hashcons.mli

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(*                                                                        *)
(*  Copyright (C) Jean-Christophe Filliatre                               *)
(*                                                                        *)
(*  This software is free software; you can redistribute it and/or        *)
(*  modify it under the terms of the GNU Library General Public           *)
(*  License version 2.1, with the special exception on linking            *)
(*  described in file LICENSE.                                            *)
(*                                                                        *)
(*  This software is distributed in the hope that it will be useful,      *)
(*  but WITHOUT ANY WARRANTY; without even the implied warranty of        *)
(*  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.                  *)
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(*s Hash tables for hash consing.

    The technique is described in this paper:
      Sylvain Conchon and Jean-Christophe Filliâtre.
      Type-Safe Modular Hash-Consing.
      In ACM SIGPLAN Workshop on ML, Portland, Oregon, September 2006.
      https://www.lri.fr/~filliatr/ftp/publis/hash-consing2.pdf

    Note: a different, more elaborated hash-consing library
          can be found in Why3 sources at http://why3.lri.fr/

    Hash consed values are of the
    following type [hash_consed]. The field [tag] contains a unique
    integer (for values hash consed with the same table). The field
    [hkey] contains the hash key of the value (without modulo) for
    possible use in other hash tables (and internally when hash
    consing tables are resized). The field [node] contains the value
    itself.

    Hash consing tables are using weak pointers, so that values that are no
    more referenced from anywhere else can be erased by the GC. *)

type +'a hash_consed = private {
  hkey : int;
  tag : int;
  node : 'a }

(*s Generic part, using ocaml generic equality and hash function. *)

type 'a t

val create : int -> 'a t
  (** [create n] creates an empty table of initial size [n]. The table
      will grow as needed. *)
val clear : 'a t -> unit
  (** Removes all elements from the table. *)
val hashcons : 'a t -> 'a -> 'a hash_consed
  (** [hashcons t n] hash-cons the value [n] using table [t] i.e. returns
      any existing value in [t] equal to [n], if any; otherwise, allocates
      a new one hash-consed value of node [n] and returns it.
      As a consequence the returned value is physically equal to
      any equal value already hash-consed using table [t]. *)
val iter : ('a hash_consed -> unit) -> 'a t -> unit
  (** [iter f t] iterates [f] over all elements of [t]. *)
val stats : 'a t -> int * int * int * int * int * int
  (** Return statistics on the table.  The numbers are, in order:
      table length, number of entries, sum of bucket lengths,
      smallest bucket length, median bucket length, biggest bucket length. *)

(*s Functorial interface. *)

module type HashedType =
  sig
    type t
    val equal : t -> t -> bool
    val hash : t -> int
  end

module type S =
  sig
    type key
    type t
    val create : int -> t
    val clear : t -> unit
    val hashcons : t -> key -> key hash_consed
    val iter : (key hash_consed -> unit) -> t -> unit
    val stats : t -> int * int * int * int * int * int
  end

module Make(H : HashedType) : (S with type key = H.t)