class Forum {
    constructor() {
        this._users = [];
        this._questions = [];
        this._id = 1;
    }

    register(username, password, repeatPassword, email) {

        if (username === '' || password === '' || repeatPassword === '' || email === '') {
            throw new Error('Input can not be empty');
        }

        if (password !== repeatPassword) {
            throw new Error('Passwords do not match');
        }

        const found = this._users.find(x => x.username === username || x.email === email);
        if (found) {
            throw new Error('This user already exists!');
        }

        this._users.push({ username, password, email, logged: false });

        return `${username} with ${email} was registered successfully!`
    }

    login(username, password) {

        const user = this._users.find(x => x.username === username);
        if (!user) {
            throw new Error('There is no such user');
        }

        if (user.password === password) {

            user.logged = true;
            return 'Hello! You have logged in successfully';
        }
    }

    logout(username, password) {

        const user = this._users.find(x => x.username === username);
        if (!user) {
            throw new Error('There is no such user');
        }

        if (user.password === password && user.logged === true) {

            user.logged = false;
            return 'You have logged out successfully';
        }
    }
    postQuestion(username, question) {

        const found = this._users.find(x => x.username === username && x.logged === true);
        if (!found) {
            throw new Error('You should be logged in to post questions');
        }

        if (question === '') {
            throw new Error('Invalid question');
        }

        const currentId = this._id;
        this._questions.push({ question, id: currentId, username, answers: [] });
        this._id += 1;

        return 'Your question has been posted successfully';
    }

    postAnswer(username, questionId, answer) {
        const found = this._users.find(x => x.username === username && x.logged === true);
        if (!found) {
            throw new Error('You should be logged in to post answers');
        }

        if (answer === '') {
            throw new Error('Invalid answer');
        }

        const question = this._questions.find(x => x.id === questionId);
        if (!question) {
            throw new Error('There is no such question');
        }

        question.answers.push({ answer, username });
        return 'Your answer has been posted successfully';
    }

    showQuestions() {
        let result = '';
        for (const question of this._questions) {
            result += `\nQuestion ${question.id} by ${question.username}: ${question.question}`
            for (const answer of question.answers) {
                result += `\n---${answer.username}: ${answer.answer}`;
            }
        }
        this._questions

        return result.trim();
    }
}