stdlib/random_r.c

   1 /*
   2  * Copyright (c) 1983 Regents of the University of California.
   3  * All rights reserved.
   4  *
   5  * Redistribution and use in source and binary forms are permitted
   6  * provided that the above copyright notice and this paragraph are
   7  * duplicated in all such forms and that any documentation,
   8  * advertising materials, and other materials related to such
   9  * distribution and use acknowledge that the software was developed
  10  * by the University of California, Berkeley.  The name of the
  11  * University may not be used to endorse or promote products derived
  12  * from this software without specific prior written permission.
  13  * THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR
  14  * IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED
  15  * WARRANTIES OF MERCHANTIBILITY AND FITNESS FOR A PARTICULAR PURPOSE.
  16  */
  17
  18 /*
  19  * This is derived from the Berkeley source:
  20  *      @(#)random.c    5.5 (Berkeley) 7/6/88
  21  * It was reworked for the GNU C Library by Roland McGrath.
  22  * Rewritten to be reentrant by Ulrich Drepper, 1995
  23  */
  24
  25 #include <errno.h>
  26 #include <limits.h>
  27 #include <stddef.h>
  28 #include <stdlib.h>
  29
  30
  31 /* An improved random number generation package.  In addition to the standard
  32    rand()/srand() like interface, this package also has a special state info
  33    interface.  The initstate() routine is called with a seed, an array of
  34    bytes, and a count of how many bytes are being passed in; this array is
  35    then initialized to contain information for random number generation with
  36    that much state information.  Good sizes for the amount of state
  37    information are 32, 64, 128, and 256 bytes.  The state can be switched by
  38    calling the setstate() function with the same array as was initialized
  39    with initstate().  By default, the package runs with 128 bytes of state
  40    information and generates far better random numbers than a linear
  41    congruential generator.  If the amount of state information is less than
  42    32 bytes, a simple linear congruential R.N.G. is used.  Internally, the
  43    state information is treated as an array of longs; the zeroth element of
  44    the array is the type of R.N.G. being used (small integer); the remainder
  45    of the array is the state information for the R.N.G.  Thus, 32 bytes of
  46    state information will give 7 longs worth of state information, which will
  47    allow a degree seven polynomial.  (Note: The zeroth word of state
  48    information also has some other information stored in it; see setstate
  49    for details).  The random number generation technique is a linear feedback
  50    shift register approach, employing trinomials (since there are fewer terms
  51    to sum up that way).  In this approach, the least significant bit of all
  52    the numbers in the state table will act as a linear feedback shift register,
  53    and will have period 2^deg - 1 (where deg is the degree of the polynomial
  54    being used, assuming that the polynomial is irreducible and primitive).
  55    The higher order bits will have longer periods, since their values are
  56    also influenced by pseudo-random carries out of the lower bits.  The
  57    total period of the generator is approximately deg*(2**deg - 1); thus
  58    doubling the amount of state information has a vast influence on the
  59    period of the generator.  Note: The deg*(2**deg - 1) is an approximation
  60    only good for large deg, when the period of the shift register is the
  61    dominant factor.  With deg equal to seven, the period is actually much
  62    longer than the 7*(2**7 - 1) predicted by this formula.  */
  63
  64
  65
  66 /* For each of the currently supported random number generators, we have a
  67    break value on the amount of state information (you need at least this many
  68    bytes of state info to support this random number generator), a degree for
  69    the polynomial (actually a trinomial) that the R.N.G. is based on, and
  70    separation between the two lower order coefficients of the trinomial.  */
  71
  72 /* Linear congruential.  */
  73 #define TYPE_0          0
  74 #define BREAK_0         8
  75 #define DEG_0           0
  76 #define SEP_0           0
  77
  78 /* x**7 + x**3 + 1.  */
  79 #define TYPE_1          1
  80 #define BREAK_1         32
  81 #define DEG_1           7
  82 #define SEP_1           3
  83
  84 /* x**15 + x + 1.  */
  85 #define TYPE_2          2
  86 #define BREAK_2         64
  87 #define DEG_2           15
  88 #define SEP_2           1
  89
  90 /* x**31 + x**3 + 1.  */
  91 #define TYPE_3          3
  92 #define BREAK_3         128
  93 #define DEG_3           31
  94 #define SEP_3           3
  95
  96 /* x**63 + x + 1.  */
  97 #define TYPE_4          4
  98 #define BREAK_4         256
  99 #define DEG_4           63
 100 #define SEP_4           1
 101
 102
 103 /* Array versions of the above information to make code run faster.
