/usr/share/Yap/prandom.yap is in yap 6.2.2-6+b2.
This file is owned by root:root, with mode 0o644.
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* *
* YAP Prolog *
* *
* Yap Prolog was developed at NCCUP - Universidade do Porto *
* *
* Copyright L.Damas, V.S.Costa and Universidade do Porto 1985-1997 *
* *
**************************************************************************
* *
* File: regexp.yap *
* Last rev: 5/15/2000 *
* mods: *
* comments: pseudo random numbers in YAP (from code by Van Gelder) *
* *
*************************************************************************/
% The following code produces the same random numbers as my previous
% ranpkg.pl, but is more accurately documented and slightly more
% efficient.
% ranpkg.pl random number package Allen Van Gelder, Stanford
% rannum produces a random non-negative integer whose low bits are not
% all that random, so it should be scaled to a smaller range in general.
% The integer is in the range 0 .. 2^(w-1) - 1,
% where w is the word size available for integers, e.g., 18 for DEC-10,
% and 16 or 32 for VAX and most IBM.
%
% ranunif produces a uniformly distributed non-negative random integer over
% a caller-specified range. If range is R, the result is in 0 .. R-1.
%
% ranstart must be called before the first use of rannum or ranunif,
% and may be called later to redefine the seed.
% ranstart/0 causes a built-in seed to be used.
% ranstart(N), N an integer, varies this, but the same N always
% produces the same sequence of numbers.
%
% According to my reading of Knuth, Vol. 2, this generator has period
% 2^(w-1) and potency w/2, i.e., 8, 9, or 16 in practice. Knuth says
% potency should be at least 5, so this looks more than adequate.
% Its drawback is the lack of randomness of low-order bits.
:- module(prandom, [
ranstart/0,
ranstart/1,
rannum/1,
ranunif/2]).
:- initialization(ranstart).
:- dynamic ranState/5.
%
% vsc: dangerous code, to change.
%
%
wsize(32) :-
yap_flag(max_tagged_integer,I), I >> 32 =:= 0, !.
wsize(64).
ranstart :- ranstart(8'365).
ranstart(N) :-
wsize(Wsize), % bits available for int.
MaxInt is \(1 << (Wsize - 1)), % all bits but sign bit are 1.
Incr is (8'154 << (Wsize - 9)) + 1, % per Knuth, v.2 p.78
Mult is 8'3655, % OK for 16-18 Wsize
Prev is Mult * (8 * N + 5) + Incr,
assert(ranState(Mult, Prev, Wsize, MaxInt, Incr) ).
rannum(Raw) :-
retract(ranState(Mult, Prev, Wsize, MaxInt, Incr)),
Curr is Mult * Prev + Incr,
assert(ranState(Mult, Curr, Wsize, MaxInt, Incr)),
( Curr > 0,
Raw is Curr
;
Curr < 0,
Raw is Curr /\ MaxInt % force positive sign bit
).
ranunif(Range, Unif) :-
Range > 0,
retract( ranState(Mult, Prev, Wsize, MaxInt, Incr) ),
Curr is Mult * Prev + Incr,
assert(ranState(Mult, Curr, Wsize, MaxInt, Incr)),
( Curr > 0,
Raw is Curr
;
Curr < 0,
Raw is Curr /\ MaxInt % force positive sign bit
),
Unif is (Raw * Range) >> (Wsize-1).
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