frequency.erl

%% 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/0,restarter/0,init/0,allocate/0,deallocate/1,stop/0]).


%%%%%% Assigment related code

% Start frequency supervisor
start() ->
    register(frequency_sub, spawn(?MODULE, restarter, [])).

% frequency process restarter.
restarter() ->
    process_flag(trap_exit, true),
    Pid = spawn_link(?MODULE, init, []),
    register(?MODULE, Pid),
    receive
        {'EXIT', Pid, normal} -> % not a crash
            ok;
        {'EXIT', Pid, shutdown} -> % manual termination, not a crash
            ok;
        {'EXIT', Pid, _} ->
            restarter()
    end.

%%%%%% End of assignment

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

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

%% The Main Loop
loop(Frequencies) ->
    receive
        {request, Pid, allocate} ->
             % Overloaded allocate
             % timer:sleep(5000),
            {NewFrequencies, Reply} = allocate(Frequencies, Pid),
            Pid ! {reply, Reply},
            loop(NewFrequencies);
        {request, Pid , {deallocate, Freq}} ->
            NewFrequencies = deallocate(Frequencies, Freq),
            Pid ! {reply, ok},
            loop(NewFrequencies);
        {request, Pid, stop} ->
            Pid ! {reply, stopped}
    end.

clear() ->
    receive
        _Msg ->
            io:format("Clear: ~n"),
            clear()
    after 0 ->
           true

    end.

%% Functional interface
%% timeout 1000ms
allocate() ->
    %clean mailbox before allocating new.
    clear(),
    frequency ! {request, self(), allocate},
    receive
        {reply, Reply} -> Reply
    after 1000 ->
              {error, timeout}
    end.

deallocate(Freq) ->
    %clean mailbox before deallocating new.
    clear(),
    frequency ! {request, self(), {deallocate, Freq}},
    receive
        {reply, Reply} -> Reply
    after 1000 ->
              {error, timeout}
    end.

stop() ->
    frequency ! {request, self(), stop},
    receive
        {reply, Reply} -> Reply
    after 1000 ->
              {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),
    {