import mongoose from 'mongoose';
import dns from 'dns';

// Ensure reliable SRV records resolution for MongoDB Atlas connection strings
try {
  dns.setServers(['8.8.8.8', '1.1.1.1']);
} catch (err) {
  console.warn('Could not override DNS servers:', err);
}


/**
 * Global is used here to maintain a cached connection across hot reloads
 * in development. This prevents connections growing exponentially
 * during API Route usage.
 */
interface GlobalWithMongoose {
  mongoose: {
    conn: mongoose.Connection | null;
    promise: Promise<mongoose.Connection> | null;
  } | undefined;
}

const globalWithMongoose = global as unknown as GlobalWithMongoose;

let cached = globalWithMongoose.mongoose;

if (!cached) {
  cached = globalWithMongoose.mongoose = { conn: null, promise: null };
}

export async function connectToDatabase(): Promise<mongoose.Connection> {
  const MONGODB_URI = process.env.MONGODB_URI;
  const JWT_SECRET = process.env.JWT_SECRET;
  const CRON_SECRET = process.env.CRON_SECRET;

  if (!MONGODB_URI || !JWT_SECRET || !CRON_SECRET) {
    throw new Error(
      'Startup validation failed: Missing critical environment variables (MONGODB_URI, JWT_SECRET, or CRON_SECRET).'
    );
  }

  if (cached && cached.conn) {
    return cached.conn;
  }

  if (cached && !cached.promise) {
    const opts = {
      bufferCommands: false,
    };

    cached.promise = mongoose.connect(MONGODB_URI, opts).then((mongooseInstance) => {
      return mongooseInstance.connection;
    });
  }

  try {
    if (cached) {
      cached.conn = await cached.promise;
    }
  } catch (e) {
    if (cached) {
      cached.promise = null;
    }
    throw e;
  }

  return cached!.conn!;
}