1		% (c) 2009-2019 Lehrstuhl fuer Softwaretechnik und Programmiersprachen,
2		% Heinrich Heine Universitaet Duesseldorf
3		% This software is licenced under EPL 1.0 (http://www.eclipse.org/org/documents/epl-v10.html)
4
5		:- module(hashing,[my_term_hash/2, efficient_index/1]).
6
7		:- use_module(library(terms),[term_hash/3]).
8
9
10		:- use_module(module_information,[module_info/2]).
11
12		:- module_info(group,animator).
13		:- module_info(description,'This module computes hash values (mainly for states) for efficient indexing.').
14
15		% --------------------------------------------
16
17		:- use_module(self_check).
18
19		:- assert_must_succeed(( hashing:my_term_hash([1,2,3],X), hashing:efficient_index(X) )).
20		:- assert_must_succeed(( hashing:my_term_hash([1,2,3],X), hashing:my_term_hash([1,2,4],Y), X\=Y )).
21
22		:- if(tools:platform_is_64_bit).
23		:- format("64-bit platform~n",[]).
24		my_term_hash(T,Res) :- super_hash(T,Res).
25		:- else.
26		:- format("32-bit platform~n",[]).
27		my_term_hash(T,Res) :- term_hash(T,
28		[range(smallint), /* will use full 32 bit on 64 bit systems, 28 bit on 32 bit system */
29		algorithm(sdbm), % other options jenkins, hsieh, default
30		depth(infinite),if_var(ignore)],Res).
31		% collisions seem with more recent versions of SICStus to no longer be a problem !?
32		% sdbm seems to generate less collisions
33		:- endif.
34
35		% check if a hash value is an efficient for first argument indexing
36		efficient_index(X) :- number(X),
37		prolog_flag(max_tagged_integer,Max), X=<Max,
38		prolog_flag(min_tagged_integer,Min), X>=Min.
39
40		% On 64-bit machines we can use the following for efficient indexing:
41		:- public super_hash/2.
42		super_hash(Term,Hash) :-
43		term_hash(Term,[range(smallint),algorithm(sdbm),depth(infinite),if_var(ignore)],H1),
44		term_hash(Term,[range(smallint32),algorithm(jenkins),depth(infinite),if_var(ignore)],H2),
45		Hash is H1 + (H2 << 32). % should be in range 0.. 2**60
46		% we could still try and use negative numbers
47