if (typeof Prototype == 'undefined' || Prototype.Version < '1.5.1') {
  throw "Behaviour needs Prototype 1.5.1";
}

var Behaviour = {
  list : [],
  
  eventObservers : [],
  
  clearedElements : [],

  addEventObserver : function(element, name, observer, useCapture){
    if (!this.clearedElements.include(element)) { // should do a hashed lookup here
      this.clearedElements.push(element);
      Behaviour.removeAllObservers(element);
    }
    
    element = $(element);
    useCapture = useCapture || false;
    Behaviour.eventObservers.push([element, name, observer, useCapture]);
    Event.observe(element, name, observer, useCapture);
  },

  removeEventObserver : function(element, name, observer, useCapture){
    element = $(element);
    useCapture = useCapture || false;
    Behaviour.eventObservers = Behaviour.eventObservers.reject(function(list) {
      return (list[0] == element && list[1] == name && list[2] == observer && list[3] == useCapture);
    });
    Event.stopObserving(element, name, observer, useCapture);
  },

  removeAllObservers : function(element){
    var el = $(element);
    var observersForElement = Behaviour.eventObservers.findAll(
                               function(observerInfo){
                                 return (observerInfo[0]==el);
                               });

    observersForElement.each(function(observerInfo){
      Behaviour.removeEventObserver(observerInfo[0], observerInfo[1], observerInfo[2], observerInfo[3]);
    });
  },

  register : function(sheet){
    Behaviour.list.push(sheet);
  },
  
  start : function(){
    Behaviour.addLoadEvent(function(){
      Behaviour.apply();
    });
  },
  
  addLoadEvent : function(func){
    var oldonload = window.onload;
    
    if (typeof window.onload != 'function') {
      window.onload = func;
    } else {
      window.onload = function() {
        oldonload();
        func();
      };
    }
  },

  apply : function(){
    this.clearedElements = [];
    Behaviour.list.each(function(sheet) {
      for (selector in sheet) {
        list = $$(selector);

        if (!list){
          continue;
        }

        list.each(function(element) {
          sheet[selector](element);
        });
      }
    });
  },

  // applySheet can be used to (re)apply a single sheet.
  // Note that any element touched by the sheet will lose any
  // event observers added by other sheets!
  // If rootElement is specified, only adds behaviour to a subtree.
  applySheet : function(sheet, rootElement) {
    this.clearedElements = [];
 
    for (selector in sheet) {
      list = rootElement ? $(rootElement).getElementsBySelector(selector) : $$(selector);

      if (!list) {
        continue;
      }
 
      list.each(function(element) {
        sheet[selector](element);
      });
    }
  }
};

Behaviour.start();