 104    Relies on fact that TYPE_i == i.  */
 105
 106 #define MAX_TYPES       5       /* Max number of types above.  */
 107
 108 struct random_poly_info
 109 {
 110   int seps[MAX_TYPES];
 111   int degrees[MAX_TYPES];
 112 };
 113
 114 static const struct random_poly_info random_poly_info =
 115 {
 116   { SEP_0, SEP_1, SEP_2, SEP_3, SEP_4 },
 117   { DEG_0, DEG_1, DEG_2, DEG_3, DEG_4 }
 118 };
 119
 120
 121
 122 \f
 123 /* Initialize the random number generator based on the given seed.  If the
 124    type is the trivial no-state-information type, just remember the seed.
 125    Otherwise, initializes state[] based on the given "seed" via a linear
 126    congruential generator.  Then, the pointers are set to known locations
 127    that are exactly rand_sep places apart.  Lastly, it cycles the state
 128    information a given number of times to get rid of any initial dependencies
 129    introduced by the L.C.R.N.G.  Note that the initialization of randtbl[]
 130    for default usage relies on values produced by this routine.  */
 131 int
 132 __srandom_r (seed, buf)
 133      unsigned int seed;
 134      struct random_data *buf;
 135 {
 136   int type;
 137   int32_t *state;
 138   long int i;
 139   long int word;
 140   int32_t *dst;
 141   int kc;
 142
 143   if (buf == NULL)
 144     goto fail;
 145   type = buf->rand_type;
 146   if ((unsigned int) type >= MAX_TYPES)
 147     goto fail;
 148
 149   state = buf->state;
 150   /* We must make sure the seed is not 0.  Take arbitrarily 1 in this case.  */
 151   if (seed == 0)
 152     seed = 1;
 153   state[0] = seed;
 154   if (type == TYPE_0)
 155     goto done;
 156
 157   dst = state;
 158   word = seed;
 159   kc = buf->rand_deg;
 160   for (i = 1; i < kc; ++i)
 161     {
 162       /* This does:
 163            state[i] = (16807 * state[i - 1]) % 2147483647;
 164          but avoids overflowing 31 bits.  */
 165       long int hi = word / 127773;
 166       long int lo = word % 127773;
 167       word = 16807 * lo - 2836 * hi;
 168       if (word < 0)
 169         word += 2147483647;
 170       *++dst = word;
 171     }
 172
 173   buf->fptr = &state[buf->rand_sep];
 174   buf->rptr = &state[0];
 175   kc *= 10;
 176   while (--kc >= 0)
 177     {
 178       int32_t discard;
 179       (void) __random_r (buf, &discard);
 180     }
 181
 182  done:
 183   return 0;
 184
 185  fail:
 186   return -1;
 187 }
 188
 189 weak_alias (__srandom_r, srandom_r)
 190 \f
 191 /* Initialize the state information in the given array of N bytes for
 192    future random number generation.  Based on the number of bytes we
 193    are given, and the break values for the different R.N.G.'s, we choose
 194    the best (largest) one we can and set things up for it.  srandom is
 195    then called to initialize the state information.  Note that on return
 196    from srandom, we set state[-1] to be the type multiplexed with the current
 197    value of the rear pointer; this is so successive calls to initstate won't
 198    lose this information and will be able to restart with setstate.
 199    Note: The first thing we do is save the current state, if any, just like
 200    setstate so that it doesn't matter when initstate is called.
 201    Returns a pointer to the old state.  */
 202 int
 203 __initstate_r (seed, arg_state, n, buf)
 204      unsigned int seed;
 205      char *arg_state;
 206      size_t n;
 207      struct random_data *buf;
 208 {
 209   int type;
 210   int degree;
 211   int separation;
 212   int32_t *state;
 213
 214   if (buf == NULL)
 215     goto fail;
 216
 217   if (n >= BREAK_3)
 218     type = n < BREAK_4 ? TYPE_3 : TYPE_4;
 219   else if (n < BREAK_1)
 220     {
 221       if (n < BREAK_0)
 222         {
 223           __set_errno (EINVAL);
 224           goto fail;
 225         }
 226       type = TYPE_0;
 227     }
 228   else
 229     type = n < BREAK_2 ? TYPE_1 : TYPE_2;
 230
 231   degree = random_poly_info.degrees[type];
 232   separation = random_poly_info.seps[type];
 233
 234   buf->rand_type = type;
 235   buf->rand_sep = separation;
 236   buf->rand_deg = degree;
 237   state = &((int32_t *) arg_state)[1];  /* First location.  */
 238   /* Must set END_PTR before srandom.  */
 239   buf->end_ptr = &state[degree];
 240
 241   buf->state = state;
 242
 243   __srandom_r (seed, buf);
 244
 245   state[-1] = TYPE_0;
 246   if (type != TYPE_0)
 247     state[-1] = (buf->rptr - state) * MAX_TYPES + type;
 248
 249   return 0;
 250
 251  fail:
 252   __set_errno (EINVAL);
 253   return -1;
 254 }
 255
 256 weak_alias (__initstate_r, initstate_r)
 257 \f
 258 /* Restore the state from the given state array.
