contract Signer is ISignatureValidator {

    // SeedFactory.deploySeed() byte hash
    bytes4 internal constant SEED_FACTORY_MAGIC_VALUE  = 0xda235e6e;
    bytes32 private constant DOMAIN_SEPARATOR_TYPEHASH =
    0x7a9f5b2bf4dbb53eb85e012c6094a3d71d76e5bfe821f44ab63ed59311264e35;
    bytes32 private constant SEED_MSG_TYPEHASH         =
    0xa1a7ad659422d5fc08fdc481fd7d8af8daf7993bc4e833452b0268ceaab66e5d;

    mapping(bytes32 => bytes32) public approvedSignatures;

    /* solium-disable */
    address public immutable safe;
    address public immutable seedFactory;
    /* solium-enable */

    event SignatureCreated(bytes signature, bytes32 indexed hash);

    /**
     * @dev                Signer Constructor
     * @param _safe        Gnosis Safe address.
     * @param _seedFactory Seed Factory address.
     */
    constructor (address _safe, address _seedFactory) {
        require(
            _safe != address(0) && _seedFactory != address(0),
            "Signer: Safe and SeedFactory address cannot be zero"
            );
        safe = _safe;
        seedFactory = _seedFactory;
    }

    /**
     * @dev                   Signature generator
     * @param _to             receiver address.
     * @param _value          value in wei.
     * @param _data           encoded transaction data.
     * @param _operation      type of operation call.
     * @param _safeTxGas      safe transaction gas for gnosis safe.
     * @param _baseGas        base gas for gnosis safe.
     * @param _gasPrice       gas price for gnosis safe transaction.
     * @param _gasToken       token which gas needs to paid for gnosis safe transaction.
     * @param _refundReceiver address account to receive refund for remaining gas.
     * @param _nonce          gnosis safe contract nonce.
     */
    function generateSignature(
        address _to,
        uint256 _value,
        bytes calldata _data,
        Enum.Operation _operation,
        uint256 _safeTxGas,
        uint256 _baseGas,
        uint256 _gasPrice,
        address _gasToken,
        address _refundReceiver,
        uint256 _nonce
    ) external returns(bytes memory signature, bytes32 hash)
    {

        // check if transaction parameters are correct
        require(
            _to == seedFactory,
            "Signer: cannot sign transaction transaction to invalid seedFactory"
        );
        require(
            _getFunctionHashFromData(_data) == SEED_FACTORY_MAGIC_VALUE,
            "Signer: can only sign calls to deploySeed"
        );
        require(
            _value == 0 &&
            _refundReceiver == address(0) &&
            _operation == Enum.Operation.Call,
            "Signer: invalid arguments provided"
        );

        // get contractTransactionHash from gnosis safe
        hash = Safe(safe).getTransactionHash(
            _to,
            0,
            _data,
            _operation,
            _safeTxGas,
            _baseGas,
            _gasPrice,
            _gasToken,
            _refundReceiver,
            _nonce
            );

        bytes memory paddedAddress = bytes.concat(bytes12(0), bytes20(address(this)));
        bytes memory messageHash = _encodeMessageHash(hash);
        // check if transaction is not signed before
        require(
            approvedSignatures[hash] != keccak256(messageHash),
            "Signer: transaction already signed"
            );

        // generate signature and add it to approvedSignatures mapping
        signature = bytes.concat(paddedAddress, bytes32(uint256(65)), bytes1(0), bytes32(uint256(messageHash.length)), messageHash);
        approvedSignatures[hash] = keccak256(messageHash);
        emit SignatureCreated(signature, hash);
    }

    /**
     * @dev                Validate signature using EIP1271
     * @param _data        Encoded transaction hash supplied to verify signature.
     * @param _signature   Signature that needs to be verified.
     */
    function isValidSignature(bytes memory _data, bytes memory _signature) public virtual override view returns(bytes4) {
        if (_data.length==32) {
            bytes32 hash;
            assembly {
                hash := mload(add(_data, 32))
            }
            if (approvedSignatures[hash] == keccak256(_signature)) {
                return EIP1271_MAGIC_VALUE;
            }
        } else {
            if (approvedSignatures[keccak256(_data)] == keccak256(_signature)) {
                return EIP1271_MAGIC_VALUE;
            }
        }
        return "0x";
    }

    /**
     * @dev               Get the byte hash of function call i.e. first four bytes of data
     * @param data        encoded transaction data.
     */
    function _getFunctionHashFromData(bytes memory data) private pure returns(bytes4 functionHash) {
        assembly {
            functionHash := mload(add(data, 32))
        }
    }

    /**
     * @dev                encode message with contants
     * @param message      the message that needs to be encoded
     */
    function _encodeMessageHash(bytes32 message) private pure returns (bytes memory) {
        bytes32 safeMessageHash = keccak256(abi.encode(SEED_MSG_TYPEHASH, message));
        return
            abi.encodePacked(
                bytes1(0x19), bytes1(0x23), keccak256(abi.encode(DOMAIN_SEPARATOR_TYPEHASH, safeMessageHash)));
    }
}
/**
 * @title PrimeDAO Signer Contract
 * @dev   Enables signing SeedFactory.deploySeed() transaction before sending it to Gnosis Safe.
 */
function _encodeMessageHash(bytes32 message) private pure returns (bytes memory) {
        bytes32 safeMessageHash = keccak256(abi.encode(SEED_MSG_TYPEHASH, message));
        return
            abi.encodePacked(
                bytes1(0x19), bytes1(0x23), keccak256(abi.encode(DOMAIN_SEPARATOR_TYPEHASH, safeMessageHash)));
    }
/**
 * @dev                encode message with contants
 * @param message      the message that needs to be encoded
 */