Flappy Bird Game Tutorial Code

package{
    import flash.display.MovieClip;
    import flash.events.KeyboardEvent;
    import flash.ui.Keyboard;
    import flash.events.Event; //used for ENTER_FRAME event

    public class Main extends MovieClip{

        //constants
        const gravity:Number = 1.5;            //gravity of the game
        const dist_btw_obstacles:Number = 300; //distance between two obstacles
        const ob_speed:Number = 8;             //speed of the obstacle
        const jump_force:Number = 15;          //force with which it jumps

        //variables
        var player:Player = new Player();
        var lastob:Obstacle = new Obstacle();  //varible to store the last obstacle in the obstacle array
        var obstacles:Array = new Array();     //an array to store all the obstacles
        var yspeed:Number = 0;                 //A variable representing the vertical speed of the bird
        var score:Number = 0;                  //A variable representing the score

        public function Main(){
            init();
        }

        function init():void {
            //initialize all the variables
            player = new Player();
            lastob = new Obstacle();
            obstacles = new Array();
            yspeed = 0;
            score = 0;

            //add player to center of the stage the stage
            player.x = stage.stageWidth/2;
            player.y = stage.stageHeight/2;
            addChild(player);

            //create 3 obstacles ()
            createObstacle();
            createObstacle();
            createObstacle();

            //Add EnterFrame EventListeners (which is called every frame) and KeyBoard EventListeners
            addEventListener(Event.ENTER_FRAME,onEnterFrameHandler);
            stage.addEventListener(KeyboardEvent.KEY_UP, key_up);
        }

        private function key_up(event:KeyboardEvent){
            if(event.keyCode == Keyboard.SPACE){
                //If space is pressed then make the bird
                yspeed = -jump_force;
            }
        }

        function restart(){
            if(contains(player))
                removeChild(player);
                for(var i:int = 0; i < obstacles.length; ++i){
                    if(contains(obstacles[i]) && obstacles[i] != null)
                    removeChild(obstacles[i]);
                    obstacles[i] = null;
                }
                obstacles.slice(0);
                init();
        }

        function onEnterFrameHandler(event:Event){
            //update player
            yspeed += gravity;
            player.y += yspeed;

            //restart if the player touches the ground
            if(player.y + player.height/2 > stage.stageHeight){
                restart();
            }

            //Don't allow the bird to go above the screen
            if(player.y - player.height/2 < 0){
                player.y = player.height/2;
            }

            //update obstacles
            for(var i:int = 0;i<obstacles.length;++i){
                updateObstacle(i);
            }

            //display the score
            scoretxt.text = String(score);
        }

        //This functions update the obstacle
        function updateObstacle(i:int){
            var ob:Obstacle = obstacles[i];

            if(ob == null)
            return;
            ob.x -= ob_speed;

            if(ob.x < -ob.width){
                //if an obstacle reaches left of the stage then change its position to the back of the last obstacle
                changeObstacle(ob);
            }

            //If the bird hits an obstacle then restart the game
            if(ob.hitTestPoint(player.x + player.width/2,player.y + player.height/2,true)
               || ob.hitTestPoint(player.x + player.width/2,player.y - player.height/2,true)
               || ob.hitTestPoint(player.x - player.width/2,player.y + player.height/2,true)
               || ob.hitTestPoint(player.x - player.width/2,player.y - player.height/2,true)){
                restart();
            }

            //If the bird got through the obstacle without hitting it then increase the score
            if((player.x - player.width/2 > ob.x + ob.width/2) && !ob.covered){
                ++score;
                ob.covered = true;
            }
        }

        //This function changes the position of the obstacle such that it will be the last obstacle and it also randomizes its y position
        function changeObstacle(ob:Obstacle){
            ob.x = lastob.x + dist_btw_obstacles;
            ob.y = 100+Math.random()*(stage.stageHeight-200);
            lastob = ob;
            ob.covered = false;
        }

        //this function creates an obstacle
        function createObstacle(){
            var ob:Obstacle = new Obstacle();
            if(lastob.x == 0)
            ob.x = 800;
            else
            ob.x = lastob.x + dist_btw_obstacles;
            ob.y = 100+Math.random()*(stage.stageHeight-200);
            addChild(ob);
            obstacles.push(ob);
            lastob = ob;
        }


    }
}