feat(gauge): add staking gauge with reward distribution

src/StakingGauge.sol

@@ -0,0 +1,131 @@
+// SPDX-License-Identifier: MIT
+pragma solidity 0.8.36;
+
+import {IErc20} from "./interfaces/IErc20.sol";
+
+/// @title Staking gauge distributing a reward token pro rata by staked share over time.
+contract StakingGauge {
+    IErc20 public immutable stakingToken;
+    IErc20 public immutable rewardToken;
+    address public owner;
+
+    uint256 public rewardRate;
+    uint256 public periodFinish;
+    uint256 public rewardsDuration = 7 days;
+    uint256 public lastUpdateTime;
+    uint256 public rewardPerTokenStored;
+
+    mapping(address => uint256) public userRewardPerTokenPaid;
+    mapping(address => uint256) public rewards;
+
+    uint256 public totalSupply;
+    mapping(address => uint256) public balanceOf;
+
+    event Staked(address indexed account, uint256 amount);
+    event Withdrawn(address indexed account, uint256 amount);
+    event RewardPaid(address indexed account, uint256 reward);
+    event RewardAdded(uint256 reward);
+    event RewardsDurationUpdated(uint256 duration);
+
+    constructor(address staking, address reward, address ownerAddress) {
+        stakingToken = IErc20(staking);
+        rewardToken = IErc20(reward);
+        owner = ownerAddress;
+    }
+
+    modifier onlyOwner() {
+        require(msg.sender == owner, "StakingGauge: FORBIDDEN");
+        _;
+    }
+
+    modifier updateReward(address account) {
+        rewardPerTokenStored = rewardPerToken();
+        lastUpdateTime = lastTimeRewardApplicable();
+        if (account != address(0)) {
+            rewards[account] = earned(account);
+            userRewardPerTokenPaid[account] = rewardPerTokenStored;
+        }
+        _;
+    }
+
+    function _safeTransfer(IErc20 token, address to, uint256 value) private {
+        (bool success, bytes memory returndata) =
+            address(token).call(abi.encodeWithSelector(IErc20.transfer.selector, to, value));
+        require(success && (returndata.length == 0 || abi.decode(returndata, (bool))), "StakingGauge: TRANSFER_FAILED");
+    }
+
+    function _safeTransferFrom(IErc20 token, address from, address to, uint256 value) private {
+        (bool success, bytes memory returndata) =
+            address(token).call(abi.encodeWithSelector(IErc20.transferFrom.selector, from, to, value));
+        require(
+            success && (returndata.length == 0 || abi.decode(returndata, (bool))), "StakingGauge: TRANSFER_FROM_FAILED"
+        );
+    }
+
+    function lastTimeRewardApplicable() public view returns (uint256) {
+        return block.timestamp < periodFinish ? block.timestamp : periodFinish;
+    }
+
+    function rewardPerToken() public view returns (uint256) {
+        if (totalSupply == 0) return rewardPerTokenStored;
+        return rewardPerTokenStored + ((lastTimeRewardApplicable() - lastUpdateTime) * rewardRate * 1e18) / totalSupply;
+    }
+
+    function earned(address account) public view returns (uint256) {
+        return (balanceOf[account] * (rewardPerToken() - userRewardPerTokenPaid[account])) / 1e18 + rewards[account];
+    }
+
+    function stake(uint256 amount) external updateReward(msg.sender) {
+        require(amount > 0, "StakingGauge: ZERO_STAKE");
+        totalSupply += amount;
+        balanceOf[msg.sender] += amount;
+        _safeTransferFrom(stakingToken, msg.sender, address(this), amount);
+        emit Staked(msg.sender, amount);
+    }
+
+    function withdraw(uint256 amount) public updateReward(msg.sender) {
+        require(amount > 0, "StakingGauge: ZERO_WITHDRAW");
+        totalSupply -= amount;
+        balanceOf[msg.sender] -= amount;
+        _safeTransfer(stakingToken, msg.sender, amount);
+        emit Withdrawn(msg.sender, amount);
+    }
+
+    function getReward() public updateReward(msg.sender) {
+        uint256 reward = rewards[msg.sender];
+        if (reward > 0) {
+            rewards[msg.sender] = 0;
+            _safeTransfer(rewardToken, msg.sender, reward);
+            emit RewardPaid(msg.sender, reward);
+        }
+    }
+
+    function exit() external {
+        withdraw(balanceOf[msg.sender]);
+        getReward();
+    }
+
+    function notifyRewardAmount(uint256 reward) external onlyOwner updateReward(address(0)) {
+        if (block.timestamp >= periodFinish) {
+            rewardRate = reward / rewardsDuration;
+        } else {
+            uint256 remaining = periodFinish - block.timestamp;
+            uint256 leftover = remaining * rewardRate;
+            rewardRate = (reward + leftover) / rewardsDuration;
+        }
+
+        uint256 balance = rewardToken.balanceOf(address(this));
+        require(rewardRate <= balance / rewardsDuration, "StakingGauge: REWARD_TOO_HIGH");
+
+        lastUpdateTime = block.timestamp;
+        periodFinish = block.timestamp + rewardsDuration;
+        emit RewardAdded(reward);
+    }
+
+    function setRewardsDuration(uint256 duration) external onlyOwner {
+        require(block.timestamp > periodFinish, "StakingGauge: PERIOD_ACTIVE");
+        require(duration > 0, "StakingGauge: ZERO_DURATION");
+        rewardsDuration = duration;
+        emit RewardsDurationUpdated(duration);
+    }
+}