(*  Title:      Pure/Concurrent/future.ML

     Author:     Makarius

 Future values, see also

 http://www4.in.tum.de/~wenzelm/papers/parallel-isabelle.pdf

 Notes:

   * Futures are similar to delayed evaluation, i.e. delay/force is

     generalized to fork/join (and variants).  The idea is to model

     parallel value-oriented computations, but *not* communicating

     processes.

   * Futures are grouped; failure of one group member causes the whole

     group to be interrupted eventually.  Groups are block-structured.

   * Forked futures are evaluated spontaneously by a farm of worker

     threads in the background; join resynchronizes the computation and

     delivers results (values or exceptions).

   * The pool of worker threads is limited, usually in correlation with

     the number of physical cores on the machine.  Note that allocation

     of runtime resources is distorted either if workers yield CPU time

     (e.g. via system sleep or wait operations), or if non-worker

     threads contend for significant runtime resources independently.

   * Promised futures are fulfilled by external means.  There is no

     associated evaluation task, but other futures can depend on them

     as usual.

 *)

 signature FUTURE =

 sig

   type task = Task_Queue.task

   type group = Task_Queue.group

   val is_worker: unit -> bool

   val worker_task: unit -> Task_Queue.task option

   val worker_group: unit -> Task_Queue.group option

   type 'a future

   val task_of: 'a future -> task

   val group_of: 'a future -> group

   val peek: 'a future -> 'a Exn.result option

   val is_finished: 'a future -> bool

   val fork_group: group -> (unit -> 'a) -> 'a future

   val fork_deps_pri: 'b future list -> int -> (unit -> 'a) -> 'a future

   val fork_deps: 'b future list -> (unit -> 'a) -> 'a future

   val fork_pri: int -> (unit -> 'a) -> 'a future

   val fork: (unit -> 'a) -> 'a future

   val join_results: 'a future list -> 'a Exn.result list

   val join_result: 'a future -> 'a Exn.result

   val join: 'a future -> 'a

   val value: 'a -> 'a future

   val map: ('a -> 'b) -> 'a future -> 'b future

   val promise_group: group -> 'a future

   val promise: unit -> 'a future

   val fulfill_result: 'a future -> 'a Exn.result -> unit

   val fulfill: 'a future -> 'a -> unit

   val interruptible_task: ('a -> 'b) -> 'a -> 'b

   val cancel_group: group -> unit

   val cancel: 'a future -> unit

   val shutdown: unit -> unit

 end;

 structure Future: FUTURE =

 struct

 (** future values **)

 (* identifiers *)

 type task = Task_Queue.task;

 type group = Task_Queue.group;

 local

   val tag = Universal.tag () : (task * group) option Universal.tag;

 in

   fun thread_data () = the_default NONE (Thread.getLocal tag);

   fun setmp_thread_data data f x =

     Library.setmp_thread_data tag (thread_data ()) (SOME data) f x;

 end;

 val is_worker = is_some o thread_data;

 val worker_task = Option.map #1 o thread_data;

 val worker_group = Option.map #2 o thread_data;

 fun new_group () = Task_Queue.new_group (worker_group ());

 (* datatype future *)

 type 'a result = 'a Exn.result Single_Assignment.var;

 datatype 'a future = Future of

  {promised: bool,

   task: task,

   group: group,

   result: 'a result};

 fun task_of (Future {task, ...}) = task;

 fun group_of (Future {group, ...}) = group;

 fun result_of (Future {result, ...}) = result;

 fun peek x = Single_Assignment.peek (result_of x);

 fun is_finished x = is_some (peek x);

 fun assign_result group result res =

   let

     val _ = Single_Assignment.assign result res;

     val ok =

       (case res of

         Exn.Exn exn => (Task_Queue.cancel_group group exn; false)

       | Exn.Result _ => true);

   in ok end;

 (** scheduling **)

 (* synchronization *)

 val scheduler_event = ConditionVar.conditionVar ();

 val work_available = ConditionVar.conditionVar ();

 val work_finished = ConditionVar.conditionVar ();

 local

   val lock = Mutex.mutex ();

 in

 fun SYNCHRONIZED name = SimpleThread.synchronized name lock;

 fun wait cond = (*requires SYNCHRONIZED*)

   Multithreading.sync_wait NONE NONE cond lock;

 fun wait_timeout timeout cond = (*requires SYNCHRONIZED*)

   Multithreading.sync_wait NONE (SOME (Time.+ (Time.now (), timeout))) cond lock;

 fun signal cond = (*requires SYNCHRONIZED*)

