-module(gaffer_queue_runner).
-moduledoc false.

-behaviour(gen_statem).

% API
-ignore_xref(start_link/1).
-export([start_link/1]).
-ignore_xref(poll/1).
-export([poll/1]).
-ignore_xref(reconfigure/1).
-export([reconfigure/1]).
-ignore_xref(info/1).
-export([info/1]).
-ignore_xref(pause/1).
-export([pause/1]).
-ignore_xref(resume/1).
-export([resume/1]).
% gen_statem Callbacks
-export([callback_mode/0]).
-export([init/1]).
-ignore_xref(active/3).
-export([active/3]).
-ignore_xref(paused/3).
-export([paused/3]).

%--- API -----------------------------------------------------------------------

-spec start_link(gaffer:queue()) -> gen_statem:start_ret().
start_link(Name) ->
    gen_statem:start_link({local, proc_name(Name)}, ?MODULE, Name, []).

-spec poll(gaffer:queue()) -> ok.
poll(Name) -> call(Name, poll).

-spec reconfigure(gaffer:queue()) -> ok.
reconfigure(Name) -> call(Name, reconfigure).

-spec info(gaffer:queue()) ->
    #{active := non_neg_integer(), status := active | paused}.
info(Name) -> call(Name, info).

-spec pause(gaffer:queue()) -> ok | {error, already_paused}.
pause(Name) -> call(Name, pause).

-spec resume(gaffer:queue()) -> ok | {error, already_active}.
resume(Name) -> call(Name, resume).

%--- gen_statem Callbacks ------------------------------------------------------

callback_mode() -> state_functions.

init(Name) ->
    Conf = gaffer_queue:conf(Name),
    Data = #{name => Name, workers => #{}},
    {ok, active, Data, initial_poll(Conf) ++ poll_timeout(Conf)}.

active(internal, poll, #{name := Name} = Data) ->
    {Data1, Actions} = do_poll(gaffer_queue:conf(Name), Data),
    {keep_state, Data1, Actions};
active(state_timeout, poll, #{name := Name} = Data) ->
    Conf = gaffer_queue:conf(Name),
    {Data1, Actions} = do_poll(Conf, Data),
    {keep_state, Data1, poll_timeout(Conf) ++ Actions};
active(info, {'DOWN', _Ref, process, Pid, Reason}, Data) ->
    {Data1, Actions} = handle_worker_down(Pid, Reason, Data),
    {keep_state, Data1, [{next_event, internal, poll} | Actions]};
active({call, From}, reconfigure, #{name := Name}) ->
    {keep_state_and_data, [
        {reply, From, ok} | poll_timeout(gaffer_queue:conf(Name))
    ]};
active({call, From}, pause, Data) ->
    {next_state, paused, Data, [{reply, From, ok}]};
active({call, From}, resume, _Data) ->
    {keep_state_and_data, [{reply, From, {error, already_active}}]};
active(EventType, Event, Data) ->
    common(EventType, Event, active, Data).

paused({call, From}, reconfigure, _Data) ->
    {keep_state_and_data, [{reply, From, ok}]};
paused({call, From}, pause, _Data) ->
    {keep_state_and_data, [{reply, From, {error, already_paused}}]};
paused({call, From}, resume, #{name := Name} = Data) ->
    Conf = gaffer_queue:conf(Name),
    {next_state, active, Data, [
        {reply, From, ok}, {next_event, internal, poll} | poll_timeout(Conf)
    ]};
paused(info, {'DOWN', _Ref, process, Pid, Reason}, Data) ->
    {Data1, Actions} = handle_worker_down(Pid, Reason, Data),
    {keep_state, Data1, Actions};
paused(EventType, Event, Data) ->
    common(EventType, Event, paused, Data).

common({call, From}, poll, _State, #{name := Name} = Data) ->
    {Data1, Actions} = do_poll(gaffer_queue:conf(Name), Data),
    {keep_state, Data1, [{reply, From, ok} | Actions]};
common({call, From}, info, State, #{workers := Workers}) ->
    Result = #{active => map_size(Workers), status => State},
    {keep_state_and_data, [{reply, From, Result}]};
common({timeout, Pid}, Reason, _State, Data) ->
    exit(Pid, Reason),
    {keep_state, Data}.

%--- Internal ------------------------------------------------------------------

call(Name, Msg) -> gen_statem:call(proc_name(Name), Msg).

initial_poll(#{poll_interval := infinity}) -> [];
initial_poll(#{}) -> [{next_event, internal, poll}].

poll_timeout(#{poll_interval := infinity}) -> [];
poll_timeout(#{poll_interval := Interval}) -> [{state_timeout, Interval, poll}].

do_poll(
    #{max_workers := MaxWorkers, worker := Worker},
    #{name := Name, workers := Workers} = Data
) ->
    case poll_limit(MaxWorkers, map_size(Workers)) of
        0 ->
            {Data, []};
        Limit ->
            Jobs = gaffer_queue:claim_jobs(Name, #{
                queue => Name, limit => Limit
            }),
            {NewWorkers, Actions} = spawn_workers(Worker, Jobs, Workers, []),
            {Data#{workers := NewWorkers}, Actions}
    end.

poll_limit(infinity, _Active) -> infinity;
poll_limit(Max, Active) -> max(0, Max - Active).

% elp:ignore W0048 - spawn_workers calls gaffer_job:execute which is no_return
-dialyzer({no_return, [spawn_workers/4]}).
spawn_workers(_Worker, [], Workers, Actions) ->
    {Workers, Actions};
spawn_workers(Worker, [#{timeout := Timeout} = Job | Rest], Workers, Actions) ->
    {Pid, _Ref} = spawn_monitor(fun() ->
        gaffer_job:execute(Worker, Job)
    end),
    Action = {{timeout, Pid}, Timeout, kill},
    spawn_workers(Worker, Rest, Workers#{Pid => Job}, [Action | Actions]).

handle_worker_down(Pid, Reason, #{name := Name, workers := Workers} = Data) ->
    case maps:take(Pid, Workers) of
        {OriginalJob, Workers1} ->
            Actions = handle_worker_result(Pid, Reason, OriginalJob, Name),
            {Data#{workers := Workers1}, Actions};
        error ->
            {Data, []}
    end.

handle_worker_result(_Pid, kill, OriginalJob, Name) ->
    {Event, Data, Job} = gaffer_job:handle_crash(
        OriginalJob, gaffer_job_timeout, runner
    ),
    _ = gaffer_queue:write_result(Name, Event, Data, Job),
    [];
handle_worker_result(Pid, {gaffer_job, Event, Data, Job}, _OriginalJob, Name) ->
    _ = gaffer_queue:write_result(Name, Event, Data, Job),
    [{{timeout, Pid}, infinity, kill}];
handle_worker_result(Pid, Reason, OriginalJob, Name) ->
    {Event, Data, Job} = gaffer_job:handle_crash(OriginalJob, Reason, worker),
    _ = gaffer_queue:write_result(Name, Event, Data, Job),
    [{{timeout, Pid}, infinity, kill}].

proc_name(Name) ->
    % elp:ignore W0023 - bounded by queue count, not user input
    binary_to_atom(
        <<"gaffer_queue_runner_", (atom_to_binary(Name))/binary>>
    ).