contract LibEIP712 {

    // EIP191 header for EIP712 prefix
    string constant internal EIP191_HEADER = "\x19\x01";

    // EIP712 Domain Name value
    string constant internal EIP712_DOMAIN_NAME = "0x Protocol";

    // EIP712 Domain Version value
    string constant internal EIP712_DOMAIN_VERSION = "2";

    // Hash of the EIP712 Domain Separator Schema
    bytes32 constant internal EIP712_DOMAIN_SEPARATOR_SCHEMA_HASH = keccak256(abi.encodePacked(
        "EIP712Domain(",
        "string name,",
        "string version,",
        "address verifyingContract",
        ")"
    ));

    // Hash of the EIP712 Domain Separator data
    // solhint-disable-next-line var-name-mixedcase
    bytes32 public EIP712_DOMAIN_HASH;

    constructor ()
        public
    {
        EIP712_DOMAIN_HASH = keccak256(abi.encodePacked(
            EIP712_DOMAIN_SEPARATOR_SCHEMA_HASH,
            keccak256(bytes(EIP712_DOMAIN_NAME)),
            keccak256(bytes(EIP712_DOMAIN_VERSION)),
            bytes32(address(this))
        ));
    }

    /// @dev Calculates EIP712 encoding for a hash struct in this EIP712 Domain.
    /// @param hashStruct The EIP712 hash struct.
    /// @return EIP712 hash applied to this EIP712 Domain.
    function hashEIP712Message(bytes32 hashStruct)
        internal
        view
        returns (bytes32 result)
    {
        bytes32 eip712DomainHash = EIP712_DOMAIN_HASH;

        // Assembly for more efficient computing:
        // keccak256(abi.encodePacked(
        //     EIP191_HEADER,
        //     EIP712_DOMAIN_HASH,
        //     hashStruct    
        // ));

        assembly {
            // Load free memory pointer
            let memPtr := mload(64)

            mstore(memPtr, 0x1901000000000000000000000000000000000000000000000000000000000000)  // EIP191 header
            mstore(add(memPtr, 2), eip712DomainHash)                                            // EIP712 domain hash
            mstore(add(memPtr, 34), hashStruct)                                                 // Hash of struct

            // Compute hash
            result := keccak256(memPtr, 66)
        }
        return result;
    }
}
// solhint-enable max-line-length
function hashEIP712Message(bytes32 hashStruct)
    internal
    view
    returns (bytes32 result)
{
    bytes32 eip712DomainHash = EIP712_DOMAIN_HASH;

    // Assembly for more efficient computing:
    // keccak256(abi.encodePacked(
    //     EIP191_HEADER,
    //     EIP712_DOMAIN_HASH,
    //     hashStruct    
    // ));

    assembly {
        // Load free memory pointer
        let memPtr := mload(64)

        mstore(memPtr, 0x1901000000000000000000000000000000000000000000000000000000000000)  // EIP191 header
        mstore(add(memPtr, 2), eip712DomainHash)                                            // EIP712 domain hash
        mstore(add(memPtr, 34), hashStruct)                                                 // Hash of struct

        // Compute hash
        result := keccak256(memPtr, 66)
    }
    return result;
}

/// @dev Calculates EIP712 encoding for a hash struct in this EIP712 Domain.
/// @param hashStruct The EIP712 hash struct.
/// @return EIP712 hash applied to this EIP712 Domain.