ocaml-containers/enum.ml

(*
Copyright (c) 2013, Simon Cruanes
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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 empty () = fun () -> raise EOG

let singleton x =
  fun () ->
    let stop = ref false in
    fun () ->
      if !stop
        then raise EOG
        else begin stop := true; x end

let start enum = enum ()

let next gen = gen ()

let junk gen = ignore (gen ())

let is_empty enum =
  try ignore ((enum ()) ()); false
  with EOG -> true

let fold f acc enum =
  let rec fold acc gen =
    let acc', stop =
      try f acc (gen ()), false
      with EOG -> acc, true 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 ()); false
      with EOG -> true 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 append e1 e2 =
  fun () ->
    let gen = ref (e1 ()) in
    let first = ref true in
    (* get next element *)
    let rec next () =
      try !gen ()
      with EOG ->
        if !first then begin
          first := false;
          gen := e2 ();  (* switch to the second generator *)
          next ()
        end else raise EOG  (* done *)
    in next

let flatten enum =
  fun () ->
    let next_gen = enum () in
    let gen = ref (fun () -> raise EOG) in
    (* get next element *)
    let rec next () =
      try !gen ()
      with EOG ->
        (* jump to next sub-enum *)
        let stop =
          try gen := (next_gen () ()); false
          with EOG -> true in
        if stop then raise EOG else next ()
    in next
      
let flatMap f enum =
  fun () ->
    let next_elem = enum () in
    let gen = ref (fun () -> raise EOG) in
    (* get next element *)
    let rec next () =
      try !gen ()
      with EOG ->
        (* enumerate f (next element) *)
        let stop =
          try
            let x = next_elem () in
            gen := (f x) (); false
          with EOG -> true in
        if stop then raise EOG else next ()
    in next

let take n enum =
  assert (n >= 0);
  fun () ->
    let gen = enum () in
    let count = ref 0 in  (* how many yielded elements *)
    fun () ->
      if !count = n then raise EOG
      else begin incr count; gen () end

let drop n enum =
  assert (n >= 0);
  fun () ->
    let gen = enum () in
    let count = ref 0 in  (* how many droped elements? *)
    let rec next () =
      if !count < n
        then begin incr count; ignore (gen ()); next () end
        else gen ()
    in next
    
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
      let x = gen () in
      x :: 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 ())

let int_range i j =
  fun () ->
    let r = ref i in
    fun () ->
      let x = !r in
      if x > j then raise EOG
        else begin
          incr r;
          x
        end

module Infix = struct
  let (@@) = append

  let (>>=) e f = flatMap f e

  let (--) = int_range
end