const mysql = require('mysql2/promise');

let pool;
const IS_SERVERLESS = [
  process.env.SERVERLESS,
  process.env.VERCEL,
  process.env.AWS_LAMBDA_FUNCTION_NAME,
  process.env.NETLIFY,
  process.env.FUNCTION_TARGET,
  process.env.K_SERVICE
].some(Boolean);

function getConnectionLimit() {
  const parsed = parseInt(process.env.MYSQL_CONNECTION_LIMIT, 10);
  if (!Number.isNaN(parsed) && parsed > 0) {
    return parsed;
  }
  return IS_SERVERLESS ? 2 : 10;
}

function createPool() {
  return mysql.createPool({
    host: process.env.MYSQL_HOST || 'localhost',
    port: parseInt(process.env.MYSQL_PORT, 10) || 3306,
    user: process.env.MYSQL_USER || 'root',
    password: process.env.MYSQL_PASSWORD || '',
    database: process.env.MYSQL_DATABASE || 'display_wall',
    waitForConnections: true,
    connectionLimit: getConnectionLimit(),
    queueLimit: 0,
    enableKeepAlive: true,
    keepAliveInitialDelay: 0
  });
}

function getPool() {
  if (!pool) {
    pool = createPool();
  }
  return pool;
}

function initPool({ force = false } = {}) {
  if (pool && !force) {
    return pool;
  }

  if (pool) {
    pool.end().catch(e => console.error('Error closing pool:', e));
  }

  pool = createPool();
  return pool;
}

async function checkHealth() {
  try {
    await getPool().query('SELECT 1');
    return { status: 'up' };
  } catch (err) {
    return { status: 'down', error: err.message };
  }
}

module.exports = {
  query: (...args) => getPool().query(...args),
  getPool,
  initPool,
  checkHealth
};