CS2030S - Lab 11 - Simulation I - Solved

## Prerequisite:

- Completed Lab 0


## Goal

In this lab, the goal is for you to practice the basic OOP
principles: encapsulation (including tell-don't-ask and
information hiding), abstraction, inheritance, and
polymorphism.

You are given six classes: five Java classes and one main
`Lab1` class.  Two of them are poorly written without
applying any of the OOP principles.  You should rewrite
these two classes using the OO principles you have learned,
removing them and creating new classes as needed.

## Background: Discrete Event Simulator

A discrete event simulator is software that simulates a
system (often modeled after the real world) with events and
states. An event occurs at a particular time and each event
alters the states of the system and may generate more
events. A discrete event simulator can be used to study many
complex real-world systems. The term discrete refers to the
fact that the states remain unchanged between two events,
and therefore, the simulator can jump from the time of one
event to another, instead of following the clock in real
time.

In Lab 1, we provide you with three very generic classes:
`Simulator`, which implements a discrete event simulator,
`Event`, which encapsulates an event (with a time), and
`Simulation`, which encapsulates the states we are
simulating.  The `Event` and `Simulation` class can be
extended to implement any actual simulation (network, road
traffic, weather, pandemic, etc).  More details of these
classes can be found below.

## Simulating a Shop

In Lab 1, we wish to simulate a shop.  A shop can have
one or more service counters.  

In the beginning, all service counters are available.  A
counter becomes unavailable when it is serving a customer,
and becomes available again after servicing a customer.

A customer, upon arrival at the shop, looks for the first
available counters for service.  If no counter is available,
the customer departs (due to COVID-19, no waiting is
allowed).  Otherwise, the customer is served, and after
being served for a given amount of time (called service
time), the customer departs.

Two classes, `ShopSimulation` (a subclass of Simulation)
and `ShopEvent` (a subclass of Event) are provided.  The
two classes implement the simulation above.

## The `Event` class

You should not edit this class.  The following is for your
info only.

The `Event` class is an abstract class with a single field
`time`, which indicates the time the event occurs.  The
`Event::toString` method returns the time as a string and
the `Event::getTime` method returns the time.

The most important thing to know about the `Event` class is
that it has an abstract method `simulate` that needs to be
overridden by its subclass to concretely define the action
to be taken when this event occurs.

Simulating an event can lead to more events being created.
`Event::simulate` returns an array of `Event` instances.

## The `Simulation` class

You should not edit this class.  The following is for your
info only.

The `Simulation` class is an abstract class with a single
method `getInitialEvents`, which returns an array of events
to simulate.  Each of these events may generate more events.

## The `Simulator` class

You should not edit this class.  The following is for your
info only.

The `Simulator` class is a class with only two methods and
it is what drives the simulation.  To run the simulator, we
initialize it with a `Simulation` instance, and then call
`run`:

``` 
Simulation sim = new SomeSimulation(); 
new Simulator(sim).run(); 
```

The `Simulation::run` method simply does the following:

- It gets the list of initial `Event` objects from the
  `Simulation` object;
- It then simulates the pool of events, one-by-one in the
  order of increasing time, by calling `Event::simulate`;
- If simulating an event resulted in one or more new events,
  the new events are added to the pool.
- Before each event is simulated, `Event::toString` is called
  and a message is printed
- The simulation stops running if there are no more events to
  simulate.

For those of you taking CS2040S, you might be interested to
know that the `Simulator` class uses a priority queue to
keep track of the events with their time as the key.

## The `ShopSimulation` class

You are expected to edit this class and create new classes.

The `ShopSimulation` class is a concrete implementation of
a `Simulation`.  This class is responsible for:

- reading the inputs from the standard inputs,

- initialize the service counters (represented with boolean
  `available` arrays)

- initialize the events corresponding to customer arrivals

- return the list of customer arrival events to the
  `Simulator` object when `getInitialEvent` is called.

Each customer has an id.  The first customer has id 0,
The next one is 1, and so on.

Each counter has an id, numbered from 0, 1, 2, onwards.

## The `ShopEvent` class

You are expected to _replace_ this class with new classes.

The `ShopEvent` class is a concrete implementation of
`Event`.  This class overrides the `simulate` method to
simulate the customer and counter behavior.

A `ShopEvent` instance can be tagged as either an arrival
event, service-begin event, service-end event, or departure
event.

- Arrival: the customer arrives.  It finds the first
  available service counter (scanning from id 0 upwards) and
  go to the counter for service immediately.  A
  service-begin event is generated.  If no counter is
  available, it departs.  A departure event is generated.

- Service-begin: the customer is being served.  A
  service-end event scheduled at the time (current time + service
  time) is generated.

- Service-end: the customer is done being served and departs
  immediately.  A departure event is generated.

- Departure: the customer departs.

## Inputs and Outputs

The main program `Lab1.java` reads the following, from the
standard inputs.

- An integer n, indicating the number of customers to simulate.

- An integer k, indicating the number of service counters the 
  shop has.

- n pairs double values, each pair corresponds to a customer.  
  The first value indicates the arrival time, the second
  indicates the service time for the customer.

The customers are sorted in increasing order of arrival time.  

## Assumptions

We assume that no two events ever occur at the same time.
As per all labs, we assume that the input is correctly
formatted.

## Your Task

The two classes, `ShopSimulation` and `ShopEvent`, are
poorly written.  They do not fully exploit OOP features and
apply the OO principles such as abstraction, encapsulation
(including information hiding and tell-don't-ask), composition,
inheritance, polymorphism, and LSP.

Rewrite these two classes (adding new ones as needed) with the 
OOP principles that you have learned:

- encapsulation to group relevant fields and methods into
  new classes

- inheritance and composition to model the relationship
  between the classes

- information hiding to hide internal details

- using polymorphism to make the code more succinct and
  extendable in the future, while adhering to LSP

Here are some hints:

- Think about the problem that you are solving: what are
the nouns? These are good candidates for new classes.  

- For each class, what are the attributes/properties relevant
to the class? These are good candidates for fields in the
class.  

- Do the classes relate to each other via IS-A or HAS-A
  relationship?

- For each class, what are their responsibilities?
What can they do? These are good candidates for methods in
the class.  

- How do the objects of each class interact?
These are good candidates for public methods.

- What are some behavior that changes depending on
  the specific type of objects?

Note that the goal of this lab and, and of CS2030S in
general, is _NOT to solve the problem with the cleverest and
the shortest piece of code possible_. For instance, you might
notice that you can solve Lab 1 with only a few variables
and an array.  But such a solution is hard to extend and
modify. In CS2030S, our goal is to produce software that
can easily evolve and be modified, with a reduced risk of
introducing bugs while doing so.

Note that Lab 1 is the first of a series of labs, where we
introduce new requirements or modify existing ones in every
lab (not unlike what software engineers face in the real
world). We will modify the behavior for the shop, the
counters, and the customers.  In particular, in the future,

- a customer may have different behavior in selecting the
  counters to join.
- a shop may have different rules about managing customers
  when the COVID-19 pandemic is over, including introducing
  different types of counters.