(*
Copyright (c) 2013, Simon Cruanes
All rights reserved.

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

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*)

(** {1 Consumable generators} *)

exception EOG
  (** End of Generation *)

type 'a t = unit -> 'a generator
  (** An enum is a generator of generators *)
and 'a generator = unit -> 'a
  (** A generator may be called several times, yielding the next value
      each time. It raises EOG when it reaches the end. *)

let start enum = enum ()

let next gen = gen ()

let junk gen = ignore (gen ())

let fold f acc enum =
  let rec fold acc gen =
    let acc', stop =
      try f acc (gen ()), true
      with EOG -> acc, false in
    if stop then acc' else fold acc' gen
  in
  fold acc (enum ())

let iter f enum =
  let rec iter gen =
    let stop =
      try f (gen ()); true
      with EOG -> false in
    if stop then () else iter gen
  in
  iter (enum ())

let length enum =
  fold (fun acc _ -> acc + 1) 0 enum
              
let map f enum =
  (* another enum *)
  fun () ->
    let gen = enum () in
    (* the mapped generator *)
    fun () ->
      try f (gen ())
      with EOG -> raise EOG
    
let of_list l =
  fun () ->
    let l = ref l in
    fun () ->
      match !l with
      | [] -> raise EOG
      | x::l' -> l := l'; x

let to_list enum =
  let rec fold gen =
    try (gen ()) :: fold gen
    with EOG -> []
  in fold (enum ())

let to_rev_list enum =
  let rec fold acc gen =
    let acc', stop =
      try gen () :: acc, true
      with EOG -> acc, false
    in if stop then acc' else fold acc' gen
  in
  fold [] (enum ())