contract License is Ownable, ApproveAndCallFallBack, SafeTransfer, Proxiable {
    uint256 public price;

    ERC20Token token;
    address burnAddress;

    struct LicenseDetails {
        uint price;
        uint creationTime;
    }

    address[] public licenseOwners;
    mapping(address => uint) public idxLicenseOwners;
    mapping(address => LicenseDetails) public licenseDetails;

    event Bought(address buyer, uint256 price);
    event PriceChanged(uint256 _price);

    bool internal _initialized;

    /**
     * @param _tokenAddress Address of token used to pay for licenses (SNT)
     * @param _price Price of the licenses
     * @param _burnAddress Address where the license fee is going to be sent
     */
    constructor(address _tokenAddress, uint256 _price, address _burnAddress) public {
        init(_tokenAddress, _price, _burnAddress);
    }

    /**
     * @dev Initialize contract (used with proxy). Can only be called once
     * @param _tokenAddress Address of token used to pay for licenses (SNT)
     * @param _price Price of the licenses
     * @param _burnAddress Address where the license fee is going to be sent
     */
    function init(
        address _tokenAddress,
        uint256 _price,
        address _burnAddress
    ) public {
        assert(_initialized == false);

        _initialized = true;

        price = _price;
        token = ERC20Token(_tokenAddress);
        burnAddress = _burnAddress;

        _setOwner(msg.sender);
    }

    function updateCode(address newCode) public onlyOwner {
        updateCodeAddress(newCode);
    }

    /**
     * @notice Check if the address already owns a license
     * @param _address The address to check
     * @return bool
     */
    function isLicenseOwner(address _address) public view returns (bool) {
        return licenseDetails[_address].price != 0 && licenseDetails[_address].creationTime != 0;
    }

    /**
     * @notice Buy a license
     * @dev Requires value to be equal to the price of the license.
     *      The msg.sender must not already own a license.
     */
    function buy() external returns(uint) {
        uint id = _buyFrom(msg.sender);
        return id;
    }

    /**
     * @notice Buy a license
     * @dev Requires value to be equal to the price of the license.
     *      The _owner must not already own a license.
     */
    function _buyFrom(address _licenseOwner) internal returns(uint) {
        require(licenseDetails[_licenseOwner].creationTime == 0, "License already bought");

        licenseDetails[_licenseOwner] = LicenseDetails({
            price: price,
            creationTime: block.timestamp
        });

        uint idx = licenseOwners.push(_licenseOwner);
        idxLicenseOwners[_licenseOwner] = idx;

        emit Bought(_licenseOwner, price);

        require(_safeTransferFrom(token, _licenseOwner, burnAddress, price), "Unsuccessful token transfer");

        return idx;
    }

    /**
     * @notice Set the license price
     * @param _price The new price of the license
     * @dev Only the owner of the contract can perform this action
    */
    function setPrice(uint256 _price) external onlyOwner {
        price = _price;
        emit PriceChanged(_price);
    }

    /**
     * @dev Get number of license owners
     * @return uint
     */
    function getNumLicenseOwners() external view returns (uint256) {
        return licenseOwners.length;
    }

    /**
     * @notice Support for "approveAndCall". Callable only by `token()`.
     * @param _from Who approved.
     * @param _amount Amount being approved, need to be equal `price()`.
     * @param _token Token being approved, need to be equal `token()`.
     * @param _data Abi encoded data with selector of `buy(and)`.
     */
    function receiveApproval(address _from, uint256 _amount, address _token, bytes memory _data) public {
        require(_amount == price, "Wrong value");
        require(_token == address(token), "Wrong token");
        require(_token == address(msg.sender), "Wrong call");
        require(_data.length == 4, "Wrong data length");

        require(_abiDecodeBuy(_data) == bytes4(0xa6f2ae3a), "Wrong method selector"); //bytes4(keccak256("buy()"))

        _buyFrom(_from);
    }

    /**
     * @dev Decodes abi encoded data with selector for "buy()".
     * @param _data Abi encoded data.
     * @return Decoded registry call.
     */
    function _abiDecodeBuy(bytes memory _data) internal pure returns(bytes4 sig) {
        assembly {
            sig := mload(add(_data, add(0x20, 0)))
        }
    }
}
/**
 * @title License
 * @dev Contract for buying a license
 */
function receiveApproval(address _from, uint256 _amount, address _token, bytes memory _data) public {
    require(_amount == price, "Wrong value");
    require(_token == address(token), "Wrong token");
    require(_token == address(msg.sender), "Wrong call");
    require(_data.length == 4, "Wrong data length");

    require(_abiDecodeBuy(_data) == bytes4(0xa6f2ae3a), "Wrong method selector"); //bytes4(keccak256("buy()"))

    _buyFrom(_from);
}

/**
 * @notice Support for "approveAndCall". Callable only by `token()`.
 * @param _from Who approved.
 * @param _amount Amount being approved, need to be equal `price()`.
 * @param _token Token being approved, need to be equal `token()`.
 * @param _data Abi encoded data with selector of `buy(and)`.
 */