var currentBannerNumber = '';

function switch_banner() {
	 //this function gets all divs in a page and checks their classes
	 //if the div has a banner class it is added to a bannerDivs array
	 //a function is then called that displays a random banner
	 
	  var divs = document.getElementsByTagName("div");
		var bannerDivs = new Array();
		var classes;

		for ( var i=0; i<divs.length; i++ ) {
				//there may be multiple class assignments
				//separates the div class into an array
				classes = divs[i].className.split(" ");
				
				//checks if div has a class of banner
				//if so, add it to bannerDivs array
				if ( inArray("banner", classes) ) {bannerDivs.push(divs[i]);}
		}
		display_banner();
}

function inArray( value, array ) {
//passed a value and an array, this function returns true
//if the value is contained with in the array,
//false if it is not

	for ( var i = 0; i < array.length; i++ )
		if ( array[i] == value )
			return true;
	return false;
} 


function display_banner() {
//displays a random banner
	var bannerCount = 11;
	var randomNumber;
	var randomNumber = Math.floor((Math.random() * bannerCount));
	randomNumber = randomNumber + 1;
	
	//if (document.getElementById("placeholder")) {
		//document.getElementById("placeholder").style.display = "none";
	//}

	if (document.getElementById("banner_" + currentBannerNumber)) {
		document.getElementById("banner_" + currentBannerNumber).style.display = "none";
	}
	
	if (document.getElementById("banner_" + randomNumber)) {
		currentBannerNumber = randomNumber;														
		document.getElementById("banner_" + randomNumber).style.display = "block";
		}
		
	setTimeout("display_banner()", 5000);	
}



//adds functions to the windows onload event
function addOnload(newFunction) {
  var oldOnload = window.onload;
    // does window.onload exist
  if (typeof oldOnload == "function") {
            //overwriting existing onload
              window.onload = function() {
                  if (oldOnload) {oldOnload();}
                  newFunction();
              }
          }
          else {
              window.onload = newFunction;
          }
} 

addOnload(switch_banner);