var req;

// This is run OnLoad
function Init ()
{
  var select = document.getElementById ("ListSelecter");
  select.onchange = UpdateBookList;
}

// This is run when ListSelector Changes
function UpdateBookList ()
{
  var select = document.getElementById ("ListSelecter");
  if (select.value != "")
    loadXMLDoc (select.value);
}

// Run When the XMLHttpRequest gets more data
function processReqChange ()
{
  if (req.readyState == 4) {
    if (req.status == 200) {
      // add the data to the interface
      ReplaceBookList (req.responseXML);
    } else {
      alert ("there was a problem retrieving XML: " + req.statusText);
    }
  }
}

function ReplaceBookList (xml)
{
  var BookList = document.getElementById ("BookList");

  // clear list
  BookList.innerHTML = "";

  var Titles = xml.getElementsByTagName ('Title');
  var Authors = xml.getElementsByTagName ('Author');
  
  for (var i = 0; i < Titles.length; i++) {
    var BookHTML = MakeBook (Titles[i].firstChild.nodeValue, Authors[i].firstChild.nodeValue);
    BookList.appendChild (BookHTML);

  }
}
  
function MakeBook (Title, Author)
{
  var ListItem = document.createElement ("li");
  ListItem.appendChild (document.createTextNode (Title + " by " + Author));
  return ListItem;
}

// Apple XMLHttpRequest func
function loadXMLDoc (url)
{
  if (window.XMLHttpRequest) {
    req = new XMLHttpRequest ();
    req.onreadystatechange = processReqChange;
    req.open ("GET", url, true);
    req.send (null);
  } else if (window.ActiveXObject) {
    req = new ActiveXObject ("Microsoft.XMLHTTP");
    if (req) {
      req.onreadystatechange = processReqChange;
      req.open ("GET", url, true);
      req.send ();
    }
  }
}