text: const Sender = require('./sender').Sender;
  
  const SQL_SERVER_BROWSER_PORT = 1434;
  const TIMEOUT = 2 * 1000;
  const RETRIES = 3;
  // There are three bytes at the start of the response, whose purpose is unknown.
  const MYSTERY_HEADER_LENGTH = 3;
  
  // Most of the functionality has been determined from from jTDS's MSSqlServerInfo class.
  class InstanceLookup {
    constructor() { }
  
    // Wrapper allows for stubbing Sender when unit testing instance-lookup.
    createSender(host, port, request) {
      return new Sender(host, port, request);
    }
  
    instanceLookup(options, callback) {
      const server = options.server;
      if (typeof server !== 'string') {
        throw new TypeError('Invalid arguments: "server" must be a string');
      }
  
      const instanceName = options.instanceName;
      if (typeof instanceName !== 'string') {
        throw new TypeError('Invalid arguments: "instanceName" must be a string');
      }
  
      const timeout = options.timeout === undefined ? TIMEOUT : options.timeout;
      if (typeof timeout !== 'number') {
        throw new TypeError('Invalid arguments: "timeout" must be a number');
      }
  
      const retries = options.retries === undefined ? RETRIES : options.retries;
      if (typeof retries !== 'number') {
        throw new TypeError('Invalid arguments: "retries" must be a number');
      }
  
      if (typeof callback !== 'function') {
        throw new TypeError('Invalid arguments: "callback" must be a function');
      }
  
      let sender, timer, retriesLeft = retries;
  
      const onTimeout = () => {
        sender.cancel();
        makeAttempt();
      };
  
      const makeAttempt = () => {
        if (retriesLeft > 0) {
          retriesLeft--;
  
          const request = Buffer.from([0x02]);
          sender = this.createSender(options.server, SQL_SERVER_BROWSER_PORT, request);
          sender.execute((err, message) => {
            clearTimeout(timer);
            if (err) {
              callback('Failed to lookup instance on ' + server + ' - ' + err.message);
              return;
            } else {
              message = message.toString('ascii', MYSTERY_HEADER_LENGTH);
              const port = this.parseBrowserResponse(message, instanceName);
  
              if (port) {
                callback(undefined, port);
              } else {
                callback('Port for ' + instanceName + ' not found in ' + options.server);
              }
            }
          });
  
          timer = setTimeout(onTimeout, timeout);
        } else {
          callback('Failed to get response from SQL Server Browser on ' + server);
        }
      };
  
      makeAttempt();
    }
  
    parseBrowserResponse(response, instanceName) {
      let getPort;
  
      const instances = response.split(';;');
      for (let i = 0, len = instances.length; i < len; i++) {
        const instance = instances[i];
        const parts = instance.split(';');
  
        for (let p = 0, partsLen = parts.length; p < partsLen; p += 2) {
          const name = parts[p];
          const value = parts[p + 1];
  
          if (name === 'tcp' && getPort) {
            const port = parseInt(value, 10);
            return port;
          }
  
          if (name === 'InstanceName') {
            if (value.toUpperCase() === instanceName.toUpperCase()) {
              getPort = true;
            } else {
              getPort = false;
            }
          }
        }
      }
    }
  }
  
  module.exports.InstanceLookup = InstanceLookup;
  