   ConditionVar.signal cond;

 fun broadcast cond = (*requires SYNCHRONIZED*)

   ConditionVar.broadcast cond;

 fun broadcast_work () = (*requires SYNCHRONIZED*)

  (ConditionVar.broadcast work_available;

   ConditionVar.broadcast work_finished);

 end;

 (* global state *)

 val queue = Unsynchronized.ref Task_Queue.empty;

 val next = Unsynchronized.ref 0;

 val scheduler = Unsynchronized.ref (NONE: Thread.thread option);

 val canceled = Unsynchronized.ref ([]: Task_Queue.group list);

 val do_shutdown = Unsynchronized.ref false;

 val max_workers = Unsynchronized.ref 0;

 val max_active = Unsynchronized.ref 0;

 val worker_trend = Unsynchronized.ref 0;

 datatype worker_state = Working | Waiting | Sleeping;

 val workers = Unsynchronized.ref ([]: (Thread.thread * worker_state Unsynchronized.ref) list);

 fun count_workers state = (*requires SYNCHRONIZED*)

   fold (fn (_, state_ref) => fn i => if ! state_ref = state then i + 1 else i) (! workers) 0;

 (* execute future jobs *)

 fun future_job group (e: unit -> 'a) =

   let

     val result = Single_Assignment.var "future" : 'a result;

     val pos = Position.thread_data ();

     fun job ok =

       let

         val res =

           if ok then

             Exn.capture (fn () =>

               Multithreading.with_attributes Multithreading.private_interrupts

                 (fn _ => Position.setmp_thread_data pos e ())) ()

           else Exn.Exn Exn.Interrupt;

       in assign_result group result res end;

   in (result, job) end;

 fun cancel_now group = (*requires SYNCHRONIZED*)

   Task_Queue.cancel (! queue) group;

 fun cancel_later group = (*requires SYNCHRONIZED*)

  (Unsynchronized.change canceled (insert Task_Queue.eq_group group);

   broadcast scheduler_event);

 fun execute (task, group, jobs) =

   let

     val valid = not (Task_Queue.is_canceled group);

     val ok = setmp_thread_data (task, group) (fn () =>

       fold (fn job => fn ok => job valid andalso ok) jobs true) ();

     val _ = SYNCHRONIZED "finish" (fn () =>

       let

         val maximal = Unsynchronized.change_result queue (Task_Queue.finish task);

         val _ =

           if ok then ()

           else if cancel_now group then ()

           else cancel_later group;

         val _ = broadcast work_finished;

         val _ = if maximal then () else signal work_available;

       in () end);

   in () end;

 (* worker threads *)

 fun worker_wait active cond = (*requires SYNCHRONIZED*)

   let

     val state =

       (case AList.lookup Thread.equal (! workers) (Thread.self ()) of

         SOME state => state

       | NONE => raise Fail "Unregistered worker thread");

     val _ = state := (if active then Waiting else Sleeping);

     val _ = wait cond;

     val _ = state := Working;

   in () end;

 fun worker_next () = (*requires SYNCHRONIZED*)

   if length (! workers) > ! max_workers then

     (Unsynchronized.change workers (AList.delete Thread.equal (Thread.self ()));

      signal work_available;

      NONE)

   else if count_workers Working > ! max_active then

     (worker_wait false work_available; worker_next ())

   else

     (case Unsynchronized.change_result queue (Task_Queue.dequeue (Thread.self ())) of

       NONE => (worker_wait false work_available; worker_next ())

     | some => (signal work_available; some));

 fun worker_loop name =

   (case SYNCHRONIZED name (fn () => worker_next ()) of

     NONE => ()

   | SOME work => (execute work; worker_loop name));

 fun worker_start name = (*requires SYNCHRONIZED*)

   Unsynchronized.change workers (cons (SimpleThread.fork false (fn () => worker_loop name),

     Unsynchronized.ref Working));

 (* scheduler *)

 val status_ticks = Unsynchronized.ref 0;

 val last_round = Unsynchronized.ref Time.zeroTime;

 val next_round = Time.fromMilliseconds 50;

 fun scheduler_next () = (*requires SYNCHRONIZED*)

   let

     val now = Time.now ();

     val tick = Time.<= (Time.+ (! last_round, next_round), now);

     val _ = if tick then last_round := now else ();

