[mercury-users] Converting time_t to int using common library?

[Ralph Becket]

Just to reinforce Peter Wang's comment: random.m is a very weak 
random number generator.  I've attached a better RNG, but if randomness
is important, you should really do a fair bit of homework before
selecting a particular algorithm to suit your application.

-- Ralph
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%-----------------------------------------------------------------------------%
% tausworthe3.m
% Ralph Becket <rafe at cs.mu.oz.au>
% Tue Feb  1 11:44:19 EST 2005
% vim: ft=mercury ts=4 sw=4 et wm=0 tw=0
%
%-----------------------------------------------------------------------------%

:- module tausworthe3.

:- interface.

:- import_module int.



:- type tausworthe3.



:- func seed_tausworthe3(int, int, int) = tausworthe3.

:- pred rand_tausworthe3(int::out, tausworthe3::in, tausworthe3::out) is det.

%-----------------------------------------------------------------------------%
%-----------------------------------------------------------------------------%
%
% Adapted from http://remus.rutgers.edu/~rhoads/Code/tausworth.c
%

:- implementation.

:- type tausworthe3
	--->	state(
                s1	::	int,
                s2	::	int,
                s3	::	int,
                tausworthe3_consts
            ).

:- type tausworthe3_consts
	--->	consts(
                shft1	::	int,
                shft2	::	int,
                shft3	::	int,
                mask1	::	int,
                mask2	::	int,
                mask3	::	int
            ).

%-----------------------------------------------------------------------------%

seed_tausworthe3(A, B, C) = R :-

	P1     = 12,
	P2     =  4,
	P3     = 17,

	K1     = 31,
	K2     = 29,
	K3     = 28,

	X      = 4294967295,

	Shft1  = K1 - P1,
	Shft2  = K2 - P2,
	Shft3  = K3 - P3,

	Mask1  = X << (32 - K1),
	Mask2  = X << (32 - K2),
	Mask3  = X << (32 - K3),

	S1     = ( if A > (1 << (32 - K1)) then A else 390451501 ),
	S2     = ( if A > (1 << (32 - K2)) then B else 613566701 ),
	S3     = ( if A > (1 << (32 - K3)) then C else 858993401 ),

	Consts = consts(Shft1, Shft2, Shft3, Mask1, Mask2, Mask3),
	R0     = state(S1, S2, S3, Consts),
	rand_tausworthe3(_, R0, R).

%-----------------------------------------------------------------------------%

rand_tausworthe3(I, R0, R) :-
	R0     = state(S1_0, S2_0, S3_0, Consts),
	Consts = consts(Shft1, Shft2, Shft3, Mask1, Mask2, Mask3),

	P1     = 12,
	P2     =  4,
	P3     = 17,

	Q1     = 13,
	Q2     =  2,
	Q3     =  3,
               
	B1     = ((S1_0 << Q1)`xor`S1_0) >> Shft1,
	S1     = ((S1_0 /\ Mask1) << P1)`xor`B1,

	B2     = ((S2_0 << Q2)`xor`S2_0) >> Shft2,
	S2     = ((S2_0 /\ Mask2) << P2)`xor`B2,

	B3     = ((S3_0 << Q3)`xor`S3_0) >> Shft3,
	S3     = ((S3_0 /\ Mask3) << P3)`xor`B3,

	I      = abs(S1`xor`S2`xor`S3),
	R      = state(S1,   S2,   S3,   Consts).

%-----------------------------------------------------------------------------%
%-----------------------------------------------------------------------------%