#define  kVelocityMultiplier      800 
#define  kFloorFrictionValue      .045
#define  kDecelerationMultiplier  .01
#define  kBouncingMultiplier     1.13

- (void)setCurPoint:(CGPoint)newPoint
{
   prevPoint = curPoint;
   curPoint  = newPoint;
   
   if (curPoint.x < 0)  {
      curPoint.x = 0;
      xBallVelocity = -(xBallVelocity * kBouncingMultiplier); //  0;
   }
   if (curPoint.y < 0)  {
      curPoint.y = 0;
      yBallVelocity = -(yBallVelocity * kBouncingMultiplier); // 0;
   }
   
   if (curPoint.x > self.bounds.size.width - image.size.width)  {
      curPoint.x = self.bounds.size.width - image.size.width;
      xBallVelocity = -(xBallVelocity * kBouncingMultiplier); //  0;
   }
   if (curPoint.y > self.bounds.size.height - image.size.height)  {
      curPoint.y = self.bounds.size.height - image.size.height;
      yBallVelocity = -(yBallVelocity * kBouncingMultiplier); // 0;
   }
   
   CGRect  curImageRect  = CGRectMake (curPoint.x, curPoint.y, image.size.width, image.size.height);
   CGRect  prevImageRect = CGRectMake (prevPoint.x, prevPoint.y, image.size.width, image.size.height);
   
   [self setNeedsDisplayInRect:CGRectUnion(curImageRect, prevImageRect)];
}

- (CGFloat)floorFriction:(CGFloat)acceValue
{
   if (acceValue > kFloorFrictionValue)
      acceValue -= kFloorFrictionValue;
   else  if (acceValue < -kFloorFrictionValue)
      acceValue += -kFloorFrictionValue;
   else
      acceValue = 0.;
   
   return (acceValue);
}

- (CGFloat)deceleration:(CGFloat)acceValue
{
   CGFloat retVal = acceValue - (acceValue * kDecelerationMultiplier);
   
   if (retVal > 20. || retVal < -20.)
      return (retVal / 10.);
   if (retVal < .001 && retVal > -.001)
       return (0.);

   return (retVal);
}

- (void)draw
{
   static  NSDate  *lastDrawTime = nil;
   
   if (lastDrawTime)  {
      NSTimeInterval  secondsSinceLastDraw = -([lastDrawTime timeIntervalSinceNow]);
      
      xBallVelocity +=  ([self floorFriction:acceleration.x] * secondsSinceLastDraw);
      xBallVelocity = [self deceleration:xBallVelocity];
      yBallVelocity += -([self floorFriction:acceleration.y] * secondsSinceLastDraw);
      yBallVelocity = [self deceleration:yBallVelocity];
      
      CGFloat xDelta = xBallVelocity * secondsSinceLastDraw * kVelocityMultiplier;
      CGFloat yDelta = yBallVelocity * secondsSinceLastDraw * kVelocityMultiplier;
      
      self.curPoint = CGPointMake (curPoint.x + xDelta, curPoint.y + yDelta);
   }
   
   [lastDrawTime release];
   lastDrawTime = [[NSDate alloc] init];
}

- (void)accelerometer:(UIAccelerometer *)accel didAccelerate:(UIAcceleration *)acceleration
{
   static long   staticCounter = 0;
   BallView     *ballView = (BallView *)self.view;
   
   [ballView setAcceleration:acceleration];
   [ballView draw];
   
   switch (++staticCounter % 6)  {
      case  0:  xPosition.text = [NSString stringWithFormat:@"%.0f", ballView.curPoint.x];  break;
      case  1:  yPosition.text = [NSString stringWithFormat:@"%.0f", ballView.curPoint.y];  break;
   
      case  2:  xAcceleration.text = [NSString stringWithFormat:@"%.2f", ballView.acceleration.x];  break;
      case  3:  yAcceleration.text = [NSString stringWithFormat:@"%.2f", ballView.acceleration.y];  break;
   
      case  4:  xVelocity.text = [NSString stringWithFormat:@"%.2f", ballView.xBallVelocity];  break;
      case  5:  yVelocity.text = [NSString stringWithFormat:@"%.2f", ballView.yBallVelocity];  break;
   }
}