main.cpp

#include <iostream>
#include <fstream>
#include <string>
#include <limits>

using namespace std;

const string USER_FILE = "users.txt";

// Check whether username already exists
bool usernameExists(const string& username) {
    ifstream file(USER_FILE);

    string storedUsername;
    string storedPassword;

    while (file >> storedUsername >> storedPassword) {
        if (storedUsername == username) {
            return true;
        }
    }

    return false;
}

// Register a new user
void registerUser() {
    string username;
    string password;

    cout << "\n========== USER REGISTRATION ==========\n";

    cout << "Enter username: ";
    cin >> username;

    if (username.empty()) {
        cout << "Error: Username cannot be empty.\n";
        return;
    }

    if (usernameExists(username)) {
        cout << "Error: Username already exists.\n";
        return;
    }

    cout << "Enter password: ";
    cin >> password;

    if (password.empty()) {
        cout << "Error: Password cannot be empty.\n";
        return;
    }

    ofstream file(USER_FILE, ios::app);

    if (!file) {
        cout << "Error: Unable to open user database.\n";
        return;
    }

    file << username << " " << password << "\n";
    file.close();

    cout << "Registration successful!\n";
}

// Login an existing user
void loginUser() {
    string username;
    string password;

    cout << "\n============== USER LOGIN ==============\n";

    cout << "Enter username: ";
    cin >> username;

    cout << "Enter password: ";
    cin >> password;

    ifstream file(USER_FILE);

    if (!file) {
        cout << "No registered users found. Please register first.\n";
        return;
    }

    string storedUsername;
    string storedPassword;

    while (file >> storedUsername >> storedPassword) {
        if (storedUsername == username &&
            storedPassword == password) {

            cout << "Login successful! Welcome, "
                 << username << ".\n";

            file.close();
            return;
        }
    }

    file.close();

    cout << "Login failed: Invalid username or password.\n";
}

// Main menu
int main() {

    int choice;

    cout << "============================================\n";
    cout << "       CODEALPHA LOGIN & REGISTRATION       \n";
    cout << "============================================\n";

    while (true) {

        cout << "\n--------------- MAIN MENU ----------------\n";
        cout << "1. Register\n";
        cout << "2. Login\n";
        cout << "3. Exit\n";
        cout << "--------------------------------------------\n";
        cout << "Enter your choice: ";

        if (!(cin >> choice)) {
            cout << "Invalid input! Please enter a number.\n";

            cin.clear();
            cin.ignore(numeric_limits<streamsize>::max(), '\n');

            continue;
        }

        switch (choice) {

            case 1:
                registerUser();
                break;

            case 2:
                loginUser();
                break;

            case 3:
                cout << "\nThank you for using the system!\n";
                cout << "Goodbye!\n";
                return 0;

            default:
                cout << "Invalid choice! Please select 1, 2, or 3.\n";
        }
    }
}