root/user/lib/libc/stdlib/random.c

DEFINITIONS

1. srandom_unlocked
2. srandom
3. initstate
4. setstate
5. random_unlocked
6. random

   1 /*
   2  * Copyright (c) 1983, 1993
   3  *      The Regents of the University of California.  All rights reserved.
   4  *
   5  * Redistribution and use in source and binary forms, with or without
   6  * modification, are permitted provided that the following conditions
   7  * are met:
   8  * 1. Redistributions of source code must retain the above copyright
   9  *    notice, this list of conditions and the following disclaimer.
  10  * 2. Redistributions in binary form must reproduce the above copyright
  11  *    notice, this list of conditions and the following disclaimer in the
  12  *    documentation and/or other materials provided with the distribution.
  13  * 3. All advertising materials mentioning features or use of this software
  14  *    must display the following acknowledgement:
  15  *      This product includes software developed by the University of
  16  *      California, Berkeley and its contributors.
  17  * 4. Neither the name of the University nor the names of its contributors
  18  *    may be used to endorse or promote products derived from this software
  19  *    without specific prior written permission.
  20  *
  21  * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
  22  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  23  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  24  * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
  25  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  26  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  27  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  28  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  29  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  30  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  31  * SUCH DAMAGE.
  32  */
  34 /*
  35  * From:
  36  *    NetBSD: random.c,v 1.19 2000/01/22 22:19:20 mycroft Exp
  37  *
  38  * Hacked gruesomely for OS/161.
  39  */
  41 #include <assert.h>
  42 #include <errno.h>
  43 #include <stdlib.h>
  45 /*
  46  * For a thread-safe libc, declare a lock for this file and change
  47  * these to be nonempty.
  48  */
  49 #define LOCKME()
  50 #define UNLOCKME()
  52 static void srandom_unlocked(unsigned long);
  53 static long random_unlocked(void);
  56 /*
  57  * random.c:
  58  *
  59  * An improved random number generation package.  In addition to the standard
  60  * rand()/srand() like interface, this package also has a special state info
  61  * interface.  The initstate() routine is called with a seed, an array of
  62  * bytes, and a count of how many bytes are being passed in; this array is
  63  * then initialized to contain information for random number generation with
  64  * that much state information.  Good sizes for the amount of state
  65  * information are 32, 64, 128, and 256 bytes.  The state can be switched by
  66  * calling the setstate() routine with the same array as was initiallized
  67  * with initstate().  By default, the package runs with 128 bytes of state
  68  * information and generates far better random numbers than a linear
  69  * congruential generator.  If the amount of state information is less than
  70  * 32 bytes, a simple linear congruential R.N.G. is used.
  71  *
  72  * Internally, the state information is treated as an array of longs; the
  73  * zeroeth element of the array is the type of R.N.G. being used (small
  74  * integer); the remainder of the array is the state information for the
  75  * R.N.G.  Thus, 32 bytes of state information will give 7 longs worth of
  76  * state information, which will allow a degree seven polynomial.  (Note:
  77  * the zeroeth word of state information also has some other information
  78  * stored in it -- see setstate() for details).
  79  * 
  80  * The random number generation technique is a linear feedback shift register
  81  * approach, employing trinomials (since there are fewer terms to sum up that
  82  * way).  In this approach, the least significant bit of all the numbers in
  83  * the state table will act as a linear feedback shift register, and will
  84  * have period 2^deg - 1 (where deg is the degree of the polynomial being
  85  * used, assuming that the polynomial is irreducible and primitive).  The
  86  * higher order bits will have longer periods, since their values are also
  87  * influenced by pseudo-random carries out of the lower bits.  The total
  88  * period of the generator is approximately deg*(2**deg - 1); thus doubling
  89  * the amount of state information has a vast influence on the period of the
  90  * generator.  Note: the deg*(2**deg - 1) is an approximation only good for
  91  * large deg, when the period of the shift register is the dominant factor.
  92  * With deg equal to seven, the period is actually much longer than the
  93  * 7*(2**7 - 1) predicted by this formula.
  94  *
  95  * Modified 28 December 1994 by Jacob S. Rosenberg.
  96  * The following changes have been made:
  97  * All references to the type u_int have been changed to unsigned long.
  98  * All references to type int have been changed to type long.  Other
  99  * cleanups have been made as well.  A warning for both initstate and
 100  * setstate has been inserted to the effect that on Sparc platforms
 101  * the 'arg_state' variable must be forced to begin on word boundaries.
