import { getApps, initializeApp, cert, type App } from 'firebase-admin/app';
import { getAuth, type Auth } from 'firebase-admin/auth';
import { getFirestore, type Firestore } from 'firebase-admin/firestore';
import { getServerFirebaseProjectId } from './firebase-config';

/**
 * Server-only singleton initialization for Firebase Admin.
 *
 * Requirements:
 * - Singleton reuse across Next.js API route invocations.
 * - Server-only execution (never imported or bundled in client-side code).
 * - Fails closed if project configuration is missing.
 * - Requires explicit FIREBASE_PROJECT_ID with no fallback to production.
 * - Supports Google Application Default Credentials (ADC), explicit service account
 *   via FIREBASE_SERVICE_ACCOUNT or GOOGLE_APPLICATION_CREDENTIALS, and Firebase emulator.
 */

let _adminApp: App | null = null;

export function resetFirebaseAdminAppForTesting(): void {
  _adminApp = null;
}

export function getFirebaseAdminApp(): App {
  if (_adminApp) {
    return _adminApp;
  }

  const existingApps = getApps();
  if (existingApps.length > 0 && existingApps[0]) {
    _adminApp = existingApps[0];
    return _adminApp;
  }

  const projectId = getServerFirebaseProjectId();
  if (process.env.NEXT_PUBLIC_FIREBASE_EMULATORS === 'true' || projectId.startsWith('demo-')) {
    process.env.FIREBASE_AUTH_EMULATOR_HOST = process.env.FIREBASE_AUTH_EMULATOR_HOST || '127.0.0.1:9099';
    process.env.FIRESTORE_EMULATOR_HOST = process.env.FIRESTORE_EMULATOR_HOST || '127.0.0.1:8088';
  }
  const serviceAccountJson = process.env.FIREBASE_SERVICE_ACCOUNT;

  let credential;
  if (serviceAccountJson) {
    try {
      credential = cert(JSON.parse(serviceAccountJson));
    } catch (err) {
      console.error('[FIREBASE_ADMIN] Failed to parse FIREBASE_SERVICE_ACCOUNT credential:', err);
      throw new Error('Invalid Firebase service account configuration.');
    }
  }

  try {
    _adminApp = initializeApp({
      projectId,
      ...(credential ? { credential } : {}),
    });
    return _adminApp;
  } catch (err: any) {
    console.error('[FIREBASE_ADMIN] Failed to initialize Firebase Admin SDK:', err?.message || err);
    throw new Error('Firebase Admin SDK initialization failed.');
  }
}

export function getAdminAuth(): Auth {
  return getAuth(getFirebaseAdminApp());
}

export function getAdminFirestore(): Firestore {
  return getFirestore(getFirebaseAdminApp());
}