====== Erlang Language Lab, Multiple Processes and Communication ======

===== General Description =====

Write a program which implements a simple Instant Messaging (IM) system.
The system consists of two components:
  * client - which allows to send and receive messages,
  * server (//name server// and //chat-room//) - which relays messages to clients.
Both components should be written in Erlang.

The IM system is built around //chat-rooms//.
//Chat-rooms// are identified by unique names, these names can be registered or discovered by the //name server//.
Any message sent by a //client// to the //chat-room// is relayed to all connected //clients//.

Grading: points are given in square brackets.

===== Details =====

==== Name Server ====
[10] There is a single //name server// process which accepts incoming ''connection_request'' messages from //clients//:
There are separate processes for each //chat-room//.
Upon incoming ''connection_request'' the //name server// should check if there is already a //chat-room// process active.
If there is none, it should be spawned.
The //name server// should reply the //client// informing it about //chat-room's// PID.
Further communication takes place between the //client// and //chat-room// processes.
Additionally the //name server// informs the //chat-room// about the new //client// sending its PID.

The //name server// can also be asked about all available //chat-rooms//.
It replies with a list of their names.

==== Chat-room ====

[10] A trusted //client// can send a ''message'' to the //chat-room//.
The //client// is trusted if its PID is obtained from the //name-server//.
Untrusted //clients'// messages are not relayed, appropriate error message is sent. 
The //chat-room// process forwards a payload of the incoming message to all connected //clients// (processes), including the sender
The //chat-room// keeps track of all connected (trusted) //clients//.
A //client// can disconnect itself from the //chat-room// by sending appropriate ''disconnect'' message.
If there are no //clients// connected the //chat-room// should terminate itself - in such a case the //name server// should be informed that the //chat room// no longer exists.

==== Client ====

[10] The //client// implements the following functions which subsequently send messages to appropriate processes:
  * ''get_chatrooms'' - returns a list of chat-rooms,
  * ''connect'' - connects to a chat-room,
  * ''disconnect'' - disconnects from a chat-room,
  * ''receive'' - returns all received messages along with their origins,
  * ''send'' - sends a message.
Decide by yourself what arguments the functions should have.

[5] Make the above functions work to provide an interactive client which can connect to a chat-room.
While connected it gets all the messages, and simultaneously it is capable of sending messages.

  * Hint: spawn a process which gets and displays messages along with another one which sends them.

===== Implementation and Testing =====

There is a single //name server//, multiple //chat-rooms// and multiple //clients//.

Make the //name server// a registered process which name is defined upon creating the process (i.e.: ''name_server'') - it would allow to extend the system to have different chat servers running.
A //client// should know the name of the //name server// to connect to (it might be the client functions' parameter).
Furthermore it allows to test the system using multiple shells or Erlang nodes easily.