/***************************************************************************
/*
/* Pure JavaScript Star Field Animation v1.0
/* 
/* Written by Alex Sirota (alex@elbrus.com), May 2004
/*
/* Copyright Alex Sirota 2004, all rights reserved
/*
/* Please ask for permission before using this code.
/*
/***************************************************************************/

function MoveObjNN4(obj,xPos,yPos) {
     obj.top = yPos;
     obj.left = xPos;
}

function MoveObjOpera(obj,xPos,yPos) {
     obj.style.pixelTop = yPos-25;
     obj.style.pixelLeft = xPos;
}

function MoveObjIE4(obj,xPos,yPos) {
     obj.style.pixelTop = yPos;
     obj.style.pixelLeft = xPos;
}

function MoveObjIE5NN6(obj,xPos,yPos) {
     obj.style.top = yPos + "px";
     obj.style.left = xPos + "px";
}

// horiz only functions for efficiency
function MoveObjHorizNN4(obj,xPos) {
     obj.left = xPos;
}

function MoveObjHorizOpera(obj,xPos) {
     obj.style.pixelLeft = xPos;

}

function MoveObjHorizIE4(obj,xPos) {
     obj.style.pixelLeft = xPos;
}

function MoveObjHorizIE5NN6(obj,xPos) {
     obj.style.left = xPos + "px";
}


var MoveObj = null;
var MoveObjHoriz = null;
function InitMoveObj() {
  if (document.layers) { //Netscape 4
  	MoveObj = MoveObjNN4; 
	MoveObjHoriz = MoveObjHorizNN4; 
  }
  else if (navigator.userAgent.indexOf("Opera") != -1) { //Opera
     MoveObj = MoveObjOpera; 
     MoveObjHoriz = MoveObjHorizOpera; 
  }
  else if (document.all && !document.getElementById) { //IE 4
     MoveObj = MoveObjIE4; 
     MoveObjHoriz = MoveObjHorizIE4; 
  }
  else if (document.getElementById) { //Netscape 6 & IE 5
     MoveObj = MoveObjIE5NN6;   
     MoveObjHoriz = MoveObjHorizIE5NN6;   
  }
}
InitMoveObj();

function GetRandomSpeed() {
	speed = Math.random();
	speed = (-1.0 / 1) * Math.log(1.0-speed); // uniform distribution becomes exponential, more slower stars
	speed = Math.round(speed*10) + 1;
	return speed;
}

function Speed2Color() {
	color =  255 - 10*(20 - speed); // the faster the brighter
	if (color > 255) color = 255;
	return color;
}

function getObjbyID(objId) {
  var myObj;
  if (document.layers) { //Netscape 4
     myObj = eval('document.' + objId);
  }
  else if (document.all && !document.getElementById) { //IE 4
     myObj = eval('document.all.' + objId);
  }
  else if (document.getElementById) { //Netscape 6 & IE 5
     myObj = document.getElementById(objId);
  }
  else {
     return null;
  }
  return myObj;
}

function SetStarColor(obj, color) {
    obj.style.color = 'rgb(' + color + "," + color + "," + color + ")";
}

var totalWidth;
var totalHeight;
var nStars;
var speeds;
var stars;
var star_tops;
var star_lefts;

function GetViewportWidth() {
  var myWidth = 0;
  if( typeof( window.innerWidth ) == 'number' ) {
    //Non-IE
    myWidth = window.innerWidth;
  } else if( document.documentElement &&
      (document.documentElement.clientWidth)) {
    //IE 6+ in 'standards compliant mode'
    myWidth = document.documentElement.clientWidth;
  } else if( document.body && ( document.body.clientWidth) ) {
    //IE 4 compatible
    myWidth = document.body.clientWidth;
  }
  return myWidth - 10;
}

function GetViewportHeight() {
  var myHeight = 0;
  if( typeof( window.innerHeight ) == 'number' ) {
    //Non-IE
    myHeight = window.innerHeight;
  } else if( document.documentElement &&
      (document.documentElement.clientHeight)) {
    //IE 6+ in 'standards compliant mode'
    myHeight = document.documentElement.clientHeight;
  } else if( document.body && ( document.body.clientHeight) ) {
    //IE 4 compatible
    myHeight = document.body.clientHeight;
  }
  return myHeight - 20;
}


function InitGlobals() {
   totalWidth = GetViewportWidth();
   totalHeight = GetViewportHeight();
   if (QueryString('nstars') != null) {
       nStars = QueryString('nstars');
   } else {
       nStars = 200;
   }
   speeds = new Array(nStars);
   stars = new Array(nStars);
   star_tops = new Array(nStars);
   star_lefts = new Array(nStars);
}

function StartStars() {
   InitStars();
   doStarAnim();
}

function InitStars() {
   InitGlobals();
   for (i=0; i < nStars; i++) {
	var x = Math.round(Math.random() * totalWidth);
	var y = Math.round(Math.random() * totalHeight);
	star_tops[i] = y;
	star_lefts[i] = x;
       
        var star = document.createElement('span');
	star.innerHTML = '&middot;';
	star.className =  'star';
	star.id =  'star_' + i;
	stars[i] = star;
        document.body.appendChild(star);

	var speed = GetRandomSpeed(); 
	speeds[i] = speed;
	var color = Speed2Color(speed);
	SetStarColor(stars[i], color);
	MoveObj(stars[i], x, y);
   }
}

function doStarAnim() {
	for (i=0; i < nStars; i++) {	
		var newStarLeft = star_lefts[i] + speeds[i];
		var newStarTop = star_tops[i];
		
		if (newStarLeft >= totalWidth) {
			newStarLeft = star_lefts[i] = 0;
			speed = GetRandomSpeed(); 
			speeds[i] = speed;
			color = Speed2Color(speed);
			SetStarColor(stars[i], color);
		}
	
		star_lefts[i] = newStarLeft;
		MoveObjHoriz(stars[i], newStarLeft);
		
	}
	timerID = setTimeout("doStarAnim()", 10);
}

function RepositionStars() {
	totalWidth = GetViewportWidth();
	totalHeight = GetViewportHeight();

	for (i=0; i < nStars; i++) {	
		star_lefts[i] = Math.round(Math.random() * totalWidth);
		star_tops[i] = Math.round(Math.random() * totalHeight);
		MoveObj(stars[i], star_lefts[i], star_tops[i]);
	}
}

window.onresize = RepositionStars;