const express = require("express");
const cors = require("cors");
const path = require("path");
const fs = require("fs").promises;

const app = express();
const PORT = 3000;

// Middleware
app.use(cors());
app.use(express.json());

// File path for todo persistence
const TODO_FILE = "todos.json";

// Load todos from file
async function loadTodos() {
  try {
    const data = await fs.readFile(TODO_FILE, "utf8");
    const parsed = JSON.parse(data);
    if (Array.isArray(parsed)) {
      return parsed;
    }
    return [];
  } catch (error) {
    // If file doesn't exist or is invalid, return empty array
    return [];
  }
}

// Save todos to file
async function saveTodos(todos) {
  try {
    await fs.writeFile(TODO_FILE, JSON.stringify(todos, null, 2));
  } catch (error) {
    console.error("Failed to save todos:", error);
  }
}

// Load todos on startup
let todos = [];
let nextId = 1;

async function initializeTodos() {
  todos = await loadTodos();
  if (todos.length > 0) {
    // Find the highest ID and set nextId accordingly
    nextId = Math.max(...todos.map(todo => todo.id)) + 1;
  }
}

// Initialize todos on startup
initializeTodos();

// GET /todos - get all todos
app.get("/todos", (req, res) => {
  res.json(todos);
});

// POST /todos - create a new todo
app.post("/todos", (req, res) => {
  const { text } = req.body;
  if (!text) {
    return res.status(400).json({ error: "Text is required" });
  }
  const newTodo = { id: nextId++, text };
  todos.push(newTodo);
  saveTodos(todos); // Save to file after adding
  res.status(201).json(newTodo);
});

// PUT /todos/:id - update a todo
app.put("/todos/:id", (req, res) => {
  const id = parseInt(req.params.id);
  const todo = todos.find((t) => t.id === id);
  if (!todo) return res.status(404).json({ error: "Todo not found" });
  const { text, completed } = req.body;
  if (text !== undefined) todo.text = text;
  if (completed !== undefined) todo.completed = completed;
  saveTodos(todos); // Save to file after updating
  res.json(todo);
});

// DELETE /todos/:id - delete a todo
app.delete("/todos/:id", (req, res) => {
  const id = parseInt(req.params.id);
  const index = todos.findIndex((t) => t.id === id);
  if (index === -1) return res.status(404).json({ error: "Todo not found" });
  const deleted = todos.splice(index, 1)[0];
  saveTodos(todos); // Save to file after deleting
  res.json(deleted);
});

// Reorder todos (client can send new order of ids)
// Expected body: { order: [1,2,3] } where each number is a todo id
app.post("/reorder", (req, res) => {
  const { order } = req.body;
  if (!Array.isArray(order)) {
    return res.status(400).json({ error: "Order must be an array of ids" });
  }
  // Build a new array in the requested order
  const orderedTodos = [];
  for (const id of order) {
    const item = todos.find((t) => t.id === id);
    if (item) orderedTodos.push(item);
  }
  // Replace the current todos array with the reordered version
  todos.length = 0; // empty the existing array
  todos.push(...orderedTodos);
  saveTodos(todos); // Save to file after reordering
  res.json(orderedTodos);
});

// Serve static files (index.html, etc.) from the same directory
app.use(express.static(path.join(__dirname)));

// Start server
app.listen(PORT, () => {
  console.log(`Todo API server running at http://localhost:${PORT}`);
});

// Create todos.json file if it doesn't exist
async function ensureTodoFileExists() {
  try {
    await fs.access(TODO_FILE);
  } catch (error) {
    // File doesn't exist, create it with empty array
    await saveTodos([]);
  }
}

ensureTodoFileExists();