%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%   Copyright 2018 by Fred Dushin <fred@dushin.net>                       %
%                                                                         %
%   This program is free software; you can redistribute it and/or modify  %
%   it under the terms of the GNU Lesser General Public License as        %
%   published by the Free Software Foundation; either version 2 of the    %
%   License, or (at your option) any later version.                       %
%                                                                         %
%   This program is distributed in the hope that it will be useful,       %
%   but WITHOUT ANY WARRANTY; without even the implied warranty of        %
%   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the         %
%   GNU General Public License for more details.                          %
%                                                                         %
%   You should have received a copy of the GNU General Public License     %
%   along with this program; if not, write to the                         %
%   Free Software Foundation, Inc.,                                       %
%   51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA .        %
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

%%-----------------------------------------------------------------------------
%% @hidden
%%-----------------------------------------------------------------------------
-module(timer_manager).

-export([start/0, start_timer/3, cancel_timer/1, get_timer_refs/0, send_after/3]).
-export([init/1, handle_call/3, handle_cast/2, handle_info/2, terminate/2]).
-export([run_timer/5, send_after_timer/3]).

-record(state, {
    timers = [] :: [{reference(), pid()}]
}).

-define(SERVER_NAME, ?MODULE).

-spec start() -> {ok, Pid::pid()} | {error, Reason::term()}.
start() ->
    gen_server:start({local, ?SERVER_NAME}, ?MODULE, [], []).

%%-----------------------------------------------------------------------------
%% @param   Time time in milliseconds after which to send the timeout message.
%% @param   Dest Pid or server name to which to send the timeout message.
%% @param   Msg Message to send to Dest after Time ms.
%% @returns a reference that can be used to cancel the timer, if desired.
%% @doc     Start a timer, and send {timeout, TimerRef, Msg} to Dest after
%%          Time ms, where TimerRef is the reference returned from this function.
%% @end
%%-----------------------------------------------------------------------------
-spec start_timer(Time::non_neg_integer(), Dest::pid(), Msg::term()) -> TimerRef::reference().
start_timer(Time, Dest, Msg) ->
    start(),
    gen_server:call(?SERVER_NAME, {Time, Dest, Msg}).

%%-----------------------------------------------------------------------------
%% @hidden
%% @param   Time time in milliseconds after which to send the timeout message.
%% @param   Dest Pid or server name to which to send the timeout message.
%% @param   Msg Message to send to Dest after Time ms.
%% @param   Options options
%% @returns a reference that can be used to cancel the timer, if desired.
%% @doc     Start a timer, and send {timeout, TimerRef, Msg} to Dest after
%%          Time ms, where TimerRef is the reference returned from this function.
%%
%%          <em><b>Note.</b>  The Options argument is currently ignored.</em>
%%-----------------------------------------------------------------------------
-spec cancel_timer(TimerRef::reference()) -> ok.
cancel_timer(TimerRef) ->
    start(),
    gen_server:call(?SERVER_NAME, {cancel, TimerRef}).

-spec get_timer_refs() -> [reference()].
get_timer_refs() ->
    start(),
    gen_server:call(?SERVER_NAME, get_timer_refs).

%%-----------------------------------------------------------------------------
%% @param   Time time in milliseconds after which to send the message.
%% @param   Dest Pid or server name to which to send the message.
%% @param   Msg Message to send to Dest after Time ms.
%% @returns a reference that can be used to cancel the timer, if desired.
%% @doc     Send Msg to Dest after Time ms.
%% @end
%%-----------------------------------------------------------------------------
-spec send_after(non_neg_integer(), pid() | atom(), term()) -> reference().
send_after(Time, Dest, Msg) ->
    TimerRef = erlang:make_ref(),
    spawn(?MODULE, send_after_timer, [Time, Dest, Msg]),
    TimerRef.


%%
%% ?GEN_SERVER callbacks
%%

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

%% @hidden
handle_call(get_timer_refs, _From, #state{timers=Timers} = State) ->
    {reply, [TimerRef || {TimerRef, _Pid} <- Timers], State};
handle_call({cancel, TimerRef}, From, #state{timers=Timers} = State) ->
    case lists:keyfind(TimerRef, 1, Timers) of
        false ->
            {reply, false, State};
        {TimerRef, Pid} ->
            Pid ! {cancel, From},
            {noreply, State}
    end;
handle_call({Time, Dest, Msg}, _From, #state{timers=Timers} = State) ->
    {TimerRef, Pid} = do_start_timer(Time, Dest, Msg),
    {reply, TimerRef, State#state{timers=[{TimerRef, Pid} | Timers]}}.

%% @hidden
handle_cast(_Request, State) ->
    {noreply, State}.

%% @hidden
handle_info({finished, TimerRef}, #state{timers=Timers} = State) ->
    {noreply, State#state{timers=lists:keydelete(TimerRef, 1, Timers)}}.

%% @hidden
terminate(_Reason, _State) ->
    ok.


%%=============================================================================
%% internal functions

%% @private
do_start_timer(Time, Dest, Msg) ->
    TimerRef = erlang:make_ref(),
    Pid = spawn(?MODULE, run_timer, [self(), Time, TimerRef, Dest, Msg]),
    {TimerRef, Pid}.

%% @private
run_timer(MgrPid, Time, TimerRef, Dest, Msg) ->
    Start = erlang:timestamp(),
    receive
        {cancel, From} ->
            gen_server:reply(From, Time - timestamp_util:delta_ms(erlang:timestamp(), Start))
    after Time ->
        Dest ! {timeout, TimerRef, Msg}
    end,
    MgrPid ! {finished, TimerRef}.

%% @private
send_after_timer(Time, Dest, Msg) ->
    TimerRef = start_timer(Time, self(), Msg),
    receive
        {timeout, TimerRef, Msg} ->
            Dest ! Msg
    end.