cashier.java

国外的数据结构与算法分析用书

import dslib.exception.ItemNotComparableUosException;
import dslib.base.FormatUos; 

/**	A Cashier for a simple waiting-line simulation. The cashier knows which
	customer is being serviced (if any), knows when the last idle time period
	started, and accumulates the total amount of idle time. */
public class Cashier
{
	/**	Customer currently being served. */
	protected Customer customer;

	/**	Is the cashier currently busy?
		Analysis : Time = O(1)	*/
	public boolean busy()
	{
		return customer != null;
	}

	/**	The total amount of time that the cashier has been idle. */
	protected int totalIdleTime; 

	/**	The time when the cashier started his/her last idle period. */
	protected int startIdleTime;

	/**	Time to serve a customer. */
	protected final int serviceTime = 20;

	/**	Create a cashier not busy, and initialize the startIdleTime and totalIdleTime. 
		Analysis : Time = O(1) */
	public Cashier()
	{
		customer = null;
		startIdleTime = 0;
		totalIdleTime = 0;
	}

	/**	Start the cashier serving the next customer.
		Analysis: Time = O(1) 
		PRECONDITION
			!Simulation.waitingLine.isEmpty()
			!busy()  */
	public void startNextService() throws WaitingLineEmptyException, CashierBusyException
	{
		if (Simulation.waitingLine.isEmpty())
			throw new WaitingLineEmptyException("Cannot start next customer if line is empty.");
		if (busy())
			throw new CashierBusyException("Cashier cannot start a new customer as the cashier is already busy.");
	
		customer = Simulation.waitingLine.first();
		Simulation.waitingLine.deleteFirst();
		totalIdleTime = totalIdleTime + Simulation.time - startIdleTime;
		traceStartService();
		FinishServiceEvent event = new FinishServiceEvent(Simulation.time + serviceTime);
		Simulation.eventslist.insert(event);
		traceScheduleFinishService();
	}

	/**	Complete the service of the present customer.
		Analysis : Time = O(1) */
	public void completeService()
	{
		traceFinishService();
		customer = null;
		startIdleTime =  Simulation.time;
		if (!Simulation.waitingLine.isEmpty())
			try
			{
				startNextService();
			} catch (WaitingLineEmptyException e) 
			{
				/*	Should not occur: !isEmpty() is checked. */
				e.printStackTrace();
				throw new Error("Error: possible fault in isEmpty()");
			} catch (CashierBusyException e) 
			{
				/*	Should not occur: the cashier was just set to the idle state. */
				e.printStackTrace();
				throw new Error("Error: possible fault in setting cashier idle");
			}
	}

	/**	Print information for the start of service of the current customer.
		Analysis: Time = O(1) */
	public void traceStartService()
	{
		System.out.println(FormatUos.pad("", 4, 'r') + FormatUos.pad("", 7, 'c') + FormatUos.pad("start service", 14, 'l') 
				+ FormatUos.pad(customer.number, 9, 'r') + FormatUos.pad("", 12, 'r') 
				+ FormatUos.pad(Simulation.time - customer.arrivalTime, 11, 'r') // customer waiting time
				+ FormatUos.pad(Simulation.time - startIdleTime, 11, 'r'));  // cashier idle time
	}

	/**	Print information for the scheduling of a finish service event.
		Analysis: Time = O(1) */	
	public void traceScheduleFinishService()
	{
		System.out.println(FormatUos.pad("", 4, 'r') + FormatUos.pad("sched", 7, 'c')
				+ FormatUos.pad("finish service", 14, 'l') 
				+ FormatUos.pad(customer.number, 9, 'r') 
				+ FormatUos.pad(Simulation.time + serviceTime, 12, 'r'));  // scheduled finish time
	}

	/**	Print information for a finish service event to facilitate tracing.
		Analysis: Time = O(1) */
	public void traceFinishService()
	{
		System.out.println(FormatUos.pad(Simulation.time, 4, 'r') + FormatUos.pad("", 7, 'c') 
				+ FormatUos.pad("finish service", 14, 'l') + FormatUos.pad(customer.number, 9, 'r'));
	}
}