     (* queue and worker status *)

     val _ =

       if tick then Unsynchronized.change status_ticks (fn i => (i + 1) mod 10) else ();

     val _ =

       if tick andalso ! status_ticks = 0 then

         Multithreading.tracing 1 (fn () =>

           let

             val {ready, pending, running, passive} = Task_Queue.status (! queue);

             val total = length (! workers);

             val active = count_workers Working;

             val waiting = count_workers Waiting;

           in

             "SCHEDULE " ^ Time.toString now ^ ": " ^

               string_of_int ready ^ " ready, " ^

               string_of_int pending ^ " pending, " ^

               string_of_int running ^ " running, " ^

               string_of_int passive ^ " passive; " ^

               string_of_int total ^ " workers, " ^

               string_of_int active ^ " active, " ^

               string_of_int waiting ^ " waiting "

           end)

       else ();

     val _ =

       if forall (Thread.isActive o #1) (! workers) then ()

       else

         let

           val  (alive, dead) = List.partition (Thread.isActive o #1) (! workers);

           val _ = workers := alive;

         in

           Multithreading.tracing 0 (fn () =>

             "SCHEDULE: disposed " ^ string_of_int (length dead) ^ " dead worker threads")

         end;

     (* worker pool adjustments *)

     val max_active0 = ! max_active;

     val max_workers0 = ! max_workers;

     val m = if ! do_shutdown then 0 else Multithreading.max_threads_value ();

     val _ = max_active := m;

     val mm =

       if ! do_shutdown then 0

       else if m = 9999 then 1

       else Int.min (Int.max (count_workers Working + 2 * count_workers Waiting, m), 4 * m);

     val _ =

       if tick andalso mm > ! max_workers then

         Unsynchronized.change worker_trend (fn w => if w < 0 then 0 else w + 1)

       else if tick andalso mm < ! max_workers then

         Unsynchronized.change worker_trend (fn w => if w > 0 then 0 else w - 1)

       else ();

     val _ =

       if mm = 0 orelse ! worker_trend > 50 orelse ! worker_trend < ~50 then

         max_workers := mm

       else if ! worker_trend > 5 andalso ! max_workers < 2 * m then

         max_workers := Int.min (mm, 2 * m)

       else ();

     val missing = ! max_workers - length (! workers);

     val _ =

       if missing > 0 then

         funpow missing (fn () =>

           ignore (worker_start ("worker " ^ string_of_int (Unsynchronized.inc next)))) ()

       else ();

     val _ =

       if ! max_active = max_active0 andalso ! max_workers = max_workers0 then ()

       else signal work_available;

     (* canceled groups *)

     val _ =

       if null (! canceled) then ()

       else

        (Multithreading.tracing 1 (fn () =>

           string_of_int (length (! canceled)) ^ " canceled groups");

         Unsynchronized.change canceled (filter_out cancel_now);

         broadcast_work ());

     (* delay loop *)

     val _ = Exn.release (wait_timeout next_round scheduler_event);

     (* shutdown *)

     val _ = if Task_Queue.all_passive (! queue) then do_shutdown := true else ();

     val continue = not (! do_shutdown andalso null (! workers));

     val _ = if continue then () else scheduler := NONE;

     val _ = broadcast scheduler_event;

   in continue end

   handle Exn.Interrupt =>

    (Multithreading.tracing 1 (fn () => "Interrupt");

     List.app cancel_later (Task_Queue.cancel_all (! queue));

     broadcast_work (); true);

 fun scheduler_loop () =

   while

     Multithreading.with_attributes

       (Multithreading.sync_interrupts Multithreading.public_interrupts)

       (fn _ => SYNCHRONIZED "scheduler" (fn () => scheduler_next ()))

   do ();

 fun scheduler_active () = (*requires SYNCHRONIZED*)

   (case ! scheduler of NONE => false | SOME thread => Thread.isActive thread);

 fun scheduler_check () = (*requires SYNCHRONIZED*)

  (do_shutdown := false;

   if scheduler_active () then ()

   else scheduler := SOME (SimpleThread.fork false scheduler_loop));

 (** futures **)

 (* fork *)

 fun fork_future opt_group deps pri e =

   let

     val group =

       (case opt_group of

         NONE => new_group ()

       | SOME group => group);

     val (result, job) = future_job group e;

     val task = SYNCHRONIZED "enqueue" (fn () =>

       let

         val (task, minimal) =

           Unsynchronized.change_result queue (Task_Queue.enqueue group deps pri job);

         val _ = if minimal then signal work_available else ();

         val _ = scheduler_check ();

       in task end);

   in Future {promised = false, task = task, group = group, result = result} end;

 fun fork_group group e = fork_future (SOME group) [] 0 e;

 fun fork_deps_pri deps pri e = fork_future NONE (map task_of deps) pri e;

 fun fork_deps deps e = fork_deps_pri deps 0 e;

 fun fork_pri pri e = fork_deps_pri [] pri e;

 fun fork e = fork_deps [] e;