 102  * This can be easily done by casting a long integer array to char *.
 103  * The overall logic has been left STRICTLY alone.  This software was
 104  * tested on both a VAX and Sun SpacsStation with exactly the same
 105  * results.  The new version and the original give IDENTICAL results.
 106  * The new version is somewhat faster than the original.  As the
 107  * documentation says:  "By default, the package runs with 128 bytes of
 108  * state information and generates far better random numbers than a linear
 109  * congruential generator.  If the amount of state information is less than
 110  * 32 bytes, a simple linear congruential R.N.G. is used."  For a buffer of
 111  * 128 bytes, this new version runs about 19 percent faster and for a 16
 112  * byte buffer it is about 5 percent faster.
 113  */
 115 /*
 116  * For each of the currently supported random number generators, we have a
 117  * break value on the amount of state information (you need at least this
 118  * many bytes of state info to support this random number generator), a degree
 119  * for the polynomial (actually a trinomial) that the R.N.G. is based on, and
 120  * the separation between the two lower order coefficients of the trinomial.
 121  */
 122 #define TYPE_0          0               /* linear congruential */
 123 #define BREAK_0         8
 124 #define DEG_0           0
 125 #define SEP_0           0
 127 #define TYPE_1          1               /* x**7 + x**3 + 1 */
 128 #define BREAK_1         32
 129 #define DEG_1           7
 130 #define SEP_1           3
 132 #define TYPE_2          2               /* x**15 + x + 1 */
 133 #define BREAK_2         64
 134 #define DEG_2           15
 135 #define SEP_2           1
 137 #define TYPE_3          3               /* x**31 + x**3 + 1 */
 138 #define BREAK_3         128
 139 #define DEG_3           31
 140 #define SEP_3           3
 142 #define TYPE_4          4               /* x**63 + x + 1 */
 143 #define BREAK_4         256
 144 #define DEG_4           63
 145 #define SEP_4           1
 147 /*
 148  * Array versions of the above information to make code run faster --
 149  * relies on fact that TYPE_i == i.
 150  */
 151 #define MAX_TYPES       5               /* max number of types above */
 153 static const int degrees[MAX_TYPES] =   { DEG_0, DEG_1, DEG_2, DEG_3, DEG_4 };
 154 static const int seps[MAX_TYPES] =      { SEP_0, SEP_1, SEP_2, SEP_3, SEP_4 };
 156 /*
 157  * Initially, everything is set up as if from:
 158  *
 159  *      initstate(1, &randtbl, 128);
 160  *
 161  * Note that this initialization takes advantage of the fact that srandom()
 162  * advances the front and rear pointers 10*rand_deg times, and hence the
 163  * rear pointer which starts at 0 will also end up at zero; thus the zeroeth
 164  * element of the state information, which contains info about the current
 165  * position of the rear pointer is just
 166  *
 167  *      MAX_TYPES * (rptr - state) + TYPE_3 == TYPE_3.
 168  */
 170 static long randtbl[DEG_3 + 1] = {
TYPE_3,
 172         (long)0x9a319039L, (long)0x32d9c024L, (long)0x9b663182L,
 173         (long)0x5da1f342L, (long)0xde3b81e0L, (long)0xdf0a6fb5L,
 174         (long)0xf103bc02L, (long)0x48f340fbL, (long)0x7449e56bL,
 175         (long)0xbeb1dbb0L, (long)0xab5c5918L, (long)0x946554fdL,
 176         (long)0x8c2e680fL, (long)0xeb3d799fL, (long)0xb11ee0b7L,
 177         (long)0x2d436b86L, (long)0xda672e2aL, (long)0x1588ca88L,
 178         (long)0xe369735dL, (long)0x904f35f7L, (long)0xd7158fd6L,
 179         (long)0x6fa6f051L, (long)0x616e6b96L, (long)0xac94efdcL,
 180         (long)0x36413f93L, (long)0xc622c298L, (long)0xf5a42ab8L,
 181         (long)0x8a88d77bL, (long)0xf5ad9d0eL, (long)0x8999220bL,
 182         (long)0x27fb47b9L,
 183 };
 185 /*
 186  * fptr and rptr are two pointers into the state info, a front and a rear
 187  * pointer.  These two pointers are always rand_sep places aparts, as they
 188  * cycle cyclically through the state information.  (Yes, this does mean we
 189  * could get away with just one pointer, but the code for random() is more
 190  * efficient this way).  The pointers are left positioned as they would be
 191  * from the call
 192  *
 193  *      initstate(1, randtbl, 128);
 194  *
 195  * (The position of the rear pointer, rptr, is really 0 (as explained above
 196  * in the initialization of randtbl) because the state table pointer is set
 197  * to point to randtbl[1] (as explained below).
