var ScrollSpy = new Class({

  /* implements */
  Implements: [Options,Events],

  /* options */
  options: {
    container: window,
    max: 0,
    min: 0,
    mode: 'vertical'/*,
    onEnter: $empty,
    onLeave: $empty,
    onScroll: $empty,
    onTick: $empty
    */
  },

  /* initialization */
  initialize: function(options) {
    /* set options */
    this.setOptions(options);
    this.container = document.id(this.options.container);
    this.enters = this.leaves = 0;
    this.max = this.options.max;
    this.inside = false;
    
    /* make it happen */
    this.addListener();
  },

  /* a method that does whatever you want */
  addListener: function() {
    /* state trackers */
    this.container.addEvent('scroll',function(e) {
      /* if it has reached the level */
      var position = this.container.getScroll(),
      xy = position[this.options.mode == 'vertical' ? 'y' : 'x'];
      /* if we reach the minimum and are still below the max... */
      if(xy >= this.options.min && (this.max == 0 || xy <= this.max)) {
        /* trigger Enter event if necessary */
        if(!this.inside) {
          /* record as inside */
          this.inside = true;
          this.enters++;
          /* fire enter event */
          this.fireEvent('enter',[position,this.enters,e]);
        }
        /* trigger the "tick", always */
        this.fireEvent('tick',[position,this.inside,this.enters,this.leaves,e]);
      }
      /* trigger leave */
      else if(this.inside){
        this.inside = false;
        this.leaves++;
        this.fireEvent('leave',[position,this.leaves,e]);
      }
      /* fire scroll event */
      this.fireEvent('scroll',[position,this.inside,this.enters,this.leaves,e]);
    }.bind(this));
  }
});