// public/script.js
// Handles frontend logic for login, registration, and todo management
// Version 3: Improved error handling message for JSON parse errors

document.addEventListener('DOMContentLoaded', () => {
    const path = window.location.pathname;

    // --- Page-Specific Logic ---
    if (path === '/login' || path.endsWith('login.html')) {
        setupLoginForm();
    } else if (path === '/register' || path.endsWith('register.html')) {
        setupRegisterForm();
    } else if (path === '/' || path.endsWith('index.html')) {
        // Main page logic: Check login status and set up UI accordingly
        checkLoginStatusAndSetupPage();
    }
});

// --- Authentication Form Handlers (Unchanged from previous version) ---

function setupLoginForm() {
    const loginForm = document.getElementById('login-form');
    const errorMessage = document.getElementById('error-message');

    if (loginForm) {
        loginForm.addEventListener('submit', async (event) => {
            event.preventDefault(); // Prevent default form submission
            errorMessage.textContent = ''; // Clear previous errors

            const username = loginForm.username.value;
            const password = loginForm.password.value;

            try {
                const response = await fetch('/api/auth/login', {
                    method: 'POST',
                    headers: { 'Content-Type': 'application/json' },
                    body: JSON.stringify({ username, password }),
                });

                const result = await response.json();

                if (response.ok) {
                    // Login successful, redirect to the main todo page
                    window.location.href = '/'; // Redirect to home page
                } else {
                    // Display error message from server
                    errorMessage.textContent = result.message || 'Login fehlgeschlagen.';
                }
            } catch (error) {
                console.error('Login request failed:', error);
                errorMessage.textContent = 'Ein Netzwerkfehler ist aufgetreten. Bitte versuchen Sie es erneut.';
            }
        });
    }
}

function setupRegisterForm() {
    const registerForm = document.getElementById('register-form');
    const errorMessage = document.getElementById('error-message');
    const successMessage = document.getElementById('success-message');


    if (registerForm) {
        registerForm.addEventListener('submit', async (event) => {
            event.preventDefault();
            errorMessage.textContent = '';
            successMessage.textContent = '';


            const username = registerForm.username.value;
            const password = registerForm.password.value;
            const confirmPassword = registerForm['confirm-password'].value;

            if (password !== confirmPassword) {
                errorMessage.textContent = 'Passwörter stimmen nicht überein.';
                return;
            }

            try {
                const response = await fetch('/api/auth/register', {
                    method: 'POST',
                    headers: { 'Content-Type': 'application/json' },
                    body: JSON.stringify({ username, password }),
                });

                const result = await response.json();

                if (response.ok) {
                     successMessage.textContent = result.message + ' Sie werden zum Login weitergeleitet...';
                     registerForm.reset(); // Clear the form
                     // Redirect to login page after a short delay
                     setTimeout(() => {
                         window.location.href = '/login';
                     }, 2000); // 2 seconds delay
                } else {
                    errorMessage.textContent = result.message || 'Registrierung fehlgeschlagen.';
                }
            } catch (error) {
                console.error('Registration request failed:', error);
                errorMessage.textContent = 'Ein Netzwerkfehler ist aufgetreten. Bitte versuchen Sie es erneut.';
            }
        });
    }
}

// --- Main Page Logic (index.html) ---

// Global references for main page elements
let todoAppContent = null;
let loginPrompt = null;
let authButtonContainer = null;
let todoList = null;
let errorMessage = null; // Error message specifically for todo actions

// Function to check login status and set up the main page UI
async function checkLoginStatusAndSetupPage() {
    // Get references to the main containers
    todoAppContent = document.getElementById('todo-app-content');
    loginPrompt = document.getElementById('login-prompt');
    authButtonContainer = document.getElementById('auth-button-container');
    todoList = document.getElementById('todo-list'); // Needed for clearing/rendering
    errorMessage = document.getElementById('error-message'); // Error display within todo-app-content

    if (!todoAppContent || !loginPrompt || !authButtonContainer || !todoList || !errorMessage) {
        console.error('Essential page elements not found!');
        return;
    }