 198  */
 199 static long *fptr = &randtbl[SEP_3 + 1];
 200 static long *rptr = &randtbl[1];
 202 /*
 203  * The following things are the pointer to the state information table, the
 204  * type of the current generator, the degree of the current polynomial being
 205  * used, and the separation between the two pointers.  Note that for efficiency
 206  * of random(), we remember the first location of the state information, not
 207  * the zeroeth.  Hence it is valid to access state[-1], which is used to
 208  * store the type of the R.N.G.  Also, we remember the last location, since
 209  * this is more efficient than indexing every time to find the address of
 210  * the last element to see if the front and rear pointers have wrapped.
 211  */
 212 static long *state = &randtbl[1];
 213 static long rand_type = TYPE_3;
 214 static int rand_deg = DEG_3;
 215 static int rand_sep = SEP_3;
 216 static long *end_ptr = &randtbl[DEG_3 + 1];
 218 /*
 219  * srandom:
 220  *
 221  * Initialize the random number generator based on the given seed.  If the
 222  * type is the trivial no-state-information type, just remember the seed.
 223  * Otherwise, initializes state[] based on the given "seed" via a linear
 224  * congruential generator.  Then, the pointers are set to known locations
 225  * that are exactly rand_sep places apart.  Lastly, it cycles the state
 226  * information a given number of times to get rid of any initial dependencies
 227  * introduced by the L.C.R.N.G.  Note that the initialization of randtbl[]
 228  * for default usage relies on values produced by this routine.
 229  */
 230 static
 231 void
srandom_unlocked(unsigned long x)
 233 {
 234         int i;
 236         if (rand_type == TYPE_0)
state[0] = x;
 238         else {
state[0] = x;
 240                 for (i = 1; i < rand_deg; i++)
state[i] = 1103515245L * state[i - 1] + 12345L;
fptr = &state[rand_sep];
rptr = &state[0];
 244                 for (i = 0; i < 10 * rand_deg; i++)
 245                         (void)random_unlocked();
 246         }
 247 }
 249 void
srandom(unsigned long x)
 251 {
LOCKME();
srandom_unlocked(x);
UNLOCKME();
 256 }
 258 /*
 259  * initstate:
 260  *
 261  * Initialize the state information in the given array of n bytes for future
 262  * random number generation.  Based on the number of bytes we are given, and
 263  * the break values for the different R.N.G.'s, we choose the best (largest)
 264  * one we can and set things up for it.  srandom() is then called to
 265  * initialize the state information.
 266  * 
 267  * Note that on return from srandom(), we set state[-1] to be the type
 268  * multiplexed with the current value of the rear pointer; this is so
 269  * successive calls to initstate() won't lose this information and will be
 270  * able to restart with setstate().
 271  * 
 272  * Note: the first thing we do is save the current state, if any, just like
 273  * setstate() so that it doesn't matter when initstate is called.
 274  *
 275  * Returns a pointer to the old state.
 276  *
 277  * Note: The Sparc platform requires that arg_state begin on a long
 278  * word boundary; otherwise a bus error will occur. Even so, lint will
 279  * complain about mis-alignment, but you should disregard these messages.
 280  */
 281 char *
initstate(
 283         unsigned long seed,             /* seed for R.N.G. */
 284         char *arg_state,                /* pointer to state array */
size_t n)                       /* # bytes of state info */
 286 {
 287         void *ostate = (void *)(&state[-1]);
 288         long *long_arg_state;
assert(arg_state != NULL);
long_arg_state = (long *)(void *)arg_state;
LOCKME();
 295         if (rand_type == TYPE_0)
state[-1] = rand_type;
 297         else
state[-1] = MAX_TYPES * (rptr - state) + rand_type;
 299         if (n < BREAK_0) {
UNLOCKME();
 301                 return (NULL);
 302         } else if (n < BREAK_1) {
rand_type = TYPE_0;
rand_deg = DEG_0;
rand_sep = SEP_0;
 306         } else if (n < BREAK_2) {
rand_type = TYPE_1;
rand_deg = DEG_1;
rand_sep = SEP_1;
 310         } else if (n < BREAK_3) {
rand_type = TYPE_2;
rand_deg = DEG_2;
rand_sep = SEP_2;
 314         } else if (n < BREAK_4) {
rand_type = TYPE_3;
rand_deg = DEG_3;
rand_sep = SEP_3;
 318         } else {
rand_type = TYPE_4;
rand_deg = DEG_4;
rand_sep = SEP_4;
 322         }
state = (long *) (long_arg_state + 1); /* first location */
end_ptr = &state[rand_deg];     /* must set end_ptr before srandom */
srandom_unlocked(seed);
 326         if (rand_type == TYPE_0)
long_arg_state[0] = rand_type;
 328         else
long_arg_state[0] = MAX_TYPES * (rptr - state) + rand_type;
UNLOCKME();
 331         return((char *)ostate);
 332 }
 334 /*
 335  * setstate:
 336  *
 337  * Restore the state from the given state array.
