-module (explain).
-export ([start/1, add/2]).

%% Lets start a process
%% This process will be given the name bound to Name
%% This is converted to a GUID and sent to the loop
%% left and right are placeholders for the left and right neighbour
start(Name) -> GUID = erlang:md5(Name),
               spawn(fun() -> loop(dict:new(),
                                   GUID,
                                   left,
                                   right)
                     end).

%% Remote Procedure Call takes care of all the communication
rpc(Pid, Request) ->
    Pid ! {self(), Request},
    receive
    {Pid, Response} ->
        Response
    end.

%% Add a contact to the given Pid
add(Pid, Contact) -> rpc(Pid, {add, Contact}).

%% Our main loop
loop(Contacts, GUID, Left, Right) ->
    receive
        %% We receive a touple
        {From, Request} ->
            %% The handle_request always returns a result, the contact list, the left and right neighbour
            {Res, Updated, NewLeft, NewRight} = handle_request(Request, Contacts, GUID, Left, Right),
            %% Lets send the result back to whom ever asked for it
            From ! {self(), Res},
            %% Lets run again
            loop(Updated, GUID, NewLeft, NewRight)
    end.

%% Handles all the requests from the remote procedure calls
handle_request(Request, Contacts, GUID, Left, Right) ->
    case Request of
        %% The add method already finds the correct peer, so we just add the contact
        {add, Contact} ->
            {Name, _, _} = Contact,
            case dict:is_key(Name, Contacts) of
                true  -> {{error, Name, is_already_there},
                          Contacts,
                          Left,
                          Right};
                false -> {ok,
                          dict:store(Name, Contact, Contacts),
                          Left,
                          Right}
            end;
        Other -> {{error, unknown_request, Other},
                  Contacts,
                  Left,
                  Right}
    end.