$(function() {
    
  var $document   = $(document);
  var $window     = $(window);
  var height      = $document.height() - $window.height();
  var timeline    = new TimelineMax();
  var scrolling   = false;
  
  // TimeScale value when Ctrl is pressed
  // https://greensock.com/docs/#/HTML5/GSAP/TweenLite/timeScale/
  var ctrlSpeed = 3;
  
  // Set your speed constant here for how many pixels you want
  // it to scroll / second
  var scrollSpeed = 200;
  var totalTime   = height / scrollSpeed;
   
  var duration;
  var ratio;
  var timeScale; 
    
  var keyboard = {
    DOWN : 40,
    UP   : 38,
    CTRL : 17
  };  
    
  // ===========================================================================
  //  KEY DOWN
  // ===========================================================================
  $document.on("keydown", function(event) {
    
    // IF 'CTRL' is pressed, speed up the timeScale to the ctrlSpeed
    if (event.ctrlKey) {
      event.preventDefault();
      timeline.timeScale(ctrlSpeed);
    }
    
    // DOWN KEY
    if (event.keyCode === keyboard.DOWN) {
      event.preventDefault();
            
      if (!scrolling) {
        
        ratio     = $window.scrollTop() / height;
        duration  = totalTime * (1 - ratio);
        scrolling = true;
        
        // IF the 'CTRL' key is pressed, change the timeScale to the ctrlSpeed
        // ELSE set the timeScale to 1
        timeScale = event.ctrlKey ? ctrlSpeed : 1;
        
        timeline
          .to($window, duration, { scrollTo: "max", ease: Power0.easeNone })
          .timeScale(timeScale);
      }
    }
    
    // UP KEY
    if (event.keyCode === keyboard.UP) {
      event.preventDefault();
      
      if (!scrolling) {
        
        ratio     = $window.scrollTop() / height;
        duration  = totalTime - (totalTime * (1 - ratio));        
        scrolling = true;        
        timeScale = event.ctrlKey ? ctrlSpeed : 1;
        
        timeline
          .to($window, duration, { scrollTo: 0,  ease: Power0.easeNone })
          .timeScale(timeScale);
      }
    }
  });
  
  // ===========================================================================
  //  KEY UP
  // ===========================================================================
  $document.on("keyup", function(event) {
    
    // Set the timeScale back to 1 when 'CTRL' is released
    if (event.keyCode === keyboard.CTRL) {
      event.preventDefault();
      timeline.timeScale(1);
    }
    
    if (event.keyCode === keyboard.UP || event.keyCode === keyboard.DOWN) {
      event.preventDefault();
      timeline.clear();      
      scrolling = false;
    }
  });  
});