BiteRebate.sol

pragma solidity >=0.5.12;

interface CatLike {
    function bite(bytes32 ilk, address urn) external;
}

interface VatLike {
    function urns(bytes32 ilk, address u) external view returns (uint ink, uint art);
}

contract BiteRebate {
    address vat;
    
    // TODO - read from regitery
    address cat;
    
    // TODO - make configurable, and also use dai instead of art
    uint public maxArtForRebate = 1500e18; // a bit more than 1500 dai

    // TODO - make configurable, and also use dai instead of art
    uint public minArtForRebate = 200e18; // a bit more than 200 dai

    // TODO - make configurable    
    uint fixedRebateInEth = 0.01 ether;
    
    // TODO - make configurable
    uint gasCostNum = 2;
    uint gasCostDen = 3;
    
    constructor(address _vat, address _cat) public {
        vat = _vat;
        cat = _cat;
    }
    
    // TODO - let DAO withdraw remaining ETH
    function deposit() public payable {}
    
    function biteWithRebate(bytes32 ilk, address urn) external {
        uint gasLeftBefore = gasleft();
        
        (, uint art) = VatLike(vat).urns(ilk, urn);
        require(art >= minArtForRebate, "urn-too-small");        
        require(art <= maxArtForRebate, "urn-too-big");
        
        CatLike(cat).bite(ilk, urn);
        
        uint gasLeftAfter = gasleft();
        // consider having a cap on gas cost to avoid attacks with gas token burning
        uint gasCost = (gasLeftBefore - gasLeftAfter) * tx.gasprice;
        
        // TODO - send dai instead of ETH ?
        (bool succ, ) = msg.sender.call.value(gasCost * gasCostNum / gasCostDen + fixedRebateInEth)("");
        require(succ, "eth-transfer-failed");
    }
}