/* chaser.js
 * by Aaron Boodman v1.0 000919
 * Copyright (c) 2000 Aaron Boodman. All Rights Reserved.
 * Created for GreatEqualizer.com (http://www.greatequalizer.com/) and
 * documented at DHTML Lab (http://www.webreference.com/dhtml/)
 * License to use is granted if and only if this entire
 * copyright notice is included.
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
*/

// The chaser object. Since we don't anticipate having more than one on a 
// page, we don't bother making this into a class definition. All necessary 
// properties are set as properties of this object to avoid global variables.
var oChaser = {
  topMargin : 375,
  callRate : 10,
  slideTime : 1200,
  maxDiff : document.all ? document.body.clientHeight : window.innerHeight,
  isIE : document.all ? true : false,
  chaserDiv : document[document.all ? "all" : "layers"]["myChaser"]
}

// Arrange to have the main loop called as often as possible, but not more
// often than every 35 milliseconds. Even though some OS's can acheive
// better than that, there's no real reason to overload them if its not
// going to improve the animation quality very much. I tested, and better
// than 35ms doesn't do very much visually.
window.setInterval("oChaser.main( )", oChaser.callRate)

// Main loop. Updates targetY, and decides whether to start the animation
// over again, continue an existing animation, or do nothing at all.
oChaser.main = function( ) {
  this.currentY = this.isIE ? this.chaserDiv.style.pixelTop : this.chaserDiv.top
  this.scrollTop = this.isIE ? document.body.scrollTop : window.pageYOffset
  var newTargetY = this.scrollTop + this.topMargin
  
  if ( this.currentY != newTargetY ) {
    if ( newTargetY != this.targetY ) {
      this.targetY = newTargetY
      this.slideInit( )
    }
    this.slide( )
  }
}

// .slideInit( ). Initializes the slide animation. Sets properties of the
// oChaser object that will represent the various paramaters for the sine
// wave function.
oChaser.slideInit = function( ) {
  var now	= new Date( )
  this.A = this.targetY - this.currentY
  this.B = Math.PI / ( 2 * this.slideTime )
  this.C = now.getTime( )

  if (Math.abs(this.A) > this.maxDiff) {
    this.D = this.A > 0 ? this.targetY - this.maxDiff : this.targetY + this.maxDiff
    this.A = this.A > 0 ? this.maxDiff : -this.maxDiff
  } else {
    this.D = this.currentY
  }
}

// .slide( ). Moves the oChaser one frame. Its rate decreases and is
// defined by a sine wave.
oChaser.slide = function( ) {
  var now = new Date( )
  var newY = this.A * Math.sin( this.B * ( now.getTime( ) - this.C ) ) + this.D
  newY = Math.round( newY )

  if ((this.A>0 && newY>this.currentY) || (this.A<0 && newY<this.currentY)) {
    if (this.isIE)
      this.chaserDiv.style.pixelTop = newY
    else
      this.chaserDiv.top = newY
	}
}