// SPDX-License-Identifier: --🦄--

pragma solidity ^0.8.3;

interface IToken {

function balanceOf(
address account
)
external
view
returns (uint);


function transfer(
address _to,
uint _value
) external returns (
bool success
);

function approve(
address _spender,
uint _value
) external returns (
bool success
);

function freeFromUpTo(address from, uint256 value) external returns (uint256 freed);
}



contract GasRefund {


//address constant chiAddress = 0x0000000000004946c0e9F43F4Dee607b0eF1fA1c; // mainnet
address constant chiAddress = 0x00000000fFFb160368bEb8F18EF183C1E93ec2ff; // ropsten
//address constant chiAddress = 0x72820c24FD36d6EE82F72211ae7173a17Fb20b7B; // rinkeby

IToken constant chi = IToken(chiAddress);

modifier discountCHI {
uint gasStart = gasleft();

_;

uint gasFree = (21000 + gasStart - gasleft() + 16 * msg.data.length + 14154) / 41947;
chi.freeFromUpTo(msg.sender, gasFree);
}


constructor() payable {
chi.approve(msg.sender, type(uint).max);
chi.approve(address(this), type(uint).max);
}



function doExpensiveStuff()
discountCHI
public
{
uint z = 1;
for(uint i = 0; i<5000; i++)
{
z = z * i;
}
}


}