/**
* Returns an XMLHttp instance to use for asynchronous
* downloading. This method will never throw an exception, but will
* return NULL if the browser does not support XmlHttp for any reason.
* @return {XMLHttpRequest|Null}
*/
function createXmlHttpRequest() {
  try {
    if (typeof ActiveXObject != 'undefined') {
      return new ActiveXObject('Microsoft.XMLHTTP');
    } else if (window["XMLHttpRequest"]) {
      return new XMLHttpRequest();
    }
  } catch (e) {
    changeStatus(e);
  }
  return null;
};

/**
* This functions wraps XMLHttpRequest open/send function.
* It lets you specify a URL and will call the callback if
* it gets a status code of 200.
* @param {String} url The URL to retrieve
* @param {Function} callback The function to call once retrieved.
*/
function downloadUrl(url, callback) {
  var status = -1;
  var request = createXmlHttpRequest();
  if (!request) {
    return false;
  }

  request.onreadystatechange = function() {
    if (request.readyState == 4) {
      try {
        status = request.status;
      } catch (e) {
        // Usually indicates request timed out in FF.
      }
      if (status == 200) {
        callback(request.responseXML, request.status);
        request.onreadystatechange = function() {};
      }
    }
  }
  request.open('GET', url, true);
  try {
    request.send(null);
  } catch (e) {
    changeStatus(e);
  }
};

function downloadUrltxt(url, callback) {
  var status = -1;
  var request = createXmlHttpRequest();
  if (!request) {
    return false;
  }

  request.onreadystatechange = function() {
    if (request.readyState == 4) {
      try {
        status = request.status;
      } catch (e) {
        // Usually indicates request timed out in FF.
      }
      if (status == 200) {
        callback(request.responseText, request.status);
        request.onreadystatechange = function() {};
      }
    }
  }
  request.open('GET', url, true);
  try {
    request.send(null);
  } catch (e) {
    changeStatus(e);
  }
};


/**
 * Parses the given XML string and returns the parsed document in a
 * DOM data structure. This function will return an empty DOM node if
 * XML parsing is not supported in this browser.
 * @param {string} str XML string.
 * @return {Element|Document} DOM.
 */
function xmlParse(str) {
  if (typeof ActiveXObject != 'undefined' && typeof GetObject != 'undefined') {
    var doc = new ActiveXObject('Microsoft.XMLDOM');
    doc.loadXML(str);
    return doc;
  }

  if (typeof DOMParser != 'undefined') {
    return (new DOMParser()).parseFromString(str, 'text/xml');
  }

  return createElement('div', null);
}

/**
 * Appends a JavaScript file to the page.
 * @param {string} url
 */
function downloadScript(url) {
  var script = document.createElement('script');
  script.src = url;
  document.body.appendChild(script);
}