%%%-------------------------------------------------------------------
%%% File    : queue.erl
%%% Author  : Sven-Olof Nystr|m <svenolof@vranx.it.uu.se>
%%% Description : Simple queues
%%%
%%% Created : 28 Oct 2011 by Sven-Olof Nystr|m <svenolof@vranx.it.uu.se>
%%%-------------------------------------------------------------------
-module(my_queue).

-behaviour(gen_server).

%% API
-export([start_link/0]).
-export([enqueue/2, dequeue/1, is_empty/1]).


%% gen_server callbacks
-export([init/1, handle_call/3, handle_cast/2, handle_info/2,
         terminate/2, code_change/3]).

-record(state, {xs=[], ys=[]}).

%%====================================================================
%% API
%%====================================================================
%%--------------------------------------------------------------------
%% Function: start_link() -> {ok,Pid} | ignore | {error,Error}
%% Description: Starts the server
%%--------------------------------------------------------------------
start_link() ->
    gen_server:start_link(?MODULE, [], []).

enqueue(Queue, Data) ->
    gen_server:cast(Queue, {enqueue, Data}).

dequeue(Queue) ->
    gen_server:call(Queue, dequeue).

is_empty(Queue) ->
    gen_server:call(Queue, is_empty).

%%====================================================================
%% gen_server callbacks
%%====================================================================

%%--------------------------------------------------------------------
%% Function: init(Args) -> {ok, State} |
%%                         {ok, State, Timeout} |
%%                         ignore               |
%%                         {stop, Reason}
%% Description: Initiates the server
%%--------------------------------------------------------------------
init([]) ->
    {ok, #state{}}.

%%--------------------------------------------------------------------
%% Function: %% handle_call(Request, From, State) -> {reply, Reply, State} |
%%                                      {reply, Reply, State, Timeout} |
%%                                      {noreply, State} |
%%                                      {noreply, State, Timeout} |
%%                                      {stop, Reason, Reply, State} |
%%                                      {stop, Reason, State}
%% Description: Handling call messages
%%--------------------------------------------------------------------
handle_call(dequeue, _From, State=#state{ys=[]}) ->
    {reply, not_found, State};
handle_call(dequeue, _From, State=#state{ys=[Y|YS]}) ->
    {reply, {ok, Y}, normalize(State#state{ys=YS})};
handle_call(is_empty, _From, State=#state{ys=[]}) ->
    {reply, true, State};
handle_call(is_empty, _From, State=#state{ys=[_|_]}) ->
    {reply, false, State}.

    

%%--------------------------------------------------------------------
%% Function: handle_cast(Msg, State) -> {noreply, State} |
%%                                      {noreply, State, Timeout} |
%%                                      {stop, Reason, State}
%% Description: Handling cast messages
%%--------------------------------------------------------------------
handle_cast({enqueue, X}, State=#state{xs=[],ys=[]}) ->
    {noreply, State#state{ys=[X]}};
handle_cast({enqueue, X}, State=#state{xs=XS}) ->
    {noreply, State#state{xs = [X|XS]}}.


%%--------------------------------------------------------------------
%% Function: handle_info(Info, State) -> {noreply, State} |
%%                                       {noreply, State, Timeout} |
%%                                       {stop, Reason, State}
%% Description: Handling all non call/cast messages
%%--------------------------------------------------------------------
handle_info(_Info, State) ->
    {noreply, State}.

%%--------------------------------------------------------------------
%% Function: terminate(Reason, State) -> void()
%% Description: This function is called by a gen_server when it is about to
%% terminate. It should be the opposite of Module:init/1 and do any necessary
%% cleaning up. When it returns, the gen_server terminates with Reason.
%% The return value is ignored.
%%--------------------------------------------------------------------
terminate(_Reason, _State) ->
    ok.

%%--------------------------------------------------------------------
%% Func: code_change(OldVsn, State, Extra) -> {ok, NewState}
%% Description: Convert process state when code is changed
%%--------------------------------------------------------------------
code_change(_OldVsn, State, _Extra) ->
    {ok, State}.

%%--------------------------------------------------------------------
%%% Internal functions
%%--------------------------------------------------------------------

normalize(State=#state{xs=[], ys=[]}) ->
    State;
normalize(State=#state{ys=[], xs=XS}) ->
    #state{ys=lists:reverse(XS)};
normalize(State) ->
    State.