    try {
        const response = await fetch('/api/auth/status');
        // Check if response is ok and content type is JSON before parsing
        if (response.ok && response.headers.get('Content-Type')?.includes('application/json')) {
            const status = await response.json();
            if (status.loggedIn) {
                updateUIForLoggedIn(status.username);
                setupTodoListeners(); // Set up listeners only if logged in
                await fetchTodos(); // Fetch todos only if logged in
            } else {
                updateUIForLoggedOut();
            }
        } else {
             // Handle non-JSON or error responses from /status endpoint
             console.error('Unexpected response from /api/auth/status:', response.status, response.statusText);
             updateUIForLoggedOut(); // Fallback to logged out state
             errorMessage.textContent = 'Fehler beim Prüfen des Login-Status (Serverantwort).';
        }
    } catch (error) {
        console.error('Error checking login status:', error);
        // Show logged out state as a fallback in case of network error etc.
        updateUIForLoggedOut();
        errorMessage.textContent = 'Fehler beim Prüfen des Login-Status (Netzwerk/Skript).';
    }
}

// Update UI for Logged-In User
function updateUIForLoggedIn(username) {
    if (!todoAppContent || !loginPrompt || !authButtonContainer) return;

    // Show Todo content, hide login prompt
    todoAppContent.style.display = 'block';
    loginPrompt.style.display = 'none';

    // Update Auth Button to "Abmelden"
    authButtonContainer.innerHTML = ''; // Clear previous button
    const logoutButton = document.createElement('button');
    logoutButton.id = 'logout-button';
    logoutButton.textContent = 'Abmelden';
    logoutButton.classList.add('btn', 'btn-secondary');
    logoutButton.addEventListener('click', handleLogout); // Add listener directly
    authButtonContainer.appendChild(logoutButton);

    // Optional: Display username (e.g., next to the button or in the header)
    const welcomeMessage = document.createElement('span');
    welcomeMessage.textContent = `Hallo, ${username}! `;
    welcomeMessage.style.marginRight = '10px';
    authButtonContainer.prepend(welcomeMessage); // Add before button
}

// Update UI for Logged-Out User
function updateUIForLoggedOut() {
    if (!todoAppContent || !loginPrompt || !authButtonContainer || !todoList) return;

     // Hide Todo content, show login prompt
    todoAppContent.style.display = 'none';
    loginPrompt.style.display = 'block';

     // Update Auth Button to "Anmelden"
    authButtonContainer.innerHTML = ''; // Clear previous button
    const loginLink = document.createElement('a'); // Use a link for navigation
    loginLink.href = '/login';
    loginLink.textContent = 'Anmelden';
    loginLink.classList.add('btn'); // Style as button
    authButtonContainer.appendChild(loginLink);

    // Clear any existing todos from the list
    todoList.innerHTML = '';
    // Clear any previous error messages related to todos
    if(errorMessage) errorMessage.textContent = '';
}


// Set up event listeners for the todo form and list (only when logged in)
function setupTodoListeners() {
    const addTodoForm = document.getElementById('add-todo-form');
    const newTodoInput = document.getElementById('new-todo-input');
    // todoList reference is already global

    // Add Todo Form Submission
    if (addTodoForm) {
        // Remove previous listener if any to prevent duplicates
        addTodoForm.onsubmit = null;
        addTodoForm.onsubmit = async (event) => {
            event.preventDefault();
            const task = newTodoInput.value.trim();
            if (task) {
                await addTodoItem(task);
                newTodoInput.value = ''; // Clear input field
            }
        };
    }

    // Todo List Click Handler (for checkboxes and delete buttons)
    if (todoList) {
        // Remove previous listener if any
        todoList.onclick = null;
        todoList.onclick = async (event) => {
            const target = event.target;
            const todoItem = target.closest('.todo-item'); // Find the parent li element

            if (!todoItem) return; // Click was not inside a todo item

            const todoId = todoItem.dataset.id;

            // Handle Checkbox Click (Toggle Completion)
            if (target.type === 'checkbox') {
                const isCompleted = target.checked;
                await updateTodoStatus(todoId, isCompleted);
            }

            // Handle Delete Button Click
            if (target.classList.contains('btn-delete')) {
                await deleteTodoItem(todoId);
            }
        };
    }
}

