package {
    import flash.display.Stage;
    import flash.events.Event;
    import flash.events.KeyboardEvent;

    public class Key {
        private static var initialized:Boolean = false;
        private static var keysDown:Object = new Object();

        public static function initialize(stage:Stage) {
            if (!initialized) {
                stage.addEventListener(KeyboardEvent.KEY_DOWN, keyPressed);
                stage.addEventListener(KeyboardEvent.KEY_UP, keyReleased);
                stage.addEventListener(Event.DEACTIVATE, clearKeys);

                initialized = true;
            }
        }

        public static function isDown(keyCode:int):Boolean {
            if (!initialized) {
                throw new Error("Key class has yet been initialized.");
            }
            return Boolean(keyCode in keysDown);
        }

        private static function keyPressed(event:KeyboardEvent):void {
            keysDown[event.keyCode] = true;
        }

        private static function keyReleased(event:KeyboardEvent):void {
            if (event.keyCode in keysDown) {
                delete keysDown[event.keyCode];
            }
        }

        private static function clearKeys(event:Event):void {
            keysDown = new Object();
        }
    }
}