const moment = require("moment");
const db = require("../dbConfig");
const crypto = require("crypto");
const cron = require("node-cron");

/**
 * Checks if a given string is a valid URL format.
 * @param {string} url - The input string to validate.
 * @returns {boolean} - Returns true if valid URL format, otherwise false.
 */
function isValidUrl(url) {
  try {
    new URL(url);
    return true;
  } catch (e) {
    return false;
  }
}

/**
 * Generates a deterministic short code based on long URL, expiry date, and user ID.
 * Uses MD5 hashing, encodes the result in base64, and slices to desired length.
 * @param {string} longUrl - Original input URL.
 * @param {number} codeLength - Desired length of output short code.
 * @param {string} expiryDate - Expiry date used in code generation.
 * @param {string} user_id - User identifier to namespace the code.
 * @returns {string} - Base64-based truncated hash used as short code.
 */
const generateShortCode = (
  longUrl,
  codeLength,
  expiryDate,
  userId,
  customizedShortnerSlug
) => {
  const dataToHash =
    longUrl + "_" + expiryDate + "_" + userId + "_" + customizedShortnerSlug;
  return crypto
    .createHash("md5")
    .update(dataToHash)
    .digest("base64")
    .substring(0, codeLength);
};

/**
 * Creates and stores a new short code mapping for a given URL.
 * Validates input values and optionally supports user-scoped or custom slugs.
 * @param {object} params - Input parameters for short code creation.
 * @param {string} params.longUrl - Original full URL.
 * @param {number} params.expiryInDays - Days until expiry (0–30 allowed).
 * @param {number} params.codeLength - Short code length (8–30 allowed).
 * @param {string} [params.user_id=""] - Optional user ID for namespace isolation.
 * @param {string} [params.customized_shortner_slug=""] - Optional custom alias.
 * @returns {Promise<string>} - Returns existing or newly created code/slug.
 * @throws {Error} - If input validation fails or database errors occur.
 */
const createShortUrl = async ({
  longUrl = "",
  expiryInDays = 30,
  codeLength = 10,
  userId = "",
  customizedShortnerSlug = "",
}) => {
  if (!String(longUrl).trim() || !isValidUrl(longUrl)) {
    throw new Error("Long url required.");
  }
  if (!Number.isInteger(codeLength)) {
    throw new Error("Code length must be valid integer value.");
  } else if (codeLength < 8 || codeLength > 30) {
    throw new Error("Code length must be between 8 to 30 characters.");
  }
  if (!Number.isInteger(expiryInDays)) {
    throw new Error("Expiry in days must be valid integer value.");
  } else if (expiryInDays < 0 || expiryInDays > 30) {
    throw new Error("Expiry in days must be between 0 to 30 days.");
  }

  const expiryDate = moment()
    .add(expiryInDays, "days")
    .endOf("day")
    .format("YYYY-MM-DD HH:mm:ss");

  // Avoid duplicates: return existing code if it already exists
  const existing = await db.findByLongUrl(
    longUrl,
    expiryDate,
    userId,
    customizedShortnerSlug
  );
  if (existing) return existing.customized_shortner_slug || existing.short_code;

  const shortCode = generateShortCode(
    longUrl,
    codeLength,
    expiryDate,
    userId,
    customizedShortnerSlug
  );
  const res = await db.insertUrl(
    longUrl,
    shortCode,
    expiryDate,
    String(userId).trim(),
    String(customizedShortnerSlug).trim()
  );

  return customizedShortnerSlug || shortCode;
};

/**
 * Retrieves the original long URL using a short code or custom slug.
 * @param {string} shortCode - Code or slug used to look up the original URL.
 * @param {string} [userId=""] - Optional user ID for user-specific resolution.
 * @returns {Promise<string>} - Returns the original URL if found and valid.
 * @throws {Error} - If lookup fails or URL is expired.
 */
const getOriginalUrl = async (shortCode, userId = "") => {
  try {
    const longUrl = await db.findByShortCode(shortCode, userId);
    return longUrl;
  } catch (error) {
    throw error;
  }
};

/**
 * Starts a cron job that runs daily at midnight.
 * Cleans up expired short URL entries from the database.
 */
const startScheduler = () => {
  console.log("Starting expiration cleanup scheduler...");
  cron.schedule("0 0 * * *", () => {
    console.log("[Scheduler] Running deletion job...");
    db.deleteAllExpired().then(console.log).catch(console.error);
  });
};

module.exports = {
  createShortUrl,
  getOriginalUrl,
  startScheduler,
  getAllData: db.getAllData,
  getPaginatedData: db.getPaginatedData,
};