99 lines
3.0 KiB
Plaintext
99 lines
3.0 KiB
Plaintext
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/*************************************************************************
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* *
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* YAP Prolog *
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* *
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* Yap Prolog was developed at NCCUP - Universidade do Porto *
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* *
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* Copyright L.Damas, V.S.Costa and Universidade do Porto 1985-1997 *
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* *
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**************************************************************************
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* *
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* File: regexp.yap *
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* Last rev: 5/15/2000 *
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* mods: *
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* comments: pseudo random numbers in YAP (from code by Van Gelder) *
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* *
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*************************************************************************/
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% The following code produces the same random numbers as my previous
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% ranpkg.pl, but is more accurately documented and slightly more
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% efficient.
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% ranpkg.pl random number package Allen Van Gelder, Stanford
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% rannum produces a random non-negative integer whose low bits are not
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% all that random, so it should be scaled to a smaller range in general.
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% The integer is in the range 0 .. 2^(w-1) - 1,
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% where w is the word size available for integers, e.g., 18 for DEC-10,
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% and 16 or 32 for VAX and most IBM.
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%
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% ranunif produces a uniformly distributed non-negative random integer over
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% a caller-specified range. If range is R, the result is in 0 .. R-1.
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%
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% ranstart must be called before the first use of rannum or ranunif,
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% and may be called later to redefine the seed.
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% ranstart/0 causes a built-in seed to be used.
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% ranstart(N), N an integer, varies this, but the same N always
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% produces the same sequence of numbers.
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%
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% According to my reading of Knuth, Vol. 2, this generator has period
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% 2^(w-1) and potency w/2, i.e., 8, 9, or 16 in practice. Knuth says
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% potency should be at least 5, so this looks more than adequate.
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% Its drawback is the lack of randomness of low-order bits.
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:- module(prandom, [
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ranstart/0,
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ranstart/1,
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rannum/1,
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rannunif/2]).
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:- initialization(ranstart).
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%
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% vsc: dangerous code, to change.
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%
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%
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wsize(32) :-
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yap_flag(max_integer,2147483647), !.
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wsize(64).
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ranstart :- ranstart(8'365).
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ranstart(N) :-
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wsize(32), % bits available for int.
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MaxInt is \(1 << (Wsize - 1)), % all bits but sign bit are 1.
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Incr is (8'154 << (Wsize - 9)) + 1, % per Knuth, v.2 p.78
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Mult is 8'3655, % OK for 16-18 Wsize
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Prev is Mult * (8 * N + 5) + Incr,
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recorda(ranState, ranState(Mult, Prev, Wsize, MaxInt, Incr), Ref).
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rannum(Raw) :-
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recorded(ranState, ranState(Mult, Prev, Wsize, MaxInt, Incr), Oldref),
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erase(Oldref),
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Curr is Mult * Prev + Incr,
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recorda(ranState, ranState(Mult, Curr, Wsize, MaxInt, Incr), Ref),
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( Curr > 0,
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Raw is Curr
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;
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Curr < 0,
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Raw is Curr /\ MaxInt % force positive sign bit
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).
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ranunif(Range, Unif) :-
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Range > 0,
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recorded(ranState, ranState(Mult, Prev, Wsize, MaxInt, Incr), Oldref),
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erase(Oldref),
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Curr is Mult * Prev + Incr,
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recorda(ranState, ranState(Mult, Curr, Wsize, MaxInt, Incr), Ref),
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( Curr > 0,
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Raw is Curr
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;
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Curr < 0,
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Raw is Curr /\ MaxInt % force positive sign bit
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),
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Unif is (Raw * Range) >> (Wsize-1).
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