// Elastic Bullets -
  // Philip Winston <winston@cs.unc.edu>
  // 10/98
  // I've tried it with IE4 and Netscape 4.5 on NT and
  // Netscape 4.04 on a Mac

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;
var GRAVITY = 50;
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 = 10;
  // 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;


init();

function init()
{
	var i = 0;
	dots = new Array();
	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
		setInitPositions()
	}

	  // 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
		  // sincey they're hiding on the real bullets
		setTimeout("startanimate()", 3000);
	}
}



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);
}



function setInitPositions()
{
  // 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++) {
		dots[i+1].X = startloc[i].offsetLeft +
					startloc[i].offsetParent.offsetLeft;
		dots[i+1].Y = startloc[i].offsetTop +
					startloc[i].offsetParent.offsetTop +
					DOTSIZE/2;
	}
	  // 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;
  Ypos = window.event.y;
}

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,
							(spring.Y + resist.Y)/ MASS + GRAVITY);

		  // 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;
		    width = window.innerWidth;
		} else {
		    height = document.body.clientHeight;
		    width = document.body.clientWidth;
		}

          // bounce of 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;
	}
}