ocaml-containers/tTree.ml

(*
copyright (c) 2013, simon cruanes
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(** {1 T-Trees} *)

(** {2 Persistent array}

The nodes of the tree are arrays, but to expose a persistent interface we
use persistent arrays. *)

module PArray = struct
  type 'a t = 'a zipper ref
  and 'a zipper =
    | Array of 'a array
    | Diff of int * 'a * 'a zipper ref

  (* XXX maybe having a snapshot of the array from point to point may help? *)

  let make size elt =
    let a = Array.create size elt in
    ref (Array a)

  (** Recover the given version of the shared array. Returns the array
      itself. *)
  let rec reroot t =
    match !t with
    | Array a -> a
    | Diff (i, v, t') ->
      begin
        let a = reroot t' in
        let v' = a.(i) in
        t' := Diff (i, v', t);
        a.(i) <- v;
        t := Array a;
        a
      end

  let get t i =
    match !t with
    | Array a -> a.(i)
    | Diff _ -> 
      let a = reroot t in
      a.(i)

  let set t i v =
    let a =
      match !t with
      | Array a -> a
      | Diff _ -> reroot t in
    let v' = a.(i) in
    if v == v'
      then t (* no change *)
      else begin
        let t' = ref (Array a) in
        a.(i) <- v;
        t := Diff (i, v', t');
        t' (* create new array *)
      end

  let fold_left f acc t =
    let a = reroot t in
    Array.fold_left f acc a

  let rec length t =
    match !t with
    | Array a -> Array.length a
    | Diff (_, _, t') -> length t'
end

(** {2 signature} *)

module type S = sig
  type key

  type 'a t

  val empty : 'a t
    (** Empty tree *)

  val add : 'a t -> key -> 'a -> 'a t
    (** Add a binding key/value. If the key already was bound to some
        value, the old binding is erased. *)

  val remove : 'a t -> key -> 'a t
    (** Remove the key *)

  val find : 'a t -> key -> 'a
    (** Find the element associated with this key.
        @raise Not_found if the key is not present *)

  val length : 'a t -> int
    (** Number of bindings *)

  val fold : 'a t -> 'b -> ('b -> key -> 'a -> 'b) -> 'b
    (** Fold on bindings *)
end

(** {2 Functor} *)

module Make(X : Set.OrderedType) = struct
  type key = X.t

  (* bucket that maps a key to a value *)
  type 'a bucket =
    | B_none
    | B_some of key * 'a

  (* recursive tree type *)
  type 'a node = {
    left : 'a node option;
    right : 'a node option;
    depth : int;
    buckets : 'a bucket PArray.t;
  }

  (* to avoid the value restriction, we need to make a special case for
      the empty tree *)
  type 'a t =
    | E
    | N of 'a node

  let empty = E

  let add tree k v = assert false

  let remove tree k = assert false

  let find tree k =
    let rec find node k = assert false (* TODO *)
    in
    match tree with
    | E -> raise Not_found
    | N node -> find node k

  let length tree = assert false

  let fold tree acc f = assert false
end