/******************************************************************************
	Elastic Trail script (By Philip Winston @ pwinston@yahoo.com, 
	URL: http://www.geocities.com/pwinston/)
	Script featured on Dynamicdrive.com
	For this and 100's more DHTML scripts, visit http://dynamicdrive.com
******************************************************************************/
var nDots = 7;
var Xpos = 0;
var Ypos = 0;
// fixed time step, no relation to real time
var DELTAT = .01;
// size of one spring in pixels
var SEGLEN = 10;
// spring constant, stiffness of springs
var SPRINGK = 10;
// all the physics is bogus, just picked stuff to
// make it look okay
var MASS = 1;
// Positive XGRAVITY pulls right, negative pulls left
// Positive YGRAVITY pulls down, negative up
var XGRAVITY = 0;
var YGRAVITY = 50;
// RESISTANCE determines a slowing force proportional to velocity
var RESISTANCE = 10;
// stopping criterea to prevent endless jittering
// doesn't work when sitting on bottom since floor
// doesn't push back so acceleration always as big
// as gravity
var STOPVEL = 0.1;
var STOPACC = 0.1;
var DOTSIZE = 11;
// BOUNCE is percent of velocity retained when 
// bouncing off a wall
var BOUNCE = 0.75;

var isNetscape = navigator.appName=="Netscape";

// always on for now, could be played with to
// let dots fall to botton, get thrown, etc.
var followmouse = true;

var dots = new Array();
init();

function init() {
  var i = 0;
  for (i = 0; i < nDots; i++) {
    dots[i] = new dot(i);
  }
  if (!isNetscape) {
    // I only know how to read the locations of the 
    // <LI> items in IE
    //skip this for now
    // setInitPositions(dots)
  }
  // set their positions
  for (i = 0; i < nDots; i++) {
    dots[i].obj.left = dots[i].X;
    dots[i].obj.top = dots[i].Y;
  }
  if (isNetscape) {
    // start right away since they are positioned
    // at 0, 0
    startanimate();
  } else {
    // let dots sit there for a few seconds
    // since they're hiding on the real bullets
    setTimeout("startanimate()", 1000);
  }
}

function dot(i) {
  this.X = Xpos;
  this.Y = Ypos;
  this.dx = 0;
  this.dy = 0;
  if (isNetscape) {	
    this.obj = eval("document.dot" + i);
  } else {
    this.obj = eval("dot" + i + ".style");
  }
}

function startanimate() {	
  setInterval("animate()", 20);
}

// This is to line up the bullets with actual LI tags on the page
// Had to add -DOTSIZE to X and 2*DOTSIZE to Y for IE 5, not sure why
// Still doesn't work great
function setInitPositions(dots) {
  // initialize dot positions to be on top 
  // of the bullets in the <ul>
  var startloc = document.all.tags("LI");
  var i = 0;
  for (i = 0; i < startloc.length && i < (nDots - 1); i++) {
    dots[i+1].X = startloc[i].offsetLeft
    startloc[i].offsetParent.offsetLeft - DOTSIZE;
    dots[i+1].Y = startloc[i].offsetTop +
    startloc[i].offsetParent.offsetTop + 2*DOTSIZE;
  }
  // put 0th dot above 1st (it is hidden)
  dots[0].X = dots[1].X;
  dots[0].Y = dots[1].Y - SEGLEN;
}

// just save mouse position for animate() to use
function MoveHandler(e) {
  Xpos = e.pageX;
  Ypos = e.pageY;	  
  return true;
}

// just save mouse position for animate() to use
function MoveHandlerIE() {
  Xpos = window.event.x + document.body.scrollLeft;
  Ypos = window.event.y + document.body.scrollTop;	  
}

if (isNetscape) {
  document.captureEvents(Event.MOUSEMOVE);
  document.onMouseMove = MoveHandler;
} else {
  document.onmousemove = MoveHandlerIE;
}

function vec(X, Y) {
  this.X = X;
  this.Y = Y;
}

// adds force in X and Y to spring for dot[i] on dot[j]
function springForce(i, j, spring) {
  var dx = (dots[i].X - dots[j].X);
  var dy = (dots[i].Y - dots[j].Y);
  var len = Math.sqrt(dx*dx + dy*dy);
  if (len > SEGLEN) {
    var springF = SPRINGK * (len - SEGLEN);
    spring.X += (dx / len) * springF;
    spring.Y += (dy / len) * springF;
  }
}


function animate() {	
  // dots[0] follows the mouse,
  // though no dot is drawn there
  var start = 0;
  if (followmouse) {
    dots[0].X = Xpos;
    dots[0].Y = Ypos;	
    start = 1;
  }
  for (i = start ; i < nDots; i++ ) {
    var spring = new vec(0, 0);
    if (i > 0) {
      springForce(i-1, i, spring);
    }
    if (i < (nDots - 1)) {
      springForce(i+1, i, spring);
    }
    // air resisitance/friction
    var resist = new vec(-dots[i].dx * RESISTANCE, -dots[i].dy * RESISTANCE);
    // compute new accel, including gravity
    var accel = new vec((spring.X + resist.X)/MASS + XGRAVITY, (spring.Y + resist.Y)/ MASS + YGRAVITY);
    // compute new velocity
    dots[i].dx += (DELTAT * accel.X);
    dots[i].dy += (DELTAT * accel.Y);
    // stop dead so it doesn't jitter when nearly still
    if (Math.abs(dots[i].dx) < STOPVEL &&
        Math.abs(dots[i].dy) < STOPVEL &&
        Math.abs(accel.X) < STOPACC &&
        Math.abs(accel.Y) < STOPACC) {
      dots[i].dx = 0;
      dots[i].dy = 0;
    }
    // move to new position
    dots[i].X += dots[i].dx;
    dots[i].Y += dots[i].dy;
    // get size of window
    var height, width;
    if (isNetscape) {
      height = window.innerHeight + window.pageYOffset;
      width = window.innerWidth + window.pageXOffset;
    } else {	
      height = document.body.clientHeight + document.body.scrollTop;
      width = document.body.clientWidth + document.body.scrollLeft;
    }
    // bounce off 3 walls (leave ceiling open)
    if (dots[i].Y >=  height - DOTSIZE - 1) {
      if (dots[i].dy > 0) {
        dots[i].dy = BOUNCE * -dots[i].dy;
      }
      dots[i].Y = height - DOTSIZE - 1;
    }
    if (dots[i].X >= width - DOTSIZE) {
      if (dots[i].dx > 0) {
        dots[i].dx = BOUNCE * -dots[i].dx;
      }
      dots[i].X = width - DOTSIZE - 1;
    }
    if (dots[i].X < 0) {
      if (dots[i].dx < 0) {
        dots[i].dx = BOUNCE * -dots[i].dx;
      }
      dots[i].X = 0;
    }
    // move img to new position
    dots[i].obj.left = dots[i].X;			
    dots[i].obj.top =  dots[i].Y;		
  }
}