gistfile1.hrl

-module(node_mgr).

-behaviour(gen_server).

%% API
-export([start_link/0, register/1, next_worker/0]).

-define(SERVER, ?MODULE).

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

-record(state,
        {nodes=[],
         lookaside=[]}).

start_link() ->
  gen_server:start_link({global, ?SERVER}, ?MODULE, [], []).

register(WorkerPid) ->
  gen_server:call({global, ?SERVER}, {register_worker, WorkerPid}).

next_worker() ->
  gen_server:call({global, ?SERVER}, next_worker).

init([]) ->
  {ok, #state{}}.

handle_call({register_worker, WorkerPid}, _From, State) ->
  link(WorkerPid),
  {reply, ok, State#state{nodes=[WorkerPid|State#state.nodes]}};

handle_call(next_worker, _From, State) when length(State#state.nodes) > 0 ->
  [NextWorker|T] = State#state.nodes,
  {reply, NextWorker, State#state{nodes=T, lookaside=[NextWorker|State#state.lookaside]}};

handle_call(next_worker, _From, State) ->
  NewState = State#state{nodes=State#state.lookaside, lookaside=[]},
  [NextWorker|T] = NewState#state.nodes,
  {reply, NextWorker, State#state{nodes=T, lookaside=[NextWorker|State#state.lookaside]}};

handle_call(_Request, _From, State) ->
  {reply, ignored, State}.

handle_cast(_Msg, State) ->
  {noreply, State}.

handle_info({'EXIT', From, _Reason}, State) ->
  #state{nodes=Nodes, lookaside=Lookaside} = State,
  {noreply, State#state{nodes=filter_worker(From, Nodes), lookaside=filter_worker(From, Lookaside)}};

handle_info(_Info, State) ->
  {noreply, State}.

terminate(_Reason, _State) ->
  ok.

code_change(_OldVsn, State, _Extra) ->
  {ok, State}.

%% Internal functions

filter_worker(WorkerPid, Workers) ->
  lists:filter(fun(WP) -> (WP =:= WorkerPid) == false end, Workers).