pragma lity ^1.2.3;

contract MyContract {

    //EVENTS
    // Create an event called HelloWorld which will allow us to write to the blockchain's event logs (immutable data)'
    event HelloWorld(address myAddress, string message, safeuint counter);
    // Create an event called ContractCreated which will allow us to record the creation of the contract with a message
    event ContractCreated(address myAddress, string creationMessage);
    // Create an event called ContractTerminated which will allow us to record the termination of the contract with a message
    event ContractTerminated(address myAddress, string terminationMessage);

    // CONTRACT VARIABLES
    // Create a contract variable called counter
    safeuint public counter;
    // Create a contract variable which records who the original owner is
    address private owner;

    // CONSTRUCTOR
    // This is the constructor which will execute internally when the contract is deployed
    constructor(string memory _creationMessage) public {
        if (isValidString(_creationMessage) == true){
        // Set the counter to zero
        counter = 0;
        // Set the owner of this contract to the person who deployed it
        owner = msg.sender;
        emit ContractCreated(msg.sender, _creationMessage);
        }else{
            revert("Invalid message string, please try again");
        }
    }

    // MODIFIERS
    // This is a modifier which enforces that the message sender must be the owner of this contract
    modifier onlyOwner() {
        require(msg.sender == owner);
        _;
    }

    // PRIVATE FUNCTIONS
    // This is an internal/private function which increments the counter
    function incrementCounter() private {
        counter += 1;
    }

    // This is a function which checks the length of a string using temporary memory
    function isValidString(string memory _theString) private pure returns(bool) {
        bytes memory theBytes = bytes(_theString);
        if (theBytes.length > 0) {
            return true;
        } else {
            return false;
        }
    }

    // PUBLIC FUNCTIONS
    // This is a public function which firstly calls our very own incrementCounter() and then emits the HelloWorld event.
    function sayHello() public {
        incrementCounter();
        emit HelloWorld(msg.sender, "Hello World!", counter);
    }

    // This is a public function that allows us to delete the contract from the blockchain
    // Whilst the counter will no longer be accessible, the log events which we emitted, will forever be accessible
    // Note how we are using the onlyOwner modifier. This is so that only the owner is able to terminate this contract
    function terminateTheContract() public onlyOwner {
        emit ContractTerminated(msg.sender, "Contract has been terminated");
        selfdestruct(msg.sender);
    }

    // PUBLIC DESTRUCTOR
    // This is a fallback function which makes it impossible for people to send funds to this contract by accident
    function() public payable {
        revert("Please do not send funds to this contract");
    }

}