pragma solidity ^0.5.4;

contract Forum {

    //----------------------------------------USER----------------------------------------
    struct User {
        string username;
        uint[] topicIDs;    // IDs of the topics the user created
        uint[] postIDs;    // IDs of the posts the user created
        uint timestamp;
        bool signedUp;    // Helper variable for hasUserSignedUp()
    }

    mapping (address => User) users;
    mapping (string => address) userAddresses;

    event UserSignedUp(string username, address userAddress);
    event UsernameUpdated(string newName, string oldName,address userAddress);

    function signUp(string memory username) public returns (bool) {
        require (!hasUserSignedUp(msg.sender), "User has already signed up.");
        require(!isUserNameTaken(username), "Username is already taken.");
        users[msg.sender] = User(username,
            new uint[](0), new uint[](0), block.timestamp, true);
        userAddresses[username] = msg.sender;
        emit UserSignedUp(username, msg.sender);
        return true;
    }

    function updateUsername(string memory newUsername) public returns (bool) {
        require (hasUserSignedUp(msg.sender), "User hasn't signed up yet.");
        require(!isUserNameTaken(newUsername), "Username is already taken.");
        string memory oldUsername = getUsername(msg.sender);
        delete userAddresses[users[msg.sender].username];
        users[msg.sender].username = newUsername;
        userAddresses[newUsername] = msg.sender;
        emit UsernameUpdated(newUsername, oldUsername, msg.sender);
        return true;
    }

    function getUsername(address userAddress) public view returns (string memory) {
        require (hasUserSignedUp(userAddress), "User hasn't signed up yet.");
        return users[userAddress].username;
    }

    function getUserAddress(string memory username) public view returns (address) {
        return userAddresses[username];
    }

    function hasUserSignedUp(address userAddress) public view returns (bool) {
        return users[userAddress].signedUp;
    }

    function isUserNameTaken(string memory username) public view returns (bool) {
        if (getUserAddress(username)!=address(0))
            return true;
        return false;
    }

    function getUserDateOfRegister(address userAddress) public view returns (uint) {
        require (hasUserSignedUp(userAddress), "User hasn't signed up yet.");
        return users[userAddress].timestamp;
    }

}