/**
 * Webex Service App OAuth singleton.
 *
 * Mirrors the collabFinder reference but ported to CommonJS + this project's
 * structured logger. Once a tokens file exists at `tokensFilePath`, this
 * singleton transparently refreshes the access token (Cisco rotates the
 * refresh token on every refresh, so we always write back the new pair).
 *
 * Bootstrap is handled out-of-band by scripts/seedWebexTokens.js — this
 * singleton refuses to call Webex without a refresh token already on disk.
 */

const axios = require('axios');
const fs = require('fs').promises;
const path = require('path');
const logger = require('../../utils/logger');

const TOKEN_URL = 'https://webexapis.com/v1/access_token';
const SAFETY_BUFFER_MS = 5 * 60 * 1000; // refresh 5 min before stated expiry

class WebexServiceAppAuth {
  static #instance = null;

  /**
   * Resolve a singleton bound to the given config. Subsequent calls ignore
   * the args and return the original instance — caller controls the lifetime
   * by calling resetForTests() between unit tests.
   */
  static getInstance(opts = {}) {
    if (!WebexServiceAppAuth.#instance) {
      WebexServiceAppAuth.#instance = new WebexServiceAppAuth(opts);
    }
    return WebexServiceAppAuth.#instance;
  }

  static resetForTests() {
    WebexServiceAppAuth.#instance = null;
  }

  constructor({
    clientId = process.env.WEBEX_CLIENT_ID,
    clientSecret = process.env.WEBEX_CLIENT_SECRET,
    tokensFilePath = process.env.WEBEX_TOKENS_PATH ||
      path.join(process.cwd(), 'tokens', 'webex-service-tokens.json'),
    httpClient = axios,
  } = {}) {
    if (!clientId) {
      throw new Error('WEBEX_CLIENT_ID is required (set it in environment variables)');
    }
    if (!clientSecret) {
      throw new Error('WEBEX_CLIENT_SECRET is required (set it in environment variables)');
    }

    this.clientId = clientId;
    this.clientSecret = clientSecret;
    // Resolve to an absolute path so logs and fs ops are unambiguous whether
    // running on the host or inside Docker.
    this.tokensFilePath = path.resolve(tokensFilePath);
    this.http = httpClient;

    this.accessToken = null;
    this.refreshToken = null;
    this.expiresAt = 0;

    logger.debug('WebexServiceAppAuth initialized', { tokensFilePath: this.tokensFilePath });
  }

  async loadTokens() {
    try {
      const raw = await fs.readFile(this.tokensFilePath, 'utf8');
      const tokens = JSON.parse(raw);

      this.accessToken = tokens.accessToken || null;
      this.refreshToken = tokens.refreshToken || null;
      this.expiresAt = tokens.expiresAt || 0;

      logger.info('Webex tokens loaded from file', { tokensFilePath: this.tokensFilePath });
    } catch (err) {
      if (err.code === 'ENOENT') {
        logger.warn('No Webex tokens file found — run `npm run webex:seed` to bootstrap', {
          tokensFilePath: this.tokensFilePath,
        });
      } else {
        logger.error('Failed to load Webex tokens file', {
          tokensFilePath: this.tokensFilePath,
          error: err.message,
        });
      }
      throw err;
    }
  }

  async saveTokens() {
    const payload = {
      accessToken: this.accessToken,
      refreshToken: this.refreshToken,
      expiresAt: this.expiresAt,
      updatedAt: new Date().toISOString(),
    };

    await fs.mkdir(path.dirname(this.tokensFilePath), { recursive: true });
    await fs.writeFile(this.tokensFilePath, JSON.stringify(payload, null, 2), 'utf8');
    logger.debug('Webex tokens saved', { tokensFilePath: this.tokensFilePath });
  }

  /**
   * Exchange the current refresh token for a fresh pair. Persists the result.
   * Throws if no refresh token is available.
   */
  async refresh() {
    if (!this.refreshToken) {
      throw new Error(
        'No refresh token available. ' + 'Bootstrap initial tokens first via `npm run webex:seed`.'
      );
    }

    const params = new URLSearchParams({
      grant_type: 'refresh_token',
      client_id: this.clientId,
      client_secret: this.clientSecret,
      refresh_token: this.refreshToken,
    });

    try {
      const response = await this.http.post(TOKEN_URL, params.toString(), {
        headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
        timeout: 10000,
      });

      const data = response.data || {};
      this.accessToken = data.access_token;
      this.refreshToken = data.refresh_token || this.refreshToken;
      this.expiresAt = Date.now() + Number(data.expires_in || 0) * 1000 - SAFETY_BUFFER_MS;

      await this.saveTokens();

      logger.info('Webex tokens refreshed', { expiresInSec: data.expires_in });
      return this.accessToken;
    } catch (err) {
      const status = err.response?.status;
      const detail = err.response?.data ? JSON.stringify(err.response.data) : err.message;
      logger.error('Webex token refresh failed', { status, detail });

      if (status === 400 || status === 401) {
        throw new Error(
          'Webex refresh token rejected (400/401). ' +
            'It may be expired or revoked — re-seed via `npm run webex:seed`.',
          { cause: err }
        );
      }
      throw err;
    }
  }

  /**
   * Return a currently valid access token. Lazy-loads the tokens file on
   * first use and refreshes automatically when within the safety buffer.
   */
  async getAccessToken() {
    if (!this.accessToken && !this.refreshToken) {
      await this.loadTokens();
    }

    if (!this.accessToken || Date.now() >= this.expiresAt) {
      logger.debug('Webex access token missing or expired — refreshing');
      return this.refresh();
    }

    return this.accessToken;
  }

  async forceRefresh() {
    logger.warn('Forcing Webex token refresh');
    return this.refresh();
  }

  clearTokens() {
    this.accessToken = null;
    this.refreshToken = null;
    this.expiresAt = 0;
  }
}

module.exports = WebexServiceAppAuth;