-module(ebb_work_manager).

-behaviour(gen_server).

%% API
-export([start/0, start/1, start_link/0, start_link/1]).
-export([start_work/1, start_work/3, work_finished/1, work_finished/2,
	 available_node/0, add_node/1, remove_node/1, remove_node/2,
	 scavenge/0, scavenge/1,
	 get_node_queue/0, get_node_list/0]).

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

-define(SERVER, ebb_work_manager).

-record(state, {node_queue}).

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

start(Args) ->
    ebb_work_supv:start_link(Args).

start_link() ->
    start_link([node()]).

start_link(scavenge) ->
    case gen_server:start_link({local, ?SERVER}, ?MODULE, [node()], []) of
	{ok, Pid} ->
	    scavenge(),
	    {ok, Pid};
	Error ->
	    Error
    end;
start_link(Nodes) ->
    gen_server:start_link({local, ?SERVER}, ?MODULE, Nodes, []).

start_work(Node) ->
    gen_server:cast(?SERVER, {start_work, Node}).

start_work(Func, Args, Receiver) ->
    gen_server:call(?SERVER, {start_work, Func, Args, Receiver}).

work_finished(Node) ->
    gen_server:cast(?SERVER, {work_finished, Node}).

work_finished(Root, Node) ->
    gen_server:cast({?SERVER, Root}, {work_finished, Node}).

available_node() ->
    gen_server:call(?SERVER, available_node).

add_node(Node) ->
    gen_server:cast(?SERVER, {add_node, Node}).

remove_node(Node) ->
    gen_server:cast(?SERVER, {remove_node, Node}).

remove_node(Root, Node) ->
    gen_server:cast({?SERVER, Root}, {remove_node, Node}).

scavenge() ->
    gen_server:cast(?SERVER, {scavenge, world}).

scavenge(Domain) ->
    gen_server:cast(?SERVER, {scavenge, Domain}).

get_node_queue() ->
    gen_server:call(?SERVER, get_node_queue).

get_node_list() ->
    priority_queue:to_list(get_node_queue()).

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

%%--------------------------------------------------------------------
%% Function: init(Args) -> {ok, State} |
%%                         {ok, State, Timeout} |
%%                         ignore               |
%%                         {stop, Reason}
%% Description: Initiates the server
%%--------------------------------------------------------------------
init([]) ->
    Queue = priority_queue:empty(),
    {ok, #state{node_queue=Queue}};
init(Nodes) ->
    Queue = priority_queue:from_orddict([{0, Nodes}]),
    {ok, #state{node_queue=Queue}}.

%%--------------------------------------------------------------------
%% 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({start_work, Func, Args, Receiver}, _From,
	    State = #state{node_queue=Queue}) ->
    {Reply, Queue2} = start_work_from_queue(Func, Args, Receiver, Queue),
    State2 = State#state{node_queue=Queue2},
    {reply, Reply, State2};
handle_call(available_node, _From, State = #state{node_queue=Queue}) ->
    {_Priority, Node, Queue2} = find_available_node(Queue),
    State2 = State#state{node_queue=Queue2},
    {reply, Node, State2};
handle_call(get_node_queue, _From, State = #state{node_queue=Queue}) ->
    Reply = Queue,
    {reply, Reply, State}.

%%--------------------------------------------------------------------
%% Function: handle_cast(Msg, State) -> {noreply, State} |
%%                                      {noreply, State, Timeout} |
%%                                      {stop, Reason, State}
%% Description: Handling cast messages
%%--------------------------------------------------------------------
handle_cast({start_work, Node}, State = #state{node_queue=Queue}) ->
    Queue2 = priority_queue:modify_priority(Node, fun(P) -> P+1 end, Queue),
    State2 = State#state{node_queue=Queue2},
    {noreply, State2};
handle_cast({work_finished, Node}, State = #state{node_queue=Queue}) ->
    Queue2 = priority_queue:modify_priority(Node, fun(P) -> P-1 end, Queue),
    State2 = State#state{node_queue=Queue2},
    {noreply, State2};
handle_cast({add_node, Node}, State = #state{node_queue=Queue}) ->
    Queue2 = new_node(Node, Queue),
    State2 = State#state{node_queue=Queue2},
    {noreply, State2};
handle_cast({remove_node, Node}, State = #state{node_queue=Queue}) ->
    Queue2 = priority_queue:delete_value(Node, Queue),
    State2 = State#state{node_queue=Queue2},
    {noreply, State2};
handle_cast({scavenge, Domain}, State = #state{node_queue=Queue}) ->
    World = case Domain of
		world ->
		    net_adm:world();
		local ->
		    net_adm:world_list([list_to_atom(net_adm:localhost())]);
		Hosts when is_list(Hosts) ->
		    net_adm:world_list(Hosts);
		Host ->
		    net_adm:world_list([Host])
	    end,
    Queue2 = new_nodes(World, Queue),
    State2 = State#state{node_queue=Queue2},
    {noreply, State2}.

%%--------------------------------------------------------------------
%% 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
%%--------------------------------------------------------------------

find_available_node(Queue) ->
    {Priority, Node, Queue2} = priority_queue:take_minimum(Queue),
    case net_adm:ping(Node) of
	pong -> {Priority, Node, Queue};
	pang -> find_available_node(Queue2)
    end.

start_work_from_queue(Func, Args, Receiver, Queue) ->
    {Priority, Node, Queue2} = find_available_node(Queue),
    Reply = ebb_worker_bridge:start_link(
	      Node, Func, Args, Receiver),
    case Reply of
	{ok, _} -> 
	    Queue3 = priority_queue:insert(Priority+1, Node, Queue2),
	    {Reply, Queue3};
	_Error ->
	    start_work_from_queue(Func, Args, Receiver, Queue2)
    end.

new_nodes(Nodes, Queue) ->
    lists:foldl(fun(Node, Q) -> new_node(Node, Q) end,
		Queue, Nodes).

new_node(Node, Queue) ->
    case priority_queue:find_value(Node, Queue) of
	{_Priority, Node} -> Queue;
	none -> priority_queue:insert(0, Node, Queue)
    end.