contract Domain {
    bytes32 private constant DOMAIN_SEPARATOR_SIGNATURE_HASH = keccak256("EIP712Domain(uint256 chainId,address verifyingContract)");
    // See https://eips.ethereum.org/EIPS/eip-191
    string private constant EIP191_PREFIX_FOR_EIP712_STRUCTURED_DATA = "\x19\x01";

    // solhint-disable var-name-mixedcase
    bytes32 private immutable _DOMAIN_SEPARATOR;
    uint256 private immutable DOMAIN_SEPARATOR_CHAIN_ID;

    /// @dev Calculate the DOMAIN_SEPARATOR
    function _calculateDomainSeparator(uint256 chainId) private view returns (bytes32) {
        return keccak256(abi.encode(DOMAIN_SEPARATOR_SIGNATURE_HASH, chainId, address(this)));
    }

    constructor() public {
        uint256 chainId;
        assembly {
            chainId := chainid()
        }
        _DOMAIN_SEPARATOR = _calculateDomainSeparator(DOMAIN_SEPARATOR_CHAIN_ID = chainId);
    }

    /// @dev Return the DOMAIN_SEPARATOR
    // It's named internal to allow making it public from the contract that uses it by creating a simple view function
    // with the desired public name, such as DOMAIN_SEPARATOR or domainSeparator.
    // solhint-disable-next-line func-name-mixedcase
    function DOMAIN_SEPARATOR() public view returns (bytes32) {
        uint256 chainId;
        assembly {
            chainId := chainid()
        }
        return chainId == DOMAIN_SEPARATOR_CHAIN_ID ? _DOMAIN_SEPARATOR : _calculateDomainSeparator(chainId);
    }

    function _getDigest(bytes32 dataHash) internal view returns (bytes32 digest) {
        digest = keccak256(abi.encodePacked(EIP191_PREFIX_FOR_EIP712_STRUCTURED_DATA, DOMAIN_SEPARATOR(), dataHash));
    }
}
// License-Identifier: MIT
// Based on code and smartness by Ross Campbell and Keno
// Uses immutable to store the domain separator to reduce gas usage
// If the chain id changes due to a fork, the forked chain will calculate on the fly.
function DOMAIN_SEPARATOR() public view returns (bytes32) {
    uint256 chainId;
    assembly {
        chainId := chainid()
    }
    return chainId == DOMAIN_SEPARATOR_CHAIN_ID ? _DOMAIN_SEPARATOR : _calculateDomainSeparator(chainId);
}

// It's named internal to allow making it public from the contract that uses it by creating a simple view function
// with the desired public name, such as DOMAIN_SEPARATOR or domainSeparator.
// solhint-disable-next-line func-name-mixedcase