(**************************************************************************)
(*                                                                        *)
(*                                 OCaml                                  *)
(*                                                                        *)
(*             Xavier Leroy, projet Cristal, INRIA Rocquencourt           *)
(*                                                                        *)
(*   Copyright 1996 Institut National de Recherche en Informatique et     *)
(*     en Automatique.                                                    *)
(*                                                                        *)
(*   All rights reserved.  This file is distributed under the terms of    *)
(*   the GNU Lesser General Public License version 2.1, with the          *)
(*   special exception on linking described in the file LICENSE.          *)
(*                                                                        *)
(**************************************************************************)

(** First-in first-out queues.

   This module implements queues (FIFOs), with in-place modification.

   {b Warning} This module is not thread-safe: each {!Queue.t} value
   must be protected from concurrent access (e.g. with a [Mutex.t]).
   Failure to do so can lead to a crash.
*)

type !'a t
(** The type of queues containing elements of type ['a]. *)


exception Empty
(** Raised when {!Queue.take} or {!Queue.peek} is applied to an empty queue. *)


val create : unit -> 'a t
(** Return a new queue, initially empty. *)

val add : 'a -> 'a t -> unit
(** [add x q] adds the element [x] at the end of the queue [q]. *)

val push : 'a -> 'a t -> unit
(** [push] is a synonym for [add]. *)

val take : 'a t -> 'a
(** [take q] removes and returns the first element in queue [q],
   or raises {!Empty} if the queue is empty. *)

val take_opt : 'a t -> 'a option
(** [take_opt q] removes and returns the first element in queue [q],
   or returns [None] if the queue is empty.
   @since 4.08 *)

val pop : 'a t -> 'a
(** [pop] is a synonym for [take]. *)

val peek : 'a t -> 'a
(** [peek q] returns the first element in queue [q], without removing
   it from the queue, or raises {!Empty} if the queue is empty. *)

val peek_opt : 'a t -> 'a option
(** [peek_opt q] returns the first element in queue [q], without removing
   it from the queue, or returns [None] if the queue is empty.
   @since 4.08 *)

val top : 'a t -> 'a
(** [top] is a synonym for [peek]. *)

val clear : 'a t -> unit
(** Discard all elements from a queue. *)

val copy : 'a t -> 'a t
(** Return a copy of the given queue. *)

val is_empty : 'a t -> bool
(** Return [true] if the given queue is empty, [false] otherwise. *)

val length : 'a t -> int
(** Return the number of elements in a queue. *)

val iter : ('a -> unit) -> 'a t -> unit
(** [iter f q] applies [f] in turn to all elements of [q],
   from the least recently entered to the most recently entered.
   The queue itself is unchanged. *)

val fold : ('b -> 'a -> 'b) -> 'b -> 'a t -> 'b
(** [fold f accu q] is equivalent to [List.fold_left f accu l],
   where [l] is the list of [q]'s elements. The queue remains
   unchanged. *)

val transfer : 'a t -> 'a t -> unit
(** [transfer q1 q2] adds all of [q1]'s elements at the end of
   the queue [q2], then clears [q1]. It is equivalent to the
   sequence [iter (fun x -> add x q2) q1; clear q1], but runs
   in constant time. *)

(** {1 Iterators} *)

val to_seq : 'a t -> 'a Seq.t
(** Iterate on the queue, in front-to-back order.
    The behavior is not specified if the queue is modified
    during the iteration.
    @since 4.07 *)

val add_seq : 'a t -> 'a Seq.t -> unit
(** Add the elements from a sequence to the end of the queue.
    @since 4.07 *)

val of_seq : 'a Seq.t -> 'a t
(** Create a queue from a sequence.
    @since 4.07 *)
