Erlang frequency server enhancements frequency.erl

frequency.erl

% frequency server enhancements using list comprehension 
% My first instinct was to solve this problem through recursion. However, I decided to use list comprehension / filter just to sample a new flavor. With list comprehension, I was able to pattern match but the process felt more like checking the length of the Frequencies/allocated list to determine if the request was valid.% If anyone can provide a hint, I had to create two functions, is_allocated and is_deallocated, the first to check if the Pid is already allocated a frequency and the second to check if a requested deallocate frequency was in fact allocated. The functions are very similar and should be one function. However, I couldn't figure out how to pass in a parameter that will inform the following line to check Client == Y or Client == X
% [X || {X, Y} <- Allocated, Client == Y].

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

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

loop(Frequencies) ->
  receive
    {request, Pid, allocate} ->
      case is_allocated(Pid, Frequencies) of 
        % empty, Pid not already allocated
        [] -> {NewFrequencies, Reply} = allocate(Frequencies, Pid);
        [X|_Xs] -> {NewFrequencies, Reply} = {Frequencies, {'Already allocated', X}}
      end,
      % io:format('free frequencies ~p~n', [NewFrequencies]),
      Pid ! {reply, Reply},
      loop(NewFrequencies);
    {request, Pid , {deallocate, Freq}} ->
      case is_deallocated2(Freq, Pid, Frequencies) of  
        % empty allocation, do not deallocate
        [] -> {NewFrequencies, Reply} = {Frequencies, {'Nothing allocated', Pid}};
        [X|_Xs] -> {NewFrequencies, Reply} = deallocate(Frequencies, Freq)
      end, 
      % io:format('deallocated, free frequencies ~p~n', [NewFrequencies]),
      Pid ! {reply, Reply},
      loop(NewFrequencies);
    {request, Pid, stop} ->
      Pid ! {reply, stopped}
  end.

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}, {ok, "Deallocated"}}.

is_allocated(Client, {Free, Allocated}) -> 
  [X || {X, Y} <- Allocated, Client == Y].

% check for frequency && Pid
is_deallocated2(Freq, Pid, {Free, Allocated}) ->
  [X || {X, Y} <- Allocated, Freq == X, Pid == Y].