Universal Gas Discounter

GasDiscounter.sol

interface IUniswap {
    function getEthToTokenInputPrice(uint256 eth_sold) external view returns(uint256);
    function getEthToTokenOutputPrice(uint256 tokens_bought) external view returns(uint256);
    function ethToTokenSwapInput(uint256 min_tokens, uint256 deadline) external payable returns(uint256);
}

interface IGasToken {
    function free(uint256 value) external returns (bool success);
    function freeUpTo(uint256 value) public returns (uint256 freed);
    function freeFromUpTo(address from, uint256 value) public returns (uint256 freed);
}

contract GasDiscounter {
    IUniswap constant private _exchange = IUniswap(0x929507CD3D90Ab11eC4822E9eB5A48eb3a178F19);
    IGasToken constant private _gasToken = IGasToken(0x0000000000b3F879cb30FE243b4Dfee438691c04);
    
    modifier gasDiscount(uint256 value, bool shouldReturnToSender) {
        uint256 initialGasLeft = gasleft();
        _;
        _getGasDiscount(value, shouldReturnToSender, initialGasLeft - gasleft());
    }
    
    function _getGasDiscount(uint256 value, bool shouldReturnToSender, uint256 gasSpent) private {
        uint256 tokens = (gasSpent + 50000 + 14154) / (2*24000 - 6870);
        
        uint256 cost = 0;
        if (tokens > 0) {        
            if (_exchange.getEthToTokenOutputPrice(tokens) < tx.gasprice * 17000 * tokens) {
                tokens -= _gasToken.freeUpTo(tokens);
                
                if (tokens > 0) {
                    tokens -= _gasToken.freeFromUpTo(msg.sender, tokens);

                    if (tokens > 0) {
                        uint256 possibleTokens = _exchange.getEthToTokenInputPrice(value);
                        uint256 buyTokens = tokens < possibleTokens ? tokens : possibleTokens;
                        cost = _exchange.getEthToTokenOutputPrice(buyTokens);
                        _exchange.ethToTokenSwapInput.value(cost)(buyTokens, now);
                        require(_gasToken.free(tokens));
                    }
                }
            }
        }
        
        if (shouldReturnToSender && cost < value) {
            msg.sender.transfer(value - cost);
        }
    }
}