 (* join *)

 local

 fun get_result x =

   (case peek x of

     NONE => Exn.Exn (SYS_ERROR "unfinished future")

   | SOME (exn as Exn.Exn Exn.Interrupt) =>

       (case Exn.flatten_list (Task_Queue.group_status (group_of x)) of

         [] => exn

       | exns => Exn.Exn (Exn.EXCEPTIONS exns))

   | SOME res => res);

 fun join_next deps = (*requires SYNCHRONIZED*)

   if null deps then NONE

   else

     (case Unsynchronized.change_result queue (Task_Queue.dequeue_towards (Thread.self ()) deps) of

       (NONE, []) => NONE

     | (NONE, deps') => (worker_wait true work_finished; join_next deps')

     | (SOME work, deps') => SOME (work, deps'));

 fun execute_work NONE = ()

   | execute_work (SOME (work, deps')) = (execute work; join_work deps')

 and join_work deps =

   execute_work (SYNCHRONIZED "join" (fn () => join_next deps));

 fun join_depend task deps =

   execute_work (SYNCHRONIZED "join" (fn () =>

     (Unsynchronized.change queue (Task_Queue.depend task deps); join_next deps)));

 in

 fun join_results xs =

   if forall is_finished xs then map get_result xs

   else if Multithreading.self_critical () then

     error "Cannot join future values within critical section"

   else

     (case worker_task () of

       SOME task => join_depend task (map task_of xs)

     | NONE => List.app (ignore o Single_Assignment.await o result_of) xs;

     map get_result xs);

 end;

 fun join_result x = singleton join_results x;

 fun join x = Exn.release (join_result x);

 (* fast-path versions -- bypassing full task management *)

 fun value (x: 'a) =

   let

     val group = Task_Queue.new_group NONE;

     val result = Single_Assignment.var "value" : 'a result;

     val _ = assign_result group result (Exn.Result x);

   in Future {promised = false, task = Task_Queue.dummy_task, group = group, result = result} end;

 fun map_future f x =

   let

     val task = task_of x;

     val group = Task_Queue.new_group (SOME (group_of x));

     val (result, job) = future_job group (fn () => f (join x));

     val extended = SYNCHRONIZED "extend" (fn () =>

       (case Task_Queue.extend task job (! queue) of

         SOME queue' => (queue := queue'; true)

       | NONE => false));

   in

     if extended then Future {promised = false, task = task, group = group, result = result}

     else fork_future (SOME group) [task] (Task_Queue.pri_of_task task) (fn () => f (join x))

   end;

 (* promised futures -- fulfilled by external means *)

 fun promise_group group : 'a future =

   let

     val result = Single_Assignment.var "promise" : 'a result;

     val task = SYNCHRONIZED "enqueue" (fn () =>

       Unsynchronized.change_result queue (Task_Queue.enqueue_passive group));

   in Future {promised = true, task = task, group = group, result = result} end;

 fun promise () = promise_group (new_group ());

 fun fulfill_result (Future {promised, task, group, result}) res =

   let

     val _ = promised orelse raise Fail "Not a promised future";

     fun job ok = assign_result group result (if ok then res else Exn.Exn Exn.Interrupt);

     val _ = execute (task, group, [job]);

   in () end;

 fun fulfill x res = fulfill_result x (Exn.Result res);

 (* cancellation *)

 fun interruptible_task f x =

   if Multithreading.available then

     Multithreading.with_attributes

       (if is_worker ()

        then Multithreading.private_interrupts

        else Multithreading.public_interrupts)

       (fn _ => f x)

   else interruptible f x;

 (*cancel: present and future group members will be interrupted eventually*)

 fun cancel_group group =

   SYNCHRONIZED "cancel" (fn () => if cancel_now group then () else cancel_later group);

 fun cancel x = cancel_group (group_of x);

 (* shutdown *)

 fun shutdown () =

   if Multithreading.available then

     SYNCHRONIZED "shutdown" (fn () =>

      while scheduler_active () do

       (wait scheduler_event; broadcast_work ()))

   else ();

 (*final declarations of this structure!*)

 val map = map_future;

 end;

 type 'a future = 'a Future.future;