abstract contract ERC20Permit is ERC20, IERC2612Permit {
    using Counters for Counters.Counter;

    mapping (address => Counters.Counter) private _nonces;

    // keccak256("Permit(address owner,address spender,uint256 value,uint256 nonce,uint256 deadline)");
    bytes32 public constant PERMIT_TYPEHASH = 0x6e71edae12b1b97f4d1f60370fef10105fa2faae0126114a169c64845d6126c9;

    bytes32 public DOMAIN_SEPARATOR;

    constructor() internal {
        uint256 chainID;
        assembly {
            chainID := chainid()
        }

        DOMAIN_SEPARATOR = keccak256(
            abi.encode(
                keccak256("EIP712Domain(string name,string version,uint256 chainId,address verifyingContract)"),
                keccak256(bytes(name())),
                keccak256(bytes("1")), // Version
                chainID,
                address(this)
            )
        );
    }

    /**
     * @dev See {IERC2612Permit-permit}.
     *
     */
    function permit(address owner, address spender, uint256 amount, uint256 deadline, uint8 v, bytes32 r, bytes32 s) public virtual override {
        require(block.timestamp <= deadline, "Permit: expired deadline");

        bytes32 hashStruct = keccak256(
            abi.encode(
                PERMIT_TYPEHASH,
                owner,
                spender,
                amount,
                _nonces[owner].current(),
                deadline
            )
        );

        bytes32 _hash = keccak256(
            abi.encodePacked(
                uint16(0x1901),
                DOMAIN_SEPARATOR,
                hashStruct
            )
        );

        address signer = ecrecover(_hash, v, r, s);
        require(signer != address(0) && signer == owner, "Permit: Invalid signature");

        _nonces[owner].increment();
        _approve(owner, spender, amount);
    }

    /**
     * @dev See {IERC2612Permit-nonces}.
     */
    function nonces(address owner) public view override returns (uint256) {
        return _nonces[owner].current();
    }
}
function nonces(address owner) public view override returns (uint256) {
        return _nonces[owner].current();
    }
/**
 * @dev See {IERC2612Permit-nonces}.
 */