-module(subscription).

-export([start/0, subscribe/1, unsubscribe/1, send/2, stop/1]).

start() ->
  Owner = self(),
  spawn_link(fun () -> loop(Owner, []) end).

subscribe(Server)     -> Server ! {add, self()}.
unsubscribe(Server)   -> Server ! {remove, self()}.
send(Server, Message) -> Server ! {deliver, self(), Message}.

stop(Server)          -> Server ! {quit, self()}, ok.

loop(Owner, Subscribers) ->
  receive
    {add, New}    -> loop(Owner, add(New, Subscribers));
    {remove, Old} -> loop(Owner, remove(Old, Subscribers));
    {deliver, Owner, Message} ->
        lists:map(fun(Each) -> Each ! Message end, Subscribers),
        loop(Owner, Subscribers);
    {quit, Owner} -> ok;
    Other ->
        io:format("Subscription: didn't understand: ~w~n", [Other]),
        exit({subscription, bad_message})
  end.

add(X, [])    -> [X];
add(X, [X|T]) -> [X|T];
add(X, [H|T]) -> [H|add(X, T)].

remove(_, [])    -> [];
remove(X, [X|T]) -> T;
remove(X, [H|T]) -> [H|remove(X, T)].