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BasicAuth.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract BasicAuth {
    struct User {
        string username;
        bool exists;
    }

    // Mapping from user's Ethereum address to their User data
    mapping(address => User) private users;

    // Event to be emitted upon successful registration
    event UserRegistered(address userAddress, string username);

    // Event to be emitted upon successful login
    event UserLoggedIn(address userAddress);

    // Function to register a new user
    function register(string calldata _username) external {
        require(!users[msg.sender].exists, "User already exists");

        users[msg.sender] = User({
            username: _username,
            exists: true
        });

        emit UserRegistered(msg.sender, _username);
    }

    // Function for a user to 'log in'
    function login() external {
        require(users[msg.sender].exists, "User does not exist");
        emit UserLoggedIn(msg.sender);
    }

    // Function to get the current user's username
    function getUsername() external view returns (string memory) {
        require(users[msg.sender].exists, "User does not exist");
        return users[msg.sender].username;
    }
}