// SPDX-License-Identifier: MIT
pragma solidity ^0.8.24;

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

contract Staking is ReentrancyGuard {
    IERC20 public stakingToken;
    uint256 public rewardRate = 100; // tokens per block
    uint256 public lastUpdateBlock;
    uint256 public rewardPerTokenStored;

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

    constructor(address _token) {
        stakingToken = IERC20(_token);
    }

    function stake(uint256 amount) external nonReentrant updateReward(msg.sender) {
        totalSupply += amount;
        balances[msg.sender] += amount;
        stakingToken.transferFrom(msg.sender, address(this), amount);
    }

    modifier updateReward(address account) {
        rewardPerTokenStored = rewardPerToken();
        lastUpdateBlock = block.number;
        if (account != address(0)) {
            rewards[account] = earned(account);
            userRewardPerTokenPaid[account] = rewardPerTokenStored;
        }
        _;
    }

    function rewardPerToken() public view returns (uint256) {
        if (totalSupply == 0) return rewardPerTokenStored;
        return rewardPerTokenStored + (rewardRate * (block.number - lastUpdateBlock) * 1e18) / totalSupply;
    }

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