//=========================
// 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, iSlideTime, iCeiling, iFloor) {

	this.chaserDiv = null
	this.layerId   = sLayerId
	this.topOffset = iTopOffset
	this.slideTime = iSlideTime
	this.ceiling   = iCeiling
	this.floor     = iFloor	

	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.registry = new Array( )

ypChaser.callRate = 10

window.setInterval("ypChaser.timer( )", ypChaser.callRate)


// 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.currentY	= this.getCurrentY()
	var scrollTop	= ypChaser.getWindowScroll() 
	var newTargetY	= scrollTop + this.topOffset
	var floor		= ypChaser.getDocumentHeight() - this.floor

	newTargetY = Math.max( newTargetY, this.ceiling)

	if (!ypChaser.isO5) newTargetY = Math.min(newTargetY, floor)

	if ( this.currentY != newTargetY ) {
		if ( newTargetY != this.targetY ) {
			this.targetY = newTargetY
			this.slideInit( )
		}
		this.slide( )
	}


}


// .slideInit( ). Initializes the slide animation.

ypChaser.prototype.slideInit = function( ) {

	this.A		= (this.targetY - this.currentY) / this.slideTime / this.slideTime
	this.startT	= (new Date()).getTime()
	this.startP	= this.getCurrentY()
	this.D		= this.targetY - this.startP

}


// .slide( ). Moves the Chaser one frame. Its rate decreases and
// is defined by a sine wave.

ypChaser.prototype.slide = function( ){

	var elapsed	= (new Date()).getTime() - this.startT

	if (elapsed < this.slideTime) {
		this.moveTo(this.D - (Math.round(Math.pow(this.slideTime - elapsed, 2) * this.A)) + this.startP)
	}

}





ypChaser.prototype.moveTo = function(ny) {

	if (ypChaser.isN4)	this.chaserDiv.top = ny
	else				this.chaserDiv.style.top = ny

}

	

ypChaser.prototype.getCurrentY = function() {

	var n = ypChaser.isN4 ? this.chaserDiv.top : parseInt(this.chaserDiv.style.top)
	return isNaN(n) ? 0 : n

}



ypChaser.getWindowScroll = function() {

	if (ypChaser.isIE)	return document.body.scrollTop
	else				return window.pageYOffset

}



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

}