(*
copyright (c) 2013, simon cruanes
all rights reserved.

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modification, are permitted provided that the following conditions are met:

redistributions of source code must retain the above copyright notice, this
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*)

(** {1 Automaton} *)

type ('s, -'i, +'o) t = 's -> 'i -> 's * 'o list
(** Transition function of an event automaton *)

type ('s, 'i, 'o) automaton = ('s, 'i, 'o) t

let map_i f a s i = a s (f i)

let map_o f a s i =
  let s', os = a s i in
  s', List.map f os

let fmap_o f a s i =
  let rec _fmap f l = match l with
    | [] -> []
    | x::l' -> f x @ _fmap f l'
  in
  let s', os = a s i in
  let os' = _fmap f os in
  s', os'

let filter_i p a s i =
  if p i
  then a s i
  else s, []

let filter_o p a s i =
  let s', os = a s i in
  s', List.filter p os

let fold f s i =
  let s' = f s i in
  s', [s']

let product f1 f2 (s1, s2) i =
  let s1', os1 = f1 s1 i in
  let s2', os2 = f2 s2 i in
  (s1', s2'), (os1 @ os2)

module I = struct
  type 'a t = 'a -> unit

  let create f = f

  let send x i = x i

  let comap f i x = i (f x)

  let filter f i x = if f x then i x
end

module O = struct
  type 'a t = {
    mutable n : int;  (* how many handlers? *)
    mutable handlers : ('a -> bool) array;
    mutable alive : keepalive;   (* keep some signal alive *)
  } (** Signal of type 'a *)

  and keepalive =
    | Keep : 'a t -> keepalive
    | NotAlive : keepalive

  let nop_handler x = true

  let create () =
    let s = {
      n = 0;
      handlers = Array.create 3 nop_handler;
      alive = NotAlive;
    } in
    s

  (* remove handler at index i *)
  let remove s i =
    (if i < s.n - 1  (* erase handler with the last one *)
      then s.handlers.(i) <- s.handlers.(s.n - 1));
    s.handlers.(s.n - 1) <- nop_handler; (* free handler *)
    s.n <- s.n - 1;
    ()

  let send s x =
    for i = 0 to s.n - 1 do
      while not (try s.handlers.(i) x with _ -> false) do
        remove s i  (* i-th handler is done, remove it *)
      done
    done

  let on s f =
    (* resize handlers if needed *)
    (if s.n = Array.length s.handlers
      then begin
        let handlers = Array.create (s.n + 4) nop_handler in
        Array.blit s.handlers 0 handlers 0 s.n;
        s.handlers <- handlers
      end);
    s.handlers.(s.n) <- f;
    s.n <- s.n + 1

  let once s f =
    on s (fun x -> ignore (f x); false)

  let propagate a b =
    on a (fun x -> send b x; true)

  let map f signal =
    let signal' = create () in
    (* weak ref *)
    let r = Weak.create 1 in
    Weak.set r 0 (Some signal');
    on signal (fun x ->
      match Weak.get r 0 with
      | None -> false
      | Some signal' -> send signal' (f x); true);
    signal'.alive <- Keep signal;
    signal'

  let filter p signal =
    let signal' = create () in
    (* weak ref *)
    let r = Weak.create 1 in
    Weak.set r 0 (Some signal');
    on signal (fun x ->
      match Weak.get r 0 with
      | None -> false
      | Some signal' -> (if p x then send signal' x); true);
    signal'.alive <- Keep signal;
    signal'
end

let connect o i =
  O.on o (fun x -> I.send i x; true)

module Instance = struct
  type ('s, 'i, 'o) t = {
    transition : ('s, 'i, 'o) automaton;
    mutable i : 'i I.t;
    o : 'o O.t;
    transitions : ('s * 'i * 's * 'o list) O.t;
    mutable state : 's;
  }

  let transition_function a = a.transition

  let i a = a.i

  let o a = a.o

  let state a = a.state

  let transitions a = a.transitions

  let send a i = I.send a.i i

  let _q = Queue.create ()

  let _process q =
    while not (Queue.is_empty q) do
      let task = Queue.pop q in
      task ()
    done

  let _schedule q task = Queue.push task q

  let _do_transition q a i =
    let s = a.state in
    let s', os = a.transition s i in
    (* update state *)
    a.state <- s';
    (* trigger the transitions asap *)
    _schedule q (fun () -> O.send a.transitions (s, i, s', os));
    List.iter
      (fun o -> _schedule q (fun () -> O.send a.o o))
      os 

  let _receive a i =
    let first = Queue.is_empty _q in
    _do_transition _q a i;
    if first then _process _q

  let create ~f init =
    let o = O.create () in
    let transitions = O.create () in
    (* create input and automaton *)
    let a = { state = init; i=Obj.magic 0; o; transition=f; transitions; } in
    a.i <- _receive a;
    a
end