SimpleStaking

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.20;

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

/**
 * @title SimpleStaking
 * @dev Users stake a Staking Token to earn a Reward Token.
 * Rewards are calculated based on time staked.
 */
contract SimpleStaking is ReentrancyGuard, Ownable {
    IERC20 public stakingToken;
    IERC20 public rewardsToken;

    // Reward rate: How many tokens distributed per second total
    uint256 public constant REWARD_RATE = 100; 
    
    uint256 public lastUpdateTime;
    uint256 public rewardPerTokenStored;

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

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

    event Staked(address indexed user, uint256 amount);
    event Withdrawn(address indexed user, uint256 amount);
    event RewardPaid(address indexed user, uint256 reward);

    constructor(address _stakingToken, address _rewardsToken) Ownable(msg.sender) {
        stakingToken = IERC20(_stakingToken);
        rewardsToken = IERC20(_rewardsToken);
    }

    /**
     * @dev Modifier to update rewards for a user before any action.
     * This ensures the math is always up to date when balances change.
     */
    modifier updateReward(address account) {
        rewardPerTokenStored = rewardPerToken();
        lastUpdateTime = block.timestamp;

        if (account != address(0)) {
            rewards[account] = earned(account);
            userRewardPerTokenPaid[account] = rewardPerTokenStored;
        }
        _;
    }

    /**
     * @dev Calculates current reward per token globally.
     */
    function rewardPerToken() public view returns (uint256) {
        if (_totalSupply == 0) {
            return rewardPerTokenStored;
        }
        return rewardPerTokenStored + (((block.timestamp - lastUpdateTime) * REWARD_RATE * 1e18) / _totalSupply);
    }

    /**
     * @dev Calculates how much a specific user has earned.
     */
    function earned(address account) public view returns (uint256) {
        return
            ((_balances[account] * (rewardPerToken() - userRewardPerTokenPaid[account])) / 1e18) +
            rewards[account];
    }

    /**
     * @notice Deposit tokens to start earning rewards.
     */
    function stake(uint