1		% (c) 2009-2017 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(random_permutations, [get_num_bits/3,
6		get_masks/3,
7		random_permutation_element/10,
8		init_random_permutations/0
9		]).
10
11		:- use_module(probsrc(module_information),[module_info/2]).
12		:- module_info(group,cbc).
13		:- module_info(description,'This module provides on-the-fly construction of random permutations of intervals.').
14
15		foreign_resource(random_permutations,[get_num_bits,draw_index]).
16		foreign(get_num_bits, c, get_num_bits(+integer,-integer,-integer)).
17		foreign(draw_index, c, draw_index(+integer,+integer,+integer,+integer,+integer,+integer,+integer,+integer,-integer,-integer)).
18
19		:- dynamic is_initialised/0.
20
21		init_random_permutations :- is_initialised,!.
22		init_random_permutations :-
23		load_foreign_resource(library(random_permutations)),
24		assert(is_initialised).
25
26		get_masks(HalfNumBits,LeftMask,RightMask) :-
27		RightMask is (1 << HalfNumBits) - 1,
28		LeftMask is RightMask << HalfNumBits.
29
30		random_permutation_element(Index,MaxIndex,From,To,Seed,NumBits,LeftMask,RightMask,RandomElement,NextIndex) :-
31		draw_index(Index,MaxIndex,Seed,NumBits,LeftMask,RightMask,From,To,DrawnElement,NextIndex),
32		% working on a 4^x long interval. thus, we might pick a number that is too large
33		% if this happens, we just pick a new one
34		% to avoid context switching overhead, this is now done inside the C code
35		RandomElement is DrawnElement + From.
36
37		% -------------------
38		% Testing predicates
39		% -------------------
40		% unused at the moment:
41		%:- public shuffle/2.
42		%:- use_module(library(system)).
43		%shuffle(From,To) :-
44		% init_random_permutations,
45		% IntervalLength is To - From + 1,
46		% get_num_bits(IntervalLength,MaxIndex,NumBits),
47		% get_masks(NumBits,LeftMask,RightMask),
48		% now(Seed),
49		% shuffle(From,0,To,MaxIndex,Seed,NumBits,LeftMask,RightMask).
50		%shuffle(From,CurIdx,To,MaxIndex,Seed,NumBits,LeftMask,RightMask) :-
51		% random_permutation_element(CurIdx,MaxIndex,From,To,Seed,NumBits,LeftMask,RightMask,Drawn,NextIdx),
52		% format('Drawing index ~w resulted in ~w~n',[CurIdx,Drawn]),
53		% shuffle(From,NextIdx,To,MaxIndex,Seed,NumBits,LeftMask,RightMask).
54		%shuffle(_,_,_,_,_,_,_,_).