// holds an instance of XMLHttpRequest
var get_subcategories_xmlHttp = get_subcategories_createXmlHttpRequestObject();

// creates an XMLHttpRequest instance
function get_subcategories_createXmlHttpRequestObject() 
{
  // will store the reference to the XMLHttpRequest object
  var get_subcategories_xmlHttp;
  // this should work for all browsers except IE6 and older
  try
  {
    // try to create XMLHttpRequest object
    get_subcategories_xmlHttp = new XMLHttpRequest();
  }
  catch(e)
  {
    // assume IE6 or older
    var XmlHttpVersions = new Array("MSXML2.XMLHTTP.6.0",
                                    "MSXML2.XMLHTTP.5.0",
                                    "MSXML2.XMLHTTP.4.0",
                                    "MSXML2.XMLHTTP.3.0",
                                    "MSXML2.XMLHTTP",
                                    "Microsoft.XMLHTTP");
    // try every prog id until one works
 
    for (var i=0; i<XmlHttpVersions.length && !get_subcategories_xmlHttp; i++) 
    {
      try 
      { 
        // try to create XMLHttpRequest object
        get_subcategories_xmlHttp = new ActiveXObject(XmlHttpVersions[i]);
      } 
      catch (e) {}
    }
  }
  // return the created object or display an error message
  if (!get_subcategories_xmlHttp)
    alert("Error creating the XMLHttpRequest object.");
  else 
    return get_subcategories_xmlHttp;
}
function get_subcategories(parent_id)
{
    if ( document.getElementById('subcategory_select_status').innerHTML != 'loading' )
    {
        document.getElementById('subcategory_select_status').innerHTML = 'loading';
        var subcategory_list = document.getElementById('subcategory_id');
        if ( subcategory_list )
        {
            get_subcategories_clearOptions(subcategory_list);
            get_subcategories_addToOptionList(subcategory_list, '', 'loading...');
        }
        get_subcategories_connect_to_server("get_subcategories.php?parent_id="+parent_id)
    }

}
// read a file from the server
function get_subcategories_connect_to_server(url_string)
{
  // only continue if get_subcategories_xmlHttp isn't void
  if ( get_subcategories_xmlHttp )
  {
    // try to connect to the server
    try
    {
      // initiate reading a file from the server
      get_subcategories_xmlHttp.open("GET", url_string, true);
      get_subcategories_xmlHttp.onreadystatechange = get_subcategories_handleRequestStateChange;
      get_subcategories_xmlHttp.send(null);
    }
    // display the error in case of failure
    catch (e)
    {
      alert("Can't connect to server:\n" + e.toString());
    }
  }
}


// function called when the state of the HTTP request changes
function get_subcategories_handleRequestStateChange() 
{
  // when readyState is 4, we are ready to read the server response
  if (get_subcategories_xmlHttp.readyState == 4) 
  {
    // continue only if HTTP status is "OK"
    if (get_subcategories_xmlHttp.status == 200) 
    {
      try
      {
        // do something with the response from the server
        get_subcategories_handleServerResponse();
      }
      catch(e)
      {
        // display error message
        alert("Error reading the response: " + e.toString());
      }
    } 
    else
    {
      // display status message
      alert("There was a problem retrieving the data:\n" + 
            get_subcategories_xmlHttp.statusText);
 
    }
  }
}

// handles the response received from the server
function get_subcategories_handleServerResponse()
{
  document.getElementById('subcategory_select_status').innerHTML = 'populated';
  
  // read the message from the server
  var xmlResponse = get_subcategories_xmlHttp.responseXML;
  // catching potential errors with IE and Opera
  if (!xmlResponse || !xmlResponse.documentElement)
    throw("Invalid XML structure:\n" + get_subcategories_xmlHttp.responseText);
  // catching potential errors with Firefox
  var rootNodeName = xmlResponse.documentElement.nodeName;
  if (rootNodeName == "parsererror") throw("Invalid XML structure");
  // obtain the XML's document element
  xmlRoot = xmlResponse.documentElement;  
  // obtain arrays with category names and ids 
  categoryArray = xmlRoot.getElementsByTagName("n");
  idArray = xmlRoot.getElementsByTagName("i");
  // generate HTML output
  var html = "";  

  var category_select = document.getElementById('subcategory_id');
  if ( category_select )
  {
    get_subcategories_clearOptions(category_select);
    get_subcategories_addToOptionList(category_select, '', 'Choose a subcategory');
      for (var i=0; i<categoryArray.length; i++)
      {
        get_subcategories_addToOptionList(category_select, idArray.item(i).firstChild.data, categoryArray.item(i).firstChild.data);
      }
  }
}

function get_subcategories_clearOptions(OptionList) {

   // Always clear an option list from the last entry to the first
   for (x = OptionList.length; x >= 0; x = x - 1) {
      OptionList[x] = null;
   }
}


function get_subcategories_addToOptionList(OptionList, OptionValue, OptionText) {
   // Add option to the bottom of the list
   OptionList[OptionList.length] = new Option(OptionText, OptionValue);
}