Frequency Server for Concurrent Programming in Erlang

frequency2.erl

-module(frequency2).
-export([start/0,allocate/0,deallocate/1,stop/0]).
-export([init/0]).

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

start() ->
    register(frequency2,
       spawn(frequency2, init, [])).

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

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

%% The Main Loop

loop(Frequencies) ->
  %% sleep for 3 sec prior to processing any message to simulate server overload
  receive
    {request, Pid, allocate} ->
      timer:sleep(3000),
      {NewFrequencies, Reply} = allocate(Frequencies, Pid),
      Pid ! {reply, Reply},
      loop(NewFrequencies);
    {request, Pid , {deallocate, Freq}} ->
      timer:sleep(3000),
      NewFrequencies = deallocate(Frequencies, Freq),
      Pid ! {reply, ok},
      loop(NewFrequencies);
    {request, Pid, stop} ->
      timer:sleep(3000),
      Pid ! {reply, stopped}
  end.

%% Functional interface

allocate() ->
    frequency2 ! {request, self(), allocate},
    receive
	{reply, Reply} -> Reply
    %% timeout after 2.5 seconds (simulate overload)
    after 2500 -> timeout
    end,

    receive
    %% periodically clear mailbox
    after 5000 -> clear()
    end.

deallocate(Freq) ->
    frequency2 ! {request, self(), {deallocate, Freq}},
    receive
	{reply, Reply} -> Reply
    %% timeout after 2.5 seconds (simulate overload)
    after 2500 -> timeout
    end,

    receive
    %% periodically clear mailbox
    after 5000 -> clear()
    end.

stop() ->
    frequency2 ! {request, self(), stop},
    receive
	{reply, Reply} -> Reply
    end.

%% Flush all messages in mailbox
clear() ->
    receive
      Msg ->
        io:format("Delayed Message: ~w~n", [Msg]),
        clear()
    after 0 -> ok
    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}.