
        periodFinish = _periodFinish;
        maxReward = _maxReward;
        rewardRate = _maxReward / (_periodFinish - block.timestamp);
    }

    /* ========== VIEWS ========== */

    function totalSupply() external view returns (uint256) {
        return _totalSupply;
    }

    function balanceOf(address account) external view returns (uint256) {
        return _balances[account];
    }

    function lastRewardTimeApplicable() public view returns (uint256) {
        return block.timestamp < periodFinish ? block.timestamp : periodFinish;
    }

    // Gives the rewards calculation till the block.timestamp
    // If the protocol has L(t) tokens staked at time t, then this function returns rewards from time t to block.timestamp
    // i.e, Rewards generated from the time where first ever staking happened in this contract, till now.
    function rewardPerToken() public view returns (uint256) {
        if (_totalSupply == 0) {
            return rewardPerTokenStored;
        }
        return
            rewardPerTokenStored +
            ((rewardRate *
                (lastRewardTimeApplicable() - lastUpdateTime) *
                1e18) / _totalSupply);
    }

    function earned(address _account) public view returns (uint256) {
        return
            ((_balances[_account] *
                (rewardPerToken() - userRewardPerTokenPaid[_account])) / 1e18) +
            rewards[_account];
    }

    /* ========== MODIFIERS ========== */

    modifier updateReward(address _account) {
        rewardPerTokenStored = rewardPerToken();
        lastUpdateTime = lastRewardTimeApplicable();
        rewards[_account] = earned(_account);
        userRewardPerTokenPaid[_account] = rewardPerTokenStored;
        _;
    }

    modifier notExpired {
        require(
            periodFinish > block.timestamp,
            "ClayStakingRewards: STAKING_PERIOD_EXPIRED"
        );
        _;
    }

    /* ========== MUTATIVE FUNCTIONS ========== */

    function stake(uint256 _amount)
        external
        nonReentrant
        whenNotPaused
        notExpired
        updateReward(msg.sender)
    {
        require(_amount > 0, "ClayStakingRewards: AMOUNT_IS_ZERO");
        _totalSupply += _amount;
        _balances[msg.sender] += _amount;
        bool success = stakingToken.transferFrom(
            msg.sender,
            address(this),
            _amount
        );
        require(success, "ClayStakingRewards: TRANSFER_FAILED");
        emit Staked(msg.sender, _amount);
    }

    function withdraw(uint256 _amount)
        public
        nonReentrant
        updateReward(msg.sender)
    {
        require(_amount > 0, "ClayStakingRewards: AMOUNT_IS_ZERO");
        require(
            _amount <= _balances[msg.sender],
            "ClayStakingRewards: INSUFFICIENT_BALANCE"
        );

        _totalSupply -= _amount;
        _balances[msg.sender] -= _amount;
        bool success = stakingToken.transfer(msg.sender, _amount);
        require(success, "ClayStakingRewards: TRANSFER_FAILED");
        emit Withdrawn(msg.sender, _amount);
    }

    function exit() external {
        withdraw(_balances[msg.sender]);
        getReward();
    }

    function getReward() public nonReentrant updateReward(msg.sender) {
        uint256 reward = rewards[msg.sender];
        require(reward > 0, "ClayStakingRewards: NO_REWARDS");
        rewards[msg.sender] = 0;
        // Sumero Owner needs to grant MINTER_ROLE for CLAY to StakingRewards
        clayToken.mint(msg.sender, reward);
        emit RewardPaid(msg.sender, reward);
    }

    /* ========== RESTRICTED FUNCTIONS ========== */

    function pause() external onlyOwner {
        _pause();
    }

    function unpause() external onlyOwner {
        _unpause();
    }

    function updateMaxReward(uint256 _maxReward) external onlyOwner notExpired {
        rewardPerTokenStored = rewardPerToken();
        require(
            ((rewardPerTokenStored * _totalSupply) / 1e18) < _maxReward,
            "ClayStakingRewards: INVALID_MAX_REWARD_AMOUNT"
        );
        lastUpdateTime = block.timestamp;
        maxReward = _maxReward;
        rewardRate =
            (_maxReward - ((rewardPerTokenStored * _totalSupply) / 1e18)) /
            (periodFinish - block.timestamp);
        emit RewardRateUpdated(rewardRate);
    }

    // Added to support recovering LP Rewards from other systems
    function recoverERC20(address tokenAddress, uint256 tokenAmount)
        external
        onlyOwner
    {
        require(
            tokenAddress != address(stakingToken),
            "Cannot withdraw the staking token"
        );
        IERC20(tokenAddress).transfer(owner(), tokenAmount);
        emit Recovered(tokenAddress, tokenAmount);
    }

    /* ========== EVENTS ========== */

    event Staked(address indexed user, uint256 amount);
    event Withdrawn(address indexed user, uint256 amount);
    event RewardPaid(address indexed user, uint256 reward);
    event RewardRateUpdated(uint256 rewardRate);
    event Recovered(address token, uint256 amount);
}

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

/**
 * @dev Interface of the Clay Token.
 */
interface IClayToken {
    function mint(address account, uint256 amount) external;

    function burn(address account, uint256 amount) external;

    function balanceOf(address account) external view returns (uint256);

    function transferFrom(
        address from,
        address to,
        uint256 amount
    ) external returns (bool);

    function totalSupply() external view returns (uint256);
}