main.cpp

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

using namespace std;

// Check if username already exists
bool userExists(const string &username) {
    ifstream fin("users.txt");
    string line, fileUser;

    if (!fin) return false;

    while (getline(fin, line)) {
        if (line.empty()) continue;

        size_t pos = line.find(':');
        if (pos == string::npos) continue;

        fileUser = line.substr(0, pos);

        if (fileUser == username) {
            return true;
        }
    }
    return false;
}

// Registration function
void registerUser() {
    string username, password;

    cout << "Enter username: ";
    getline(cin, username);

    cout << "Enter password: ";
    getline(cin, password);

    if (username.empty() || password.empty()) {
        cout << "Error: Fields cannot be empty!\n";
        return;
    }

    if (username.length() < 3 || password.length() < 3) {
        cout << "Error: Minimum 3 characters required!\n";
        return;
    }

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

    ofstream fout("users.txt", ios::app);

    if (!fout) {
        cout << "Error: File cannot be opened!\n";
        return;
    }

    fout << username << ":" << password << endl;

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

// Login function
bool loginUser() {
    string username, password, line;
    string fileUser, filePass;

    cout << "Enter username: ";
    getline(cin, username);

    cout << "Enter password: ";
    getline(cin, password);

    ifstream fin("users.txt");

    if (!fin) {
        cout << "Error: No user database found!\n";
        return false;
    }

    while (getline(fin, line)) {
        if (line.empty()) continue;

        size_t pos = line.find(':');
        if (pos == string::npos) continue;

        fileUser = line.substr(0, pos);
        filePass = line.substr(pos + 1);

        if (fileUser == username && filePass == password) {
            cout << "Login successful!\n";
            return true;
        }
    }

    cout << "Invalid username or password!\n";
    return false;
}

int main() {
    int choice;

    do {
        cout << "\n1. Register\n";
        cout << "2. Login\n";
        cout << "3. Exit\n";
        cout << "Enter choice: ";

        cin >> choice;
        cin.ignore();

        switch (choice) {
            case 1:
                registerUser();
                break;

            case 2:
                loginUser();
                break;

            case 3:
                cout << "Exiting program...\n";
                break;

            default:
                cout << "Invalid choice!\n";
        }

    } while (choice != 3);

    return 0;
}