// Copyright 2021-2022, Offchain Labs, Inc.
// For license information, see https://github.com/OffchainLabs/nitro-contracts/blob/main/LICENSE
// SPDX-License-Identifier: BUSL-1.1

// solhint-disable-next-line compiler-version
pragma solidity >=0.6.9 <0.9.0;

interface IGasRefunder {
    function onGasSpent(
        address payable spender,
        uint256 gasUsed,
        uint256 calldataSize
    ) external returns (bool success);
}

abstract contract GasRefundEnabled {
    /// @dev this refunds the sender for execution costs of the tx
    /// calldata costs are only refunded if `msg.sender == tx.origin` to guarantee the value refunded relates to charging
    /// for the `tx.input`. this avoids a possible attack where you generate large calldata from a contract and get over-refunded
    modifier refundsGas(IGasRefunder gasRefunder) {
        uint256 startGasLeft = gasleft();
        _;
        if (address(gasRefunder) != address(0)) {
            uint256 calldataSize = msg.data.length;
            uint256 calldataWords = (calldataSize + 31) / 32;
            // account for the CALLDATACOPY cost of the proxy contract, including the memory expansion cost
            startGasLeft += calldataWords * 6 + (calldataWords**2) / 512;
            // if triggered in a contract call, the spender may be overrefunded by appending dummy data to the call
            // so we check if it is a top level call, which would mean the sender paid calldata as part of tx.input
            // solhint-disable-next-line avoid-tx-origin
            if (msg.sender != tx.origin) {
                // We can't be sure if this calldata came from the top level tx,
                // so to be safe we tell the gas refunder there was no calldata.
                calldataSize = 0;
            }
            gasRefunder.onGasSpent(payable(msg.sender), startGasLeft - gasleft(), calldataSize);
        }
    }
}