 338  *
 339  * Note: it is important that we also remember the locations of the pointers
 340  * in the current state information, and restore the locations of the pointers
 341  * from the old state information.  This is done by multiplexing the pointer
 342  * location into the zeroeth word of the state information.
 343  *
 344  * Note that due to the order in which things are done, it is OK to call
 345  * setstate() with the same state as the current state.
 346  *
 347  * Returns a pointer to the old state information.
 348  *
 349  * Note: The Sparc platform requires that arg_state begin on a long
 350  * word boundary; otherwise a bus error will occur. Even so, lint will
 351  * complain about mis-alignment, but you should disregard these messages.
 352  */
 353 char *
setstate(char *arg_state)               /* pointer to state array */
 355 {
 356         long *new_state;
 357         int type;
 358         int rear;
 359         void *ostate = (void *)(&state[-1]);
assert(arg_state != NULL);
new_state = (long *)(void *)arg_state;
type = (int)(new_state[0] % MAX_TYPES);
rear = (int)(new_state[0] / MAX_TYPES);
LOCKME();
 368         if (rand_type == TYPE_0)
state[-1] = rand_type;
 370         else
state[-1] = MAX_TYPES * (rptr - state) + rand_type;
 372         switch(type) {
 373         case TYPE_0:
 374         case TYPE_1:
 375         case TYPE_2:
 376         case TYPE_3:
 377         case TYPE_4:
rand_type = type;
rand_deg = degrees[type];
rand_sep = seps[type];
 381                 break;
 382         default:
UNLOCKME();
 384                 return (NULL);
 385         }
state = (long *) (new_state + 1);
 387         if (rand_type != TYPE_0) {
rptr = &state[rear];
fptr = &state[(rear + rand_sep) % rand_deg];
 390         }
end_ptr = &state[rand_deg];             /* set end_ptr too */
UNLOCKME();
 393         return((char *)ostate);
 394 }
 396 /*
 397  * random:
 398  *
 399  * If we are using the trivial TYPE_0 R.N.G., just do the old linear
 400  * congruential bit.  Otherwise, we do our fancy trinomial stuff, which is
 401  * the same in all the other cases due to all the global variables that have
 402  * been set up.  The basic operation is to add the number at the rear pointer
 403  * into the one at the front pointer.  Then both pointers are advanced to
 404  * the next location cyclically in the table.  The value returned is the sum
 405  * generated, reduced to 31 bits by throwing away the "least random" low bit.
 406  *
 407  * Note: the code takes advantage of the fact that both the front and
 408  * rear pointers can't wrap on the same call by not testing the rear
 409  * pointer if the front one has wrapped.
 410  *
 411  * Returns a 31-bit random number.
 412  */
 413 static
 414 long
random_unlocked(void)
 416 {
 417         long i;
 418         long *f, *r;
 420         if (rand_type == TYPE_0) {
i = state[0];
state[0] = i = (i * 1103515245L + 12345L) & 0x7fffffff;
 423         } else {
 424                 /*
 425                  * Use local variables rather than static variables for speed.
 426                  */
f = fptr; r = rptr;
 428                 *f += *r;
 429                 /* chucking least random bit */
i = ((unsigned long)*f >> 1) & 0x7fffffff;
 431                 if (++f >= end_ptr) {
f = state;
 433                         ++r;
 434                 }
 435                 else if (++r >= end_ptr) {
r = state;
 437                 }
fptr = f; rptr = r;
 440         }
 441         return(i);
 442 }
 444 long
random(void)
 446 {
 447         long r;
LOCKME();
r = random_unlocked();
UNLOCKME();
 452         return (r);
 453 }