lulz.net Captcha Refresher [2012-10-10]

// ==UserScript==
// @name           lulz.net Captcha Refresher
// @namespace      http://lulz.net/
// @version        1.0.0
// @description    Checks if the captcha is still valid before submitting a post. If not, it will refresh the captcha and prompt the user to retype it. Also allows to refresh the captcha manually by clicking on it. This script should work with Opera, Firefox and Chrome. Install guide, updates, changelog and other scripts @ http://pastebin.com/u/lulzScript
// @include http://lulz.net/*
// ==/UserScript==

//Global variables
var capInp;
var capImg;
var submBtn;
var postfrm;
var statusText;

function newCaptchaURL()
{
  //Change the dummy parameter of the captcha image source URL to bypass caching of the captcha image and return the new URL
  return (capImg.getAttribute('src').replace(/dummy=\d*/, 'dummy=' + Math.floor((Math.random()*10000))));
}

function onCaptchaRefresh()
{
  //Clear status text after captcha has been refreshed.
  statusText.data = '';
  //Unregister onLoad
  capImg.onload = null;
}

function onCaptchaClick(event)
{
  statusText.data = 'Refreshing captcha...';
  //Get URL for new Captcha.
  var isrc = newCaptchaURL();
  //Calls onCaptchaRefresh after new captcha has been loaded to clear status text.
  capImg.onload = onCaptchaRefresh;
  //Set the new image source to trigger the captcha refresh.
  capImg.setAttribute('src', isrc);
}

function activateCaptchaScript()
{
  //Get the "input" html tag of the captcha
  capInp = document.getElementsByName('captcha');
  if ( capInp && (capInp.length > 0) )
  {
    //Get the "img" html tag of the captcha
    imgs = capInp[0].parentNode.getElementsByTagName('img');
    if ( imgs && (imgs.length > 0) )
    {
      //Save img object of Captcha
      capImg = imgs[0];

      //Get postform to global variable
      postfrm = document.forms['postform'];
      //Find submit button
      var inps = postfrm.getElementsByTagName('input');
      for (var i=0; i < inps.length; i++)
      {
        //if the type attribute = "submit" then inps[i] is the submit button
        if ( inps[i].getAttribute('type') == 'submit' )
        {
          //Save submit button object
          submBtn = inps[i];

          //Insert elements into the document to display status messages
          //Enclose everything in a div container
          var statusDiv = document.createElement('div');
          //Insert a line break
          statusDiv.innerHTML = '<br/>';
          //Insert a text node for the status text. initial text is empty ('').
          //The global variable statusText will refer to this text node below the captcha
          statusText = document.createTextNode('');
          //Insert the text node into the div container
          statusDiv.insertBefore(statusText, null);
          //Insert the div container into the document (below the captcha image)
          capImg.parentNode.insertBefore(statusDiv, null);

          //At this point the submit button and the captcha image have been found and the click event listeners for both are registered.
          capImg.addEventListener("click", onCaptchaClick, false);
          submBtn.addEventListener("click", onSubmitClick, false);

          //The script is now successfully activated. Set flag to "true"
          captchaScriptActivated = true;
        }
      }
    }
  }
}

//This function converts an image into a data string using a canvas element.
//The data strings can be used to easily compared two images and to determine if they are different or not.
//
//img:          The image object to be converted to a data string
//return value: The data string
function imgToDataStr(img)
{
  //Create a canvas element
  var cnv = document.createElement('canvas');

  //Set canvas width and height to the values of the image
  cnv.width = img.width;
  cnv.height = img.height;

  //Draw the image onto the canvas
  cnv.getContext('2d').drawImage(img, 0, 0);

  //Convert the canvas data (the image) into a data string and return it
  return( cnv.toDataURL() );
}

//Called after the captch has successfully loaded
function captchaLoadOK()
{
  //Captcha has successufully loaded. Set status text.
  statusText.data = 'Captcha loaded.';

  //Compare the current captcha image on the page with the refreshed one
  if ( imgToDataStr(this) == imgToDataStr(capImg) )
  {
    //If the captcha images are identical, the current captcha hasn't times out yet. So the post can be submitted.
    statusText.data = 'Captcha still valid. Submitting...';
    postfrm.submit();
  }
  else
  {
    //If the captcha images are different, the current captcha has timed out. Update the captcha on the page with the refreshed one.
    capImg.src = this.src;
    //Clear the captcha text box
    capInp.value = '';
    //Ask user to retype the captcha
    statusText.data = 'Captcha has changed. Please retype.';
  }
}

//Called if the captcha failed to load.
function captchaLoadError()
{
  //Display error message if the captcha couldn't be loaded
  statusText.data = 'Error loading captcha.';
}

//Called when the submit button is clicked.
function onSubmitClick(event)
{
  //First prevent the default submit action of the form.
  event.preventDefault();

  //Create an image object
  var newCaptcha = new Image();

  //Set event handlers for successful and unsuccessful captcha loading.
  newCaptcha.onload = captchaLoadOK;
  newCaptcha.onerror = captchaLoadError;

  //Set status text
  statusText.data = 'Refreshing captcha...';
  //Trigger loading of a new captcha (or a copy of the current one if it hasn't timed out yet) into the newCaptcha object
  newCaptcha.src = newCaptchaURL();
  //Script continues at captchaLoadOK() or captchaLoadError()
}

//Flags to indicate whether the script has been activated or not. Initial value is "false".
var captchaScriptActivated = false;

//Try to activate the captcha script, if the page has loaded already at this point.
if ( !captchaScriptActivated )
  activateCaptchaScript();

document.addEventListener("DOMContentLoaded",
  function Hook_load(e)
  {
    //Try to activate the captcha script, if it isn't active already. At this point the DOM should be avilable.
    if ( !captchaScriptActivated )
      activateCaptchaScript();

  }, false
);

//Additional, faster way for Opera to activate the captcha script.
//Inline scripts sometimes seem to block the DOMContentLoaded event until they have executed completely.
//This code will activate the captcha script as soon as (but before) an inline script is called
//
//Check if the opera object exists. If yes, this script runs in an opera browser.
if (window.opera)
{
  window.opera.addEventListener ('BeforeScript',
    function onDCL(e)
    {
      //Try to activate the captcha script.
      if ( !captchaScriptActivated )
        activateCaptchaScript();

      //Check if the captcha script is activated (either by this function or by the DOMContentLoaded event listener above)
      if ( captchaScriptActivated )
      {
        //Remove this event listener again if button was inserted successfully.
        window.opera.removeEventListener ('BeforeScript', onDCL, false);
      }

    }, false
  );
}