%% Based on code from
%%   Erlang Programming
%%   Francecso Cesarini and Simon Thompson
%%   O'Reilly, 2008
%%   http://oreilly.com/catalog/9780596518189/
%%   http://www.erlangprogramming.org/
%%   (c) Francesco Cesarini and Simon Thompson

-module(frequency).
-export([start/1,allocate/0,deallocate/1,stop/0]).
-export([test_non_overloaded_server/0, test_overloaded_server/0]).
-export([init/1]).

%% These are the start functions used to create and
%% initialize the server.

%% Additional WorkTime parameter allows to easily simulate overloaded server,
%% It is passed to init and loop
start(WorkTime) ->
    register(frequency,
             spawn(frequency, init, [WorkTime])).

init(WorkTime) ->
    Frequencies = {get_frequencies(), []},
    loop(Frequencies, WorkTime).

                                                % Hard Coded
get_frequencies() -> [10,11,12,13,14,15].

%% The Main Loop

loop(Frequencies, WorkTime) ->
    timer:sleep(WorkTime),
    receive
        {request, Pid, allocate} ->
            {NewFrequencies, Reply} = allocate(Frequencies, Pid),
            Pid ! {reply, Reply},
            loop(NewFrequencies, WorkTime);
        {request, Pid , {deallocate, Freq}} ->
            NewFrequencies = deallocate(Frequencies, Freq),
            Pid ! {reply, ok},
            loop(NewFrequencies, WorkTime);
        {request, Pid, stop} ->
            Pid ! {reply, stopped}
    end.

%% Functional interface

allocate() ->
    frequency ! {request, self(), allocate},
    receive
        {reply, Reply} -> Reply
    after 10 -> {error, timeout}
    end.

deallocate(Freq) ->
    frequency ! {request, self(), {deallocate, Freq}},
    receive
        {reply, Reply} -> Reply
    after 10 -> {error, timeout}
    end.

stop() ->
    frequency ! {request, self(), stop},
    receive
        {reply, Reply} -> Reply
    after 10 -> {error, timeout}
    end.

%% The Internal Help Functions used to allocate and
%% deallocate frequencies.

allocate({[], Allocated}, _Pid) ->
    {{[], Allocated}, {error, no_frequency}};
allocate({[Freq|Free], Allocated}, Pid) ->
    {{Free, [{Freq, Pid}|Allocated]}, {ok, Freq}}.

deallocate({Free, Allocated}, Freq) ->
    NewAllocated=lists:keydelete(Freq, 1, Allocated),
    {[Freq|Free],  NewAllocated}.

clear() ->
    receive
        Msg ->
            io:format("~p~n", [Msg]),
            clear()
    after 0 -> ok
    end.

%% tests
test_non_overloaded_server() ->
    start(0),
    {ok, 10} = allocate(),
    stop(). % should return stopped

test_overloaded_server() ->
    start(100),
    {error, timeout} = allocate(),
    timer:sleep(100),
    clear(), % should print {ok, 10} that came too late
    stop(). % should print {error, timeout}, because stop operation also takes too long.