/**
 * Configuration and Constants
 */
const CHARSET = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789';
const BASE_SLUG_LENGTH = 4;

const CORS_HEADERS = {
  'Access-Control-Allow-Origin': '*',
};
const OPTIONS_HEADERS = {
  ...CORS_HEADERS,
  'Access-Control-Allow-Methods': 'POST',
  'Access-Control-Allow-Headers': 'Content-Type',
};
const JSON_HEADERS = {
  ...CORS_HEADERS,
  'Content-Type': 'application/json',
};

/**
 * Generates a random string of a given length from the character set.
 * @param {number} length The desired length of the slug.
 * @returns {string} The generated slug.
 */
const generateSlug = (length) => {
  return [...Array(length)]
    .map(() => CHARSET[Math.floor(Math.random() * CHARSET.length)])
    .join('');
};

/**
 * Tries to parse a string into a valid URL, prepending 'https://' if necessary.
 * @param {string} urlString The string to normalize.
 * @returns {string|undefined} The normalized URL as a string, or undefined if invalid.
 */
const normalizeUrl = (urlString) => {
  try {
    return new URL(urlString).href;
  } catch {
    try {
      return new URL('https://' + urlString).href;
    } catch {
      return undefined;
    }
  }
};

/**
 * Verifies the user's reCAPTCHA token with Google's API.
 * @param {string} token The reCAPTCHA token from the client.
 * @param {string} secretKey The reCAPTCHA secret key.
 * @returns {Promise<boolean>} A promise that resolves to true if the token is valid.
 */
async function verifyRecaptcha(token, secretKey) {
  if (!token || !secretKey) {
    return false;
  }
  const response = await fetch('https://www.google.com/recaptcha/api/siteverify', {
    method: 'POST',
    headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
    body: `secret=${secretKey}&response=${token}`,
  });
  const data = await response.json();
  return data.success;
}

/**
 * Handles the creation of a new shortened link.
 * @param {Request} request The incoming request object.
 * @param {object} env The environment bindings.
 * @returns {Promise<Response>} A promise that resolves to a Response object.
 */
async function handleCreateRequest(request, env) {
  try {
    const { url: targetUrl, token } = await request.json();

    if (!await verifyRecaptcha(token, env.RECAPCHA_KEY)) {
      return new Response('CAPTCHA verification failed.', { status: 403, headers: CORS_HEADERS });
    }

    const normalizedTarget = normalizeUrl(targetUrl);
    if (!normalizedTarget) {
      return new Response('Invalid URL provided.', { status: 400, headers: CORS_HEADERS });
    }

    let slugLength = BASE_SLUG_LENGTH;
    let slug;
    // Keep generating slugs until a unique one is found.
    // If a collision occurs, increase the slug length and try again.
    while (true) {
      slug = generateSlug(slugLength);
      if (!(await env.EV.get(slug))) {
        break; // Found a unique slug.
      }
      slugLength++; // Collision detected, increase length for the next attempt.
    }

    await env.EV.put(slug, normalizedTarget);

    const { origin } = new URL(request.url);
    const responsePayload = {
      slug,
      target: normalizedTarget,
      shortUrl: `${origin}/${slug}`,
    };

    return new Response(JSON.stringify(responsePayload), { headers: JSON_HEADERS });
  } catch {
    return new Response('Invalid request payload.', { status: 400, headers: CORS_HEADERS });
  }
}

/**
 * Handles redirecting a short URL to its target URL.
 * @param {string} pathname The pathname from the request URL.
 * @param {object} env The environment bindings.
 * @returns {Promise<Response|null>} A redirect Response or null if the slug is not found.
 */
async function handleRedirectRequest(pathname, env) {
  const slug = pathname.slice(1);
  if (slug) {
    const targetUrl = await env.EV.get(slug);
    if (targetUrl) {
      return Response.redirect(targetUrl, 302);
    }
  }
  return null;
}

/**
 * Main request handler for the Cloudflare Worker.
 */
export async function onRequest({ request, env, next }) {
  if (request.method === 'OPTIONS') {
    return new Response(null, { headers: OPTIONS_HEADERS });
  }

  const url = new URL(request.url);

  if (url.pathname === '/api/create' && request.method === 'POST') {
    return handleCreateRequest(request, env);
  }

  const redirectResponse = await handleRedirectRequest(url.pathname, env);
  if (redirectResponse) {
    return redirectResponse;
  }

  return next();
}