//=========================
// ypChaser class definition
//===========================================================
// sLayerId   : the ID attribute from the chaser div element
// iTopOffset : the amount of space the chaser will maintain 
//              from the top of the page
// iSlideTime : the amount of time the chaser should take to
//              perform the slide animation.
// iCeiling   : optional parameter which specifies a space
//              at the top of the document which the chaser
//              will not animate over.
// iFloor     : optional parameter which specifies a space
//              at the bottom of the document which the 
//              chaser will not animate over.
//
//
// Note:
// the floor feature does not yet work under Opera, as there
// is no way under Opera's DOM to query the pixel height
// of a document. If the floor feature is used in an Opera
// browser, it will be ignored - but otherwise the script
// will function.
//===========================================================
function ypChaser(sLayerId, iTopOffset, iLeftOffset, iSlideTime, iCeiling, iFloor, iMinLeft, iMinRight) {
	this.chaserDiv	= null
	this.layerId	= sLayerId
	this.topOffset	= iTopOffset
	this.leftOffset	= iLeftOffset
	this.slideTime	= iSlideTime
	this.ceiling  	= iCeiling
	this.floor 	= iFloor
	this.minLeft	= iMinLeft
	this.minRight	= iMinRight
	
	ypChaser.registry[ypChaser.registry.length] = this
}

ypChaser.isIE = window.clientInformation ? true : false
ypChaser.isN4 = document.layers ? true : false
ypChaser.isN6	= navigator.appName == "Netscape" && parseInt(navigator.appVersion) >= 5
ypChaser.isO5 = navigator.userAgent.indexOf("Opera") != -1 && parseInt(navigator.appVersion) >= 4
ypChaser.isMAC = navigator.platform.indexOf("Mac") != -1
ypChaser.registry = new Array( )
ypChaser.callRate = 10
// this is the main loop of the program. To avoid having multiple timers
// running concurrently (CPU-intensive) this goes through ypChaser.registry[ ]
// and calls the .main( ) method of each instance of ypChaser in turn.
ypChaser.timer = function() {
	for (var i = 0; chObj = this.registry[i]; i++) {
	
		if (!chObj.chaserDiv) chObj.attemptLoad()
		if (chObj.chaserDiv)  chObj.main()
	}
}


// attempts to find the target layer.
// sets the chaserDiv property of the parent object to
// found layer, or null if none is found.
ypChaser.prototype.attemptLoad = function() {
	var chDiv = null

	if (ypChaser.isN6 || ypChaser.isO5)	chDiv = document.getElementById(this.layerId)
	else if (ypChaser.isIE)			chDiv = document.all[this.layerId]
	else if (ypChaser.isN4)			chDiv = document.layers[this.layerId]

	if (chDiv && chDiv != null) {
		this.chaserDiv = chDiv
	}
}
	


// Main function. gets called very often by ypChaser.timer().
// Updates targetY, and decides whether to start
// the animation over again, continue an existing animation,
// or do nothing at all.
ypChaser.prototype.main = function( )
{
	this.currentX	= this.getCurrentX()
	this.currentY	= this.getCurrentY()
	var scrollTop	= ypChaser.getWindowScroll() 
	var scrollLeft	= ypChaser.getWindowScrollX()
	var newTargetY	= scrollTop + this.topOffset
	var newTargetX	= scrollLeft + this.leftOffset
	var floor	= ypChaser.getDocumentHeight() - this.floor
	var maxLeft	= ypChaser.getDocumentWidth() - this.minRight

	newTargetY = Math.max( newTargetY, this.ceiling)
	newTargetX = Math.max( newTargetX, this.minLeft)
	if (!ypChaser.isO5) newTargetY = Math.min(newTargetY, floor)
	//alert (newTargetY + "floor" + floor);
	if (!ypChaser.isO5) newTargetX = Math.min(newTargetX, maxLeft)
	if ( this.currentY != newTargetY ) {
		if ( newTargetY != this.targetY ) {
			this.targetY = newTargetY
			this.slideInitY( )
		}

		this.slideY();
	}
}


// .slideInit( ). Initializes the slide animation.
ypChaser.prototype.slideInitY = function( )
{
	this.Ay		= (this.targetY - this.currentY) / this.slideTime / this.slideTime
	this.startTy	= (new Date()).getTime()
	this.startPy	= this.getCurrentY()
	this.Dy		= this.targetY - this.startPy
}

ypChaser.prototype.slideInitX = function( )
{
	this.Ax		= (this.targetX - this.currentX) / this.slideTime / this.slideTime
	this.startTx	= (new Date()).getTime()
	this.startPx	= this.getCurrentX()
	this.Dx		= this.targetX - this.startPx
}

// .slide( ). Moves the Chaser one frame. Its rate decreases and
// is defined by a sine wave.
ypChaser.prototype.slideY = function( )
{
	var elapsed	= (new Date()).getTime() - this.startTy
	if (elapsed < this.slideTime) {
		this.setY(this.Dy - (Math.round(Math.pow(this.slideTime - elapsed, 2) * this.Ay)) + this.startPy)
	}
}

ypChaser.prototype.slideX = function( )
{
	var elapsed	= (new Date()).getTime() - this.startTx
	if (elapsed < this.slideTime) {
		this.setX(this.Dx - (Math.round(Math.pow(this.slideTime - elapsed, 2) * this.Ax)) + this.startPx)
	}
}

ypChaser.prototype.setY = function(ny) {
	if (ypChaser.isN4)	this.chaserDiv.top = ny
	else				this.chaserDiv.style.top = ny
}
	
ypChaser.prototype.setX = function(nx) {
	if (ypChaser.isN4)	this.chaserDiv.left = nx
	else			this.chaserDiv.style.left = nx
}
	
ypChaser.prototype.getCurrentY = function() {
	var n = ypChaser.isN4 ? this.chaserDiv.top : parseInt(this.chaserDiv.style.top)
	return isNaN(n) ? 0 : n
}

ypChaser.prototype.getCurrentX = function() {
	var n = ypChaser.isN4 ? this.chaserDiv.left : parseInt(this.chaserDiv.style.left)
	return isNaN(n) ? 0 : n
}

ypChaser.getWindowScroll = function() {
	if (ypChaser.isIE)	return document.body.scrollTop
	else				return window.pageYOffset
}

ypChaser.getWindowScrollX = function() {
	if (ypChaser.isIE)	return document.body.scrollLeft
	else				return window.pageXOffset
}

ypChaser.getDocumentHeight = function() {
	if (ypChaser.isO5)		return 0 // opera can't retreive this property.
	else if (ypChaser.isIE)	return Math.max(document.body.scrollHeight, document.body.offsetHeight)
	else					return window.document.height
}

ypChaser.getDocumentWidth = function() {
   return 3000;
}


// call main function
if (ypChaser.isMAC == true && navigator.appName == "Microsoft Internet Explorer") {
 	 ypChaser.timer( );
} else {
	 window.setInterval("ypChaser.timer( )", ypChaser.callRate);
}