-module(frequency).
-export([start/0, stop/0, allocate/0, deallocate/2]).
-export([init/0]).
%% These are the start functions used to create and
%% initialize the server.
start() ->
	register(frequency, spawn(frequency, init, [])).
init() ->
	Frequencies = {get_frequencies(), []},
	loop(Frequencies).
% Hard Coded
get_frequencies() -> [10,11,12,13,14,15].
%% The client Functions
stop() -> call(stop).
allocate() -> call(allocate).
deallocate(Freq, Pid) -> call({deallocate, Freq, Pid}).
%% We hide all message passing and the message
%% protocol in a functional interface.
call(Message) ->
	frequency ! {request, self(), Message},
	receive
		{reply, Reply} -> Reply
	end.
%% The Main Loop
loop(Frequencies) ->
	receive
		{request, Pid, allocate} ->
			{NewFrequencies, Reply} = allocate(Frequencies, Pid),
			reply(Pid, Reply),
			loop(NewFrequencies);
		{request, Pid , {deallocate, Freq}} ->
			NewFrequencies = deallocate(Frequencies, Freq, Pid),
			reply(Pid, ok),
			loop(NewFrequencies);
		{request, Pid, stop} ->
			reply(Pid, ok)
	end.
reply(Pid, Reply) ->
	Pid ! {reply, Reply}.
allocate({[], Allocated}, _Pid) ->
    erlang:display(Allocated),
    {{[], Allocated}, {error, no_frequency}};
allocate({[Freq|Free], Allocated}, Pid) ->
    erlang:display(Allocated),
    {{Free, [{Freq, Pid}|Allocated]}, {ok, Freq}}.
deallocate({Free, Allocated}, Freq, Pid) ->
	case lists:member({Freq,Pid}, Allocated) of
		true -> NewAllocated=lists:keydelete(Freq, 1, Allocated), {[Freq|Free], NewAllocated};
		false -> {[Free], Allocated}
	end.