// Handle Logout Process
async function handleLogout() {
    if(errorMessage) errorMessage.textContent = ''; // Clear errors
    try {
        const response = await fetch('/api/auth/logout', { method: 'POST' });
        if (response.ok) {
            // Logout successful, update UI to logged-out state
            updateUIForLoggedOut();
        } else {
             // Try to parse error response as JSON
             let result = { message: 'Logout fehlgeschlagen (unbekannter Fehler).' };
             try {
                 result = await response.json();
             } catch (e) {
                 console.error("Could not parse logout error response as JSON");
             }
             if(errorMessage) errorMessage.textContent = result.message || 'Logout fehlgeschlagen.';
        }
    } catch (error) {
        console.error('Logout failed:', error);
         if(errorMessage) errorMessage.textContent = 'Logout fehlgeschlagen. Netzwerkfehler.';
    }
}


// --- API Call Functions (Modified Error Handling) ---

// Fetch Todos from Server
async function fetchTodos() {
    if(errorMessage) errorMessage.textContent = '';
    try {
        const response = await fetch('/api/todos'); // GET request by default

        // Check for auth errors specifically (though status check should prevent this)
        if (response.status === 401 || response.status === 403) {
             console.warn('Auth error during fetchTodos - should have been caught by status check.');
             updateUIForLoggedOut(); // Update UI to logged out state
             return; // Stop processing
         }

        if (!response.ok) {
            // Handle other potential errors (e.g., 500 server error)
            const errorData = await response.json().catch(() => ({ message: 'Unbekannter Serverfehler beim Laden der Todos' }));
            throw new Error(errorData.message || `HTTP error! status: ${response.status}`);
        }

        // Ensure response is JSON before parsing
        if (!response.headers.get('Content-Type')?.includes('application/json')) {
             throw new Error('Unerwartete Serverantwort (kein JSON) beim Laden der Todos.');
        }

        const todos = await response.json();
        renderTodoList(todos);
    } catch (error) {
        console.error('Error fetching todos:', error);
        if(errorMessage) errorMessage.textContent = `Fehler beim Laden der Todos: ${error.message}`;
        // Do NOT switch to logged-out UI here, as it might be a temporary server issue
    }
}

// Add a New Todo Item
async function addTodoItem(task) {
    if(errorMessage) errorMessage.textContent = '';
    let response;
    console.log('Adding todo item:', task);
    try {
        response = await fetch('/api/todos/newEntry', {
            method: 'POST',
            headers: { 
                'Content-Type': 'application/json',
                'Accept': 'application/json' // Explicitly request JSON response
            },
            credentials: 'same-origin', // Explicitly include cookies/credentials
            body: JSON.stringify({ task }),
        });

        console.log('Response status:', response.status);
        // If we got redirected to the login page, the status might still be 200
        // but the content type will be text/html instead of application/json
        const contentType = response.headers.get('Content-Type');
        console.log('Response headers:', contentType);
        
        if (contentType && contentType.includes('text/html')) {
            // We got HTML instead of JSON - this means auth failed and we were redirected
            console.warn('Auth failed - received HTML instead of JSON');
            updateUIForLoggedOut();
            return;
        }

        if (response.status === 401 || response.status === 403) {
            console.warn('Auth error during addTodoItem.');
            updateUIForLoggedOut();
            return;
        }

        // Clone and log response body for debugging
        console.log('Response body:', await response.clone().text());
        console.log('Response ok:', response.ok);
        
        if (!response.ok) {
            const result = await response.json().catch(() => ({ 
                message: `Serverfehler (${response.status}) beim Hinzufügen.` 
            }));
            throw new Error(result.message);
        }

        // We already checked content type above, so just parse JSON now
        const newTodo = await response.json();
        addTodoToDOM(newTodo);
    } catch (error) {
        console.error('Error adding todo:', error);
        let displayError = `Fehler beim Hinzufügen: ${error.message}`;
        if (error instanceof SyntaxError) {
            displayError = 'Fehler beim Hinzufügen: Unerwartete Antwort vom Server.';
        }
        if(errorMessage) errorMessage.textContent = displayError;
    }
}

// Update Todo Completion Status
async function updateTodoStatus(id, isCompleted) {
    if(errorMessage) errorMessage.textContent = '';
    let response;
    try {
        response = await fetch(`/api/todos/${id}`, {
            method: 'PUT',
            headers: { 'Content-Type': 'application/json' },
            body: JSON.stringify({ is_completed: isCompleted }),
        });
         if (response.status === 401 || response.status === 403) {
             console.warn('Auth error during updateTodoStatus.');
             updateUIForLoggedOut();
             return;
         }
        if (!response.ok) {
             const result = await response.json().catch(() => ({ message: `Serverfehler (${response.status}) beim Aktualisieren.` }));
             throw new Error(result.message);
        }

