3 * $Id: lcrand.c,v 1.5 2004/04/08 01:36:15 mdw Exp $
5 * Simple linear congruential generator
7 * (c) 1999 Straylight/Edgeware
10 /*----- Licensing notice --------------------------------------------------*
12 * This file is part of Catacomb.
14 * Catacomb is free software; you can redistribute it and/or modify
15 * it under the terms of the GNU Library General Public License as
16 * published by the Free Software Foundation; either version 2 of the
17 * License, or (at your option) any later version.
19 * Catacomb is distributed in the hope that it will be useful,
20 * but WITHOUT ANY WARRANTY; without even the implied warranty of
21 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
22 * GNU Library General Public License for more details.
24 * You should have received a copy of the GNU Library General Public
25 * License along with Catacomb; if not, write to the Free
26 * Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
30 /*----- Header files ------------------------------------------------------*/
37 #include <mLib/bits.h>
43 /*----- Magic numbers -----------------------------------------------------*/
45 /* --- The generator parameters --- */
47 #define P LCRAND_P /* Modulus */
48 #define A LCRAND_A /* Multiplier (primitive mod @p@) */
49 #define C LCRAND_C /* Additive constant */
51 /* --- Precomputed values for modular reduction --- */
53 #define D 5 /* %$p = 2^{32} - d$% */
55 /* --- Other useful bits --- */
57 #define P256 4294967040u /* Highest multiple of 256 < %$p$% */
59 /*----- Main code ---------------------------------------------------------*/
63 * Arguments: @uint32 x@ = seed value
65 * Returns: New state of the generator.
67 * Use: Steps the generator. Returns %$ax + c \bmod p$%.
70 uint32
lcrand(uint32 x
)
75 /* --- Unpack things into the arrays --- */
77 a
[0] = U16(A
); a
[1] = U16(A
>> 16);
78 xx
[0] = U16(x
); xx
[1] = U16(x
>> 16);
80 /* --- Multiply everything together --- *
82 * This is plain old long multiplication, although it looks a bit strange.
83 * I set up the top and bottom partial products directly where they're
84 * supposed to be. The cross terms I add together, with the low 16 bits in
85 * @q@ and the high 32 bits in @p@. These I then add into the product.
96 p
= ((q
< p
) << 16) + (q
>> 16);
103 p
+= (q
>> 16) >> 16;
109 /* --- Now reduce mod p --- *
111 * I'm using shifts and adds to do the multiply step here. This needs to
112 * be changed if @D@ ever becomes something other than 5.
116 # error "Change shift sequence!"
133 y
+= (q
>> 16) >> 16;
141 /* --- Now add on the constant --- */
152 /* --- @lcrand_range@ --- *
154 * Arguments: @uint32 *x@ = pointer to seed value (updated)
155 * @uint32 m@ = limit allowable
157 * Returns: A uniformly distributed pseudorandom integer in the interval
161 uint32
lcrand_range(uint32
*x
, uint32 m
)
164 uint32 r
= P
- P
% m
;
165 do xx
= lcrand(xx
); while (xx
>= r
);
170 /*----- Generic interface -------------------------------------------------*/
172 typedef struct gctx
{
177 static void gdestroy(grand
*r
)
183 static int gmisc(grand
*r
, unsigned op
, ...)
192 switch (va_arg(ap
, unsigned)) {
195 case GRAND_SEEDUINT32
:
205 g
->x
= va_arg(ap
, unsigned);
207 case GRAND_SEEDUINT32
:
208 g
->x
= va_arg(ap
, uint32
);
210 case GRAND_SEEDRAND
: {
211 grand
*rr
= va_arg(ap
, grand
*);
213 do x
= rr
->ops
->word(rr
); while (x
>= P
|| x
== LCRAND_FIXEDPT
);
225 static uint32
graw(grand
*r
)
232 static octet
gbyte(grand
*r
)
236 do x
= lcrand(x
); while (x
>= P256
);
238 return (x
/ (P256
/ 256));
241 static uint32
grange(grand
*r
, uint32 l
)
244 return (lcrand_range(&g
->x
, l
));
247 static const grand_ops gops
= {
251 graw
, gbyte
, grand_word
, grange
, grand_fill
254 /* --- @lcrand_create@ --- *
256 * Arguments: @uint32 x@ = initial seed
258 * Returns: Pointer to a generic generator.
260 * Use: Constructs a generic generator interface over a linear
261 * congruential generator.
264 grand
*lcrand_create(uint32 x
)
266 gctx
*g
= CREATE(gctx
);
272 /*----- Test rig ----------------------------------------------------------*/
276 #include <mLib/testrig.h>
278 static int verify(dstr
*v
)
280 uint32 x
= *(uint32
*)v
[0].buf
;
281 uint32 y
= *(uint32
*)v
[1].buf
;
282 uint32 z
= lcrand(x
);
286 "\n*** lcrand failed. lcrand(%lu) = %lu, expected %lu\n",
287 (unsigned long)x
, (unsigned long)z
, (unsigned long)y
);
293 static test_chunk tests
[] = {
294 { "lcrand", verify
, { &type_uint32
, &type_uint32
, 0 } },
298 int main(int argc
, char *argv
[])
300 test_run(argc
, argv
, tests
, SRCDIR
"/tests/lcrand");
306 /*----- That's all, folks -------------------------------------------------*/