//stores the reference to the XMLHttpRequest object
var xmlHttp = createXmlHttpRequestObject();

//retrieves the XMLHttpRequest object
function createXmlHttpRequestObject()
{
  //will store the reference to th eXMLHttpRequest object
  var xmlHttp;
  //if running Internet Explorer
  if(window.ActiveXObject)
  {
    try
    {
      xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
    }
    catch (e)
    {
      xmlHttp = false;
    }
  }
  //if running Mozilla or other browswer
  else
  {
    try
    {
      xmlHttp = new XMLHttpRequest();
    }
    catch (e)
    {
      xmlHttp = false;
    }
  }
  //return the created object or display an error message
  if(!xmlHttp)
    alert("Error creating the XMLHttpRequest object.");
  else
    return xmlHttp;
}

//make asynchronous HTTP request using the XMLHttpRequest object
function busList(letter)
{
  //proceed only if the xmlHttp object isn't busy
  if(xmlHttp.readyState == 4 || xmlHttp.readyState == 0)
  {
    //retrieve the letter selected by the user
    letter = letter;
    //execute the buslist.php page from the server
    xmlHttp.open("GET", "buslist.php?letter=" + letter, true);
    //define the method to handle server responses
    xmlHttp.onreadystatechange = handleServerResponse;
    //make the server request
    xmlHttp.send(null);
  }
  else
    //if the connection is busy, try again after one second
    setTimeout('busList()', 1000);
}

//make asynchronous HTTP request using the XMLHttpRequest object
function busListing()
{
  //proceed only if the xmlHttp object isn't busy
  if(xmlHttp.readyState == 4 || xmlHttp.readyState == 0)
  {
    //retrieve the letter selected by the user
    type= encodeURIComponent(document.getElementById("bus_type").value);
    //execute the buslist.php page from the server
    xmlHttp.open("GET", "buslist.php?type=" + type, true);
    //define the method to handle server responses
    xmlHttp.onreadystatechange = handleServerResponse;
    //make the server request
    xmlHttp.send(null);
  }
  else
    //if the connection is busy, try again after one second
    setTimeout('busListing()', 1000);
}

//make asynchronous HTTP request using the XMLHttpRequest object
function busName()
{
  //proceed only if the xmlHttp object isn't busy
  if(xmlHttp.readyState == 4 || xmlHttp.readyState == 0)
  {
    //retrieve the letter selected by the user
    name= encodeURIComponent(document.getElementById("bus_name").value);
    //execute the buslist.php page from the server
    xmlHttp.open("GET", "buslist.php?name=" + name, true);
    //define the method to handle server responses
    xmlHttp.onreadystatechange = handleServerResponse;
    //make the server request
    xmlHttp.send(null);
  }
  else
    //if the connection is busy, try again after one second
    setTimeout('busName()', 1000);
}

//executed automatically when a message is received from the server
function handleServerResponse()
{
  //move forward only if the transaction has completed
  if(xmlHttp.readyState == 4)
  {
    //status of 200 indicates the transaction completed successfully
    if(xmlHttp.status == 200)
    {
      //extract the XML retrieved from the server
      xmlResponse = xmlHttp.responseXML;
      //obtain the document element (the root element) of the XML structure
      xmlDocumentElement = xmlResponse.documentElement;
      //get the business data, which is in the first child of the document element
      busInfo = xmlDocumentElement.firstChild.data;
      //update the client display using the data received from the server
      document.getElementById("busData").innerHTML = busInfo;
    }
    //a HTTP status different than 200 signals an error
    else
    {
      alert("There was a problem accessing the server: " + xmlHttp.statusText);
    }
  }
}