path: root/lib/hashcons.ml

(************************************************************************)
(*  v      *   The Coq Proof Assistant  /  The Coq Development Team     *)
(* <O___,, *   INRIA - CNRS - LIX - LRI - PPS - Copyright 1999-2012     *)
(*   \VV/  **************************************************************)
(*    //   *      This file is distributed under the terms of the       *)
(*         *       GNU Lesser General Public License Version 2.1        *)
(************************************************************************)

(* Hash consing of datastructures *)

(* The generic hash-consing functions (does not use Obj) *)

(* [t] is the type of object to hash-cons
 * [u] is the type of hash-cons functions for the sub-structures
 *   of objects of type t (u usually has the form (t1->t1)*(t2->t2)*...).
 * [hashcons u x] is a function that hash-cons the sub-structures of x using
 *   the hash-consing functions u provides.
 * [equal] is a comparison function. It is allowed to use physical equality
 *   on the sub-terms hash-consed by the hashcons function.
 * [hash] is the hash function given to the Hashtbl.Make function
 *
 * Note that this module type coerces to the argument of Hashtbl.Make.
 *)

module type HashconsedType =
  sig
    type t
    type u
    val hashcons :  u -> t -> t
    val equal : t -> t -> bool
    val hash : t -> int
  end

(* The output is a function [generate] such that
 * [generate ()] has the side-effect of creating (internally) a hash-table of the
 *   hash-consed objects. The result is a function taking the sub-hashcons
 *   functions and an object, and hashcons it. It does not really make sense
 *   to call generate() with different sub-hcons functions. That's why we use the
 *   wrappers simple_hcons, recursive_hcons, ... The latter just take as
 *   argument the sub-hcons functions (the tables are created at that moment),
 *   and returns the hcons function for t.
 *)

module type S =
  sig
    type t
    type u
    val generate : unit -> (u -> t -> t)
  end

module Make (X : HashconsedType) =
  struct
    type t = X.t
    type u = X.u

    (* We create the type of hashtables for t, with our comparison fun.
     * An invariant is that the table never contains two entries equals
     * w.r.t (=), although the equality on keys is X.equal. This is
     * granted since we hcons the subterms before looking up in the table.
     *)
    module Htbl = Hashset.Make(X)

    (* The table is created when () is applied.
     * Hashconsing is then very simple:
     *  1- hashcons the subterms using hashcons and u
     *  2- look up in the table, if we do not get a hit, we add it
     *)
    let generate () =
      let tab = Htbl.create 97 in
        (fun u x ->
           let y = X.hashcons u x in
            Htbl.repr (X.hash y) y tab)

  end

(* A few usefull wrappers:
 * takes as argument the function [generate] above and build a function of type
 * u -> t -> t that creates a fresh table each time it is applied to the
 * sub-hcons functions. *)

(* For non-recursive types it is quite easy. *)
let simple_hcons h u =
  let h = h () in
  fun x -> h u x

(* For a recursive type T, we write the module of sig Comp with u equals
 * to (T -> T) * u0
 * The first component will be used to hash-cons the recursive subterms
 * The second one to hashcons the other sub-structures.
 * We just have to take the fixpoint of h
 *)
let recursive_hcons h u =
  let hc = h () in
  let rec hrec x = hc (hrec,u) x in
  hrec

(* For 2 mutually recursive types *)
let recursive2_hcons h1 h2 u1 u2 =
  let hc1 = h1 () in
  let hc2 = h2 () in
  let rec hrec1 x = hc1 (hrec1,hrec2,u1) x
  and hrec2 x = hc2 (hrec1,hrec2,u2) x
  in (hrec1,hrec2)

(* A set of global hashcons functions *)
let hashcons_resets = ref []
let init() = List.iter (fun f -> f()) !hashcons_resets

(* [register_hcons h u] registers the hcons function h, result of the above
 *   wrappers. It returns another hcons function that always uses the same
 *   table, which can be reinitialized by init()
 *)
let register_hcons h u =
  let hf = ref (h u) in
  let reset() = hf := h u in
  hashcons_resets := reset :: !hashcons_resets;
  (fun x -> !hf x)

(* Basic hashcons modules for string and obj. Integers do not need be
   hashconsed.  *)

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

(* list *)
module Hlist (D:HashedType) =
  Make(
    struct
      type t = D.t list
      type u = (t -> t) * (D.t -> D.t)
      let hashcons (hrec,hdata) = function
        | x :: l -> hdata x :: hrec l
        | l -> l
      let equal l1 l2 =
        l1 == l2 ||
          match l1, l2 with
          | [], [] -> true
          | x1::l1, x2::l2 -> x1==x2 && l1==l2
          | _ -> false
      let rec hash accu = function
      | [] -> accu
      | x :: l ->
        let accu = Hashset.Combine.combine (D.hash x) accu in
        hash accu l
      let hash l = hash 0 l
    end)

(* string *)
module Hstring = Make(
  struct
    type t = string
    type u = unit
    let hashcons () s =(* incr accesstr;*) s
    external equal : string -> string -> bool = "caml_string_equal" "noalloc"
    (** Copy from CString *)
    let rec hash len s i accu =
      if i = len then accu
      else
        let c = Char.code (String.unsafe_get s i) in
        hash len s (succ i) (accu * 19 + c)

    let hash s =
      let len = String.length s in
      hash len s 0 0
  end)

(* Obj.t *)
exception NotEq

(* From CAMLLIB/caml/mlvalues.h *)
let no_scan_tag = 251
let tuple_p obj = Obj.is_block obj && (Obj.tag obj < no_scan_tag)

let comp_obj o1 o2 =
  if tuple_p o1 && tuple_p o2 then
    let n1 = Obj.size o1 and n2 = Obj.size o2 in
      if n1=n2 then
        try
          for i = 0 to pred n1 do
            if not (Obj.field o1 i == Obj.field o2 i) then raise NotEq
          done; true
        with NotEq -> false
      else false
  else o1=o2

let hash_obj hrec o =
  begin
    if tuple_p o then
      let n = Obj.size o in
        for i = 0 to pred n do
          Obj.set_field o i (hrec (Obj.field o i))
        done
  end;
  o

module Hobj = Make(
  struct
    type t = Obj.t
    type u = (Obj.t -> Obj.t) * unit
    let hashcons (hrec,_) = hash_obj hrec
    let equal = comp_obj
    let hash = Hashtbl.hash
  end)

(* Hashconsing functions for string and obj. Always use the same
 * global tables. The latter can be reinitialized with init()
 *)
(* string : string -> string *)
(* obj : Obj.t -> Obj.t *)
let string = register_hcons (simple_hcons Hstring.generate) ()
let obj = register_hcons (recursive_hcons Hobj.generate) ()

(* The unsafe polymorphic hashconsing function *)
let magic_hash (c : 'a) =
  init();
  let r = obj (Obj.repr c) in
  init();
  (Obj.magic r : 'a)