contract PoolToken is IPoolToken, ImpermaxERC20 {
   	uint internal constant initialExchangeRate = 1e18;
	address public underlying;
	address public factory;
	uint public totalBalance;
	uint public constant MINIMUM_LIQUIDITY = 1000;
	
	event Mint(address indexed sender, address indexed minter, uint mintAmount, uint mintTokens);
	event Redeem(address indexed sender, address indexed redeemer, uint redeemAmount, uint redeemTokens);
	event Sync(uint totalBalance);
	
	/*** Initialize ***/
	
	// called once by the factory
	function _setFactory() external {
		require(factory == address(0), "Impermax: FACTORY_ALREADY_SET");
		factory = msg.sender;
	}
	
	/*** PoolToken ***/
	
	function _update() internal {
		totalBalance = IERC20(underlying).balanceOf(address(this));
		emit Sync(totalBalance);
	}

	function exchangeRate() public returns (uint) 
	{
		uint _totalSupply = totalSupply; // gas savings
		uint _totalBalance = totalBalance; // gas savings
		if (_totalSupply == 0 || _totalBalance == 0) return initialExchangeRate;
		return _totalBalance.mul(1e18).div(_totalSupply);
	}
	
	// this low-level function should be called from another contract
	function mint(address minter) external nonReentrant update returns (uint mintTokens) {
		uint balance = IERC20(underlying).balanceOf(address(this));
		uint mintAmount = balance.sub(totalBalance);
		mintTokens = mintAmount.mul(1e18).div(exchangeRate());

		if(totalSupply == 0) {
			// permanently lock the first MINIMUM_LIQUIDITY tokens
			mintTokens = mintTokens.sub(MINIMUM_LIQUIDITY);
			_mint(address(0), MINIMUM_LIQUIDITY);
		}
		require(mintTokens > 0, "Impermax: MINT_AMOUNT_ZERO");
		_mint(minter, mintTokens);
		emit Mint(msg.sender, minter, mintAmount, mintTokens);
	}

	// this low-level function should be called from another contract
	function redeem(address redeemer) external nonReentrant update returns (uint redeemAmount) {
		uint redeemTokens = balanceOf[address(this)];
		redeemAmount = redeemTokens.mul(exchangeRate()).div(1e18);

		require(redeemAmount > 0, "Impermax: REDEEM_AMOUNT_ZERO");
		require(redeemAmount <= totalBalance, "Impermax: INSUFFICIENT_CASH");
		_burn(address(this), redeemTokens);
		_safeTransfer(redeemer, redeemAmount);
		emit Redeem(msg.sender, redeemer, redeemAmount, redeemTokens);		
	}

	// force real balance to match totalBalance
	function skim(address to) external nonReentrant {
		_safeTransfer(to, IERC20(underlying).balanceOf(address(this)).sub(totalBalance));
	}

	// force totalBalance to match real balance
	function sync() external nonReentrant update {}
	
	/*** Utilities ***/
	
	// same safe transfer function used by UniSwapV2 (with fixed underlying)
	bytes4 private constant SELECTOR = bytes4(keccak256(bytes("transfer(address,uint256)")));
	function _safeTransfer(address to, uint amount) internal {
		(bool success, bytes memory data) = underlying.call(abi.encodeWithSelector(SELECTOR, to, amount));
		require(success && (data.length == 0 || abi.decode(data, (bool))), "Impermax: TRANSFER_FAILED");
	}
	
	// prevents a contract from calling itself, directly or indirectly.
	bool internal _notEntered = true;
	modifier nonReentrant() {
		require(_notEntered, "Impermax: REENTERED");
		_notEntered = false;
		_;
		_notEntered = true;
	}
	
	// update totalBalance with current balance
	modifier update() {
		_;
		_update();
	}
}
function mint(address minter) external nonReentrant update returns (uint mintTokens) {
	uint balance = IERC20(underlying).balanceOf(address(this));
	uint mintAmount = balance.sub(totalBalance);
	mintTokens = mintAmount.mul(1e18).div(exchangeRate());

	if(totalSupply == 0) {
		// permanently lock the first MINIMUM_LIQUIDITY tokens
		mintTokens = mintTokens.sub(MINIMUM_LIQUIDITY);
		_mint(address(0), MINIMUM_LIQUIDITY);
	}
	require(mintTokens > 0, "Impermax: MINT_AMOUNT_ZERO");
	_mint(minter, mintTokens);
	emit Mint(msg.sender, minter, mintAmount, mintTokens);
}

// this low-level function should be called from another contract