1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
|
(************************************************************************)
(* 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 *)
(************************************************************************)
(* The following module is a specialized version of [Hashtbl] that is
a better space saver. Actually, [Hashcons] instanciates [Hashtbl.t]
with [constr] used both as a key and as an image. Thus, in each
cell of the internal bucketlist, there are two representations of
the same value. In this implementation, there is only one.
Besides, the responsibility of computing the hash function is now
given to the caller, which makes possible the interleaving of the
hash key computation and the hash-consing. *)
module type EqType = sig
type t
val equal : t -> t -> bool
end
module type S = sig
type elt
type t
val create : int -> t
val repr : int -> elt -> t -> elt
end
module Make (E : EqType) =
struct
type elt = E.t
type bucketlist = Empty | Cons of elt * int * bucketlist
type t = {
mutable size : int;
mutable data : bucketlist array; }
let create s =
let s = min (max 1 s) Sys.max_array_length in
{ size = 0; data = Array.make s Empty }
let resize table =
let odata = table.data in
let osize = Array.length odata in
let nsize = min (2 * osize + 1) Sys.max_array_length in
if nsize <> osize then begin
let ndata = Array.create nsize Empty in
let rec insert_bucket = function
| Empty -> ()
| Cons (key, hash, rest) ->
let nidx = hash mod nsize in
let obucket = ndata.(nidx) in
ndata.(nidx) <- Cons (key, hash, obucket);
insert_bucket rest
in
for i = 0 to osize - 1 do insert_bucket odata.(i) done;
table.data <- ndata
end
let add hash key table =
let odata = table.data in
let osize = Array.length odata in
let i = hash mod osize in
odata.(i) <- Cons (key, hash, odata.(i));
table.size <- table.size + 1;
if table.size > osize lsl 1 then resize table
let find_rec hash key table bucket =
let rec aux = function
| Empty ->
add hash key table; key
| Cons (k, h, rest) ->
if hash == h && E.equal key k then k else aux rest
in
aux bucket
let repr hash key table =
let odata = table.data in
let osize = Array.length odata in
let i = hash mod osize in
match odata.(i) with
| Empty -> add hash key table; key
| Cons (k1, h1, rest1) ->
if hash == h1 && E.equal key k1 then k1 else
match rest1 with
| Empty -> add hash key table; key
| Cons (k2, h2, rest2) ->
if hash == h2 && E.equal key k2 then k2 else
match rest2 with
| Empty -> add hash key table; key
| Cons (k3, h3, rest3) ->
if hash == h2 && E.equal key k3 then k3
else find_rec hash key table rest3
end
module Combine = struct
(* These are helper functions to combine the hash keys in a similar
way as [Hashtbl.hash] does. The constants [alpha] and [beta] must
be prime numbers. There were chosen empirically. Notice that the
problem of hashing trees is hard and there are plenty of study on
this topic. Therefore, there must be room for improvement here. *)
let alpha = 65599
let beta = 7
let combine x y = x * alpha + y
let combine3 x y z = combine x (combine y z)
let combine4 x y z t = combine x (combine3 y z t)
let combinesmall x y = beta * x + y
end
|
