{ Compile it, run it, figure out how it works and how to stop it.
  Complain about the flickering.  Try to get it to bounce from side
  to side instead of up and down.  Can you get the colors to change
  gradually from red to blue instead of abruptly?
}

program bounce(Input, Output);

import graphics;

const	radius = 40;
		windowWidth = 400;
		windowHeight = 400;
		bottom = 100;
		left = 100;

var		x, y : integer;
		red, blue, green : real;
		goingUp : boolean;

procedure StartGraphics;
begin
  { open a window }
 	initializegraphics;
	createwindow(left,bottom,left+windowWidth,bottom+windowHeight);
	
  { flood the window with a black background }
	setrgbcolor(0,0,0);
	flood; 
	flushgraphics
end;

procedure NewCenter( var horiz, vert : integer );
begin
	if goingUp then
		vert := vert + 1
	else
		vert := vert - 1;
	
	if vert >= windowHeight - radius then
	begin
		goingUp := false;
		red := 1.0;
		blue := 0
	end

	else if vert <= radius then
	begin
		goingUp := true;
		red := 0;
		blue := 1.0
	end

end;

begin	
	StartGraphics;

	x := windowWidth div 2;
	y := radius;
	goingUp := true;
	red := 0.0;
	blue := 1.0;
	green := 0.0;

	while true do
	begin
		setrgbcolor( red, green, blue );
		circle( x, y, radius );
		flushgraphics;
		
		setrgbcolor( 0, 0, 0 );
		circle( x, y, radius );
		flushgraphics;
		NewCenter( x, y )
	end;
	readln;

end.