// EventQueue class

// EventQueue is queue of events. This is an abstract class. It has got one pure virtual 
// function, draw. This is implemented by the classes that are derived from it. This is 
// a queue which has just the 'front' pointer. 

#ifndef EVENTQUEUE_H
#define EVENTQUEUE_H

#include "Event.h"

class EventQueue {
public:
	// the constructor, creates an empty queue of Events
	EventQueue ();

	// = true if the queue is empty
	virtual bool isEmpty () const;

	// add a new event e, at the end of the queue
	virtual void enqueue (Event *e);

	// = returns a pointer to event object at the front of the queue. And the object
	// is removed from the queue. If the queue is empty, it returns NULL
	virtual Event *dequeue ();

	// = length of the queue
	virtual int getLength () const;

	// draw out the queue
	virtual void draw () = 0;

	// the destructor for the queue
	virtual ~EventQueue ();

protected:
	// node in the queue
	struct QueueNode {
		Event *e;			// the event
		QueueNode *next;	// pointer to the next node in the queue
	};

	QueueNode *front;		// pointer to the first element in the queue
	QueueNode *rear;		// pointer to the last element of the queue
	int length;				// length of the queue

	// returns the i-the element (counting starts from 'front') of the queue
	virtual Event *getIthEvent (int i);

private:
	// a helper function used by the destructor. This function calls itself
	// recursively to delete the queue
	void deleteQueue (QueueNode *n);
};

#endif