This commit was generated by cvs2svn to compensate for changes in r4,

which included commits to RCS files with non-trunk default branches. git-svn-id: https://yap.svn.sf.net/svnroot/yap/trunk@5 b08c6af1-5177-4d33-ba66-4b1c6b8b522a
2001-04-09 19:54:03 +00:00
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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,
+	rannunif/2]).
+
+:- initialization(ranstart).
+
+%
+% vsc: dangerous code, to change.
+%
+%
+wsize(32) :-
+	yap_flag(max_integer,2147483647), !.
+wsize(64).
+
+ranstart :- ranstart(8'365).
+ 
+ranstart(N) :-
+	wsize(32),				% 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,
+	recorda(ranState, ranState(Mult, Prev, Wsize, MaxInt, Incr), Ref).
+ 
+rannum(Raw) :-
+	recorded(ranState, ranState(Mult, Prev, Wsize, MaxInt, Incr), Oldref),
+	erase(Oldref),
+	Curr is Mult * Prev + Incr,
+	recorda(ranState, ranState(Mult, Curr, Wsize, MaxInt, Incr), Ref),
+	(	Curr > 0,
+		Raw is Curr
+	;
+		Curr < 0,
+		Raw is Curr /\ MaxInt		% force positive sign bit
+	).
+ 
+ranunif(Range, Unif) :-
+	Range > 0,
+	recorded(ranState, ranState(Mult, Prev, Wsize, MaxInt, Incr), Oldref),
+	erase(Oldref),
+	Curr is Mult * Prev + Incr,
+	recorda(ranState, ranState(Mult, Curr, Wsize, MaxInt, Incr), Ref),
+	(	Curr > 0,
+		Raw is Curr
+	;
+		Curr < 0,
+		Raw is Curr /\ MaxInt		% force positive sign bit
+	),
+	Unif is (Raw * Range) >> (Wsize-1).
+
+