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(* Open Source License                                                       *)
(* Copyright (c) 2018 Dynamic Ledger Solutions, Inc. <contact@tezos.com>     *)
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open Lwt.Syntax

type t = {
  mutable pending_tasks : unit Lwt.u list;
  mutable pending_idle : (unit -> unit Lwt.t) list;
  mutable running_tasks : int;
  mutable running_idle : bool;
  mutable prevent_tasks : bool;
}

let create () =
  {
    pending_tasks = [];
    pending_idle = [];
    running_tasks = 0;
    running_idle = false;
    prevent_tasks = false;
  }

let rec may_run_idle_tasks w =
  if w.running_tasks = 0 && not w.running_idle then
    match w.pending_idle with
    | [] -> ()
    | pending_idle ->
        w.running_idle <- true ;
        w.prevent_tasks <- false ;
        w.pending_idle <- [] ;
        (* TODO: can we safely remove async and ignore the created
           thread? *)
        Lwt.async (fun () ->
            let pending_idle = List.rev pending_idle in
            let* () = Lwt_list.iter_s (fun f -> f ()) pending_idle in
            w.running_idle <- false ;
            let pending_tasks = List.rev w.pending_tasks in
            w.pending_tasks <- [] ;
            List.iter (fun u -> Lwt.wakeup u ()) pending_tasks ;
            may_run_idle_tasks w ;
            Lwt.return_unit)

let wrap_error f =
  Lwt.catch (fun () -> Lwt_result.ok @@ f ()) (fun exn -> Lwt.return_error exn)

let unwrap_error = function Ok r -> Lwt.return r | Error exn -> Lwt.fail exn

let wakeup_error u = function
  | Ok r -> Lwt.wakeup u r
  | Error exn -> Lwt.wakeup_exn u exn

let rec task w f =
  if w.running_idle || w.prevent_tasks then (
    let t, u = Lwt.task () in
    w.pending_tasks <- u :: w.pending_tasks ;
    let* () = t in
    task w f)
  else (
    w.running_tasks <- w.running_tasks + 1 ;
    let* res = wrap_error f in
    w.running_tasks <- w.running_tasks - 1 ;
    may_run_idle_tasks w ;
    unwrap_error res)

let when_idle w f =
  let t, u = Lwt.task () in
  let canceled = ref false in
  Lwt.on_cancel t (fun () -> canceled := true) ;
  let f () =
    if !canceled then Lwt.return_unit
    else
      let* res = wrap_error f in
      wakeup_error u res ;
      Lwt.return_unit
  in
  w.pending_idle <- f :: w.pending_idle ;
  may_run_idle_tasks w ;
  t

let force_idle w f =
  w.prevent_tasks <- true ;
  when_idle w f