        if (!response.headers.get('Content-Type')?.includes('application/json')) {
             throw new Error('Unerwartete Serverantwort (kein JSON) nach dem Aktualisieren.');
        }

        const updatedTodo = await response.json();
        // Update the specific todo item in the DOM
        const todoElement = todoList.querySelector(`li[data-id="${id}"]`);
        if (todoElement) {
            todoElement.classList.toggle('completed', updatedTodo.is_completed);
            const checkbox = todoElement.querySelector('input[type="checkbox"]');
            if(checkbox) checkbox.checked = updatedTodo.is_completed;
        }
    } catch (error) {
        console.error('Error updating todo status:', error);
        let displayError = `Fehler beim Aktualisieren: ${error.message}`;
         if (error instanceof SyntaxError) {
             displayError = 'Fehler beim Aktualisieren: Unerwartete Antwort vom Server.';
         }
        if(errorMessage) errorMessage.textContent = displayError;

         // Optional: Revert checkbox state on error
         const todoElement = todoList.querySelector(`li[data-id="${id}"]`);
         if(todoElement) {
             const checkbox = todoElement.querySelector('input[type="checkbox"]');
             if(checkbox) checkbox.checked = !isCompleted; // Revert state
         }
    }
}

// Delete a Todo Item
async function deleteTodoItem(id) {
    if(errorMessage) errorMessage.textContent = '';
    try {
        const response = await fetch(`/api/todos/${id}`, {
            method: 'DELETE',
        });
        if (response.status === 401 || response.status === 403) {
            console.warn('Auth error during deleteTodoItem.');
            updateUIForLoggedOut();
            return;
        }
         // Allow 204 No Content (which might not have a JSON body)
         if (!response.ok && response.status !== 204) {
             const result = await response.json().catch(() => ({ message: `Serverfehler (${response.status}) beim Löschen.` }));
             throw new Error(result.message);
        }
        // Remove the todo item from the DOM
        const todoElement = todoList.querySelector(`li[data-id="${id}"]`);
        if (todoElement) {
            todoElement.remove();
            // Check if list is now empty and show message if needed
            if (todoList.children.length === 0) {
                renderTodoList([]); // Will add the "empty" message
            }
        }
    } catch (error) {
        console.error('Error deleting todo:', error);
         let displayError = `Fehler beim Löschen: ${error.message}`;
         if (error instanceof SyntaxError) { // Should not happen for DELETE usually, but good practice
             displayError = 'Fehler beim Löschen: Unerwartete Antwort vom Server.';
         }
        if(errorMessage) errorMessage.textContent = displayError;
    }
}


// --- DOM Manipulation (Rendering Todos) ---

// Render the entire list of todos
function renderTodoList(todos) {
    if (!todoList) return;
    todoList.innerHTML = ''; // Clear existing list
    if (todos.length === 0) {
        // Display a message within the todo list area if empty
        const emptyMsg = document.createElement('p');
        emptyMsg.textContent = 'Noch keine Aufgaben vorhanden.';
        emptyMsg.style.textAlign = 'center';
        emptyMsg.style.color = '#888';
        emptyMsg.style.padding = '20px 0';
        todoList.appendChild(emptyMsg);
    } else {
        todos.forEach(todo => addTodoToDOM(todo));
    }
}

// Add a single todo item to the DOM
function addTodoToDOM(todo) {
     if (!todoList) return;

    // Remove the "no tasks" message if it exists and we are adding the first item
    const emptyMsg = todoList.querySelector('p');
    if (emptyMsg && todoList.children.length === 1 && emptyMsg.parentNode === todoList) {
        emptyMsg.remove();
    }


    const li = document.createElement('li');
    li.classList.add('todo-item');
    li.dataset.id = todo.id; // Store todo ID on the element
    if (todo.is_completed) {
        li.classList.add('completed');
    }

    const checkbox = document.createElement('input');
    checkbox.type = 'checkbox';
    checkbox.checked = todo.is_completed;

    const span = document.createElement('span');
    span.textContent = todo.task;

    const deleteButton = document.createElement('button');
    deleteButton.textContent = 'Löschen';
    deleteButton.classList.add('btn-delete'); // Use specific class for deletion

    li.appendChild(checkbox);
    li.appendChild(span);
    li.appendChild(deleteButton);

    // Prepend to show newest todos first
    todoList.prepend(li);
}