 259    Note: It is important that we also remember the locations of the pointers
 260    in the current state information, and restore the locations of the pointers
 261    from the old state information.  This is done by multiplexing the pointer
 262    location into the zeroth word of the state information. Note that due
 263    to the order in which things are done, it is OK to call setstate with the
 264    same state as the current state
 265    Returns a pointer to the old state information.  */
 266 int
 267 __setstate_r (arg_state, buf)
 268      char *arg_state;
 269      struct random_data *buf;
 270 {
 271   int32_t *new_state = 1 + (int32_t *) arg_state;
 272   int type;
 273   int old_type;
 274   int32_t *old_state;
 275   int degree;
 276   int separation;
 277
 278   if (arg_state == NULL || buf == NULL)
 279     goto fail;
 280
 281   old_type = buf->rand_type;
 282   old_state = buf->state;
 283   if (old_type == TYPE_0)
 284     old_state[-1] = TYPE_0;
 285   else
 286     old_state[-1] = (MAX_TYPES * (buf->rptr - old_state)) + old_type;
 287
 288   type = new_state[-1] % MAX_TYPES;
 289   if (type < TYPE_0 || type > TYPE_4)
 290     goto fail;
 291
 292   buf->rand_deg = degree = random_poly_info.degrees[type];
 293   buf->rand_sep = separation = random_poly_info.seps[type];
 294   buf->rand_type = type;
 295
 296   if (type != TYPE_0)
 297     {
 298       int rear = new_state[-1] / MAX_TYPES;
 299       buf->rptr = &new_state[rear];
 300       buf->fptr = &new_state[(rear + separation) % degree];
 301     }
 302   buf->state = new_state;
 303   /* Set end_ptr too.  */
 304   buf->end_ptr = &new_state[degree];
 305
 306   return 0;
 307
 308  fail:
 309   __set_errno (EINVAL);
 310   return -1;
 311 }
 312
 313 weak_alias (__setstate_r, setstate_r)
 314 \f
 315 /* If we are using the trivial TYPE_0 R.N.G., just do the old linear
 316    congruential bit.  Otherwise, we do our fancy trinomial stuff, which is the
 317    same in all the other cases due to all the global variables that have been
 318    set up.  The basic operation is to add the number at the rear pointer into
 319    the one at the front pointer.  Then both pointers are advanced to the next
 320    location cyclically in the table.  The value returned is the sum generated,
 321    reduced to 31 bits by throwing away the "least random" low bit.
 322    Note: The code takes advantage of the fact that both the front and
 323    rear pointers can't wrap on the same call by not testing the rear
 324    pointer if the front one has wrapped.  Returns a 31-bit random number.  */
 325
 326 int
 327 __random_r (buf, result)
 328      struct random_data *buf;
 329      int32_t *result;
 330 {
 331   int32_t *state;
 332
 333   if (buf == NULL || result == NULL)
 334     goto fail;
 335
 336   state = buf->state;
 337
 338   if (buf->rand_type == TYPE_0)
 339     {
 340       int32_t val = state[0];
 341       val = ((state[0] * 1103515245) + 12345) & 0x7fffffff;
 342       state[0] = val;
 343       *result = val;
 344     }
 345   else
 346     {
 347       int32_t *fptr = buf->fptr;
 348       int32_t *rptr = buf->rptr;
 349       int32_t *end_ptr = buf->end_ptr;
 350       int32_t val;
 351
 352       val = *fptr += *rptr;
 353       /* Chucking least random bit.  */
 354       *result = (val >> 1) & 0x7fffffff;
 355       ++fptr;
 356       if (fptr >= end_ptr)
 357         {
 358           fptr = state;
 359           ++rptr;
 360         }
 361       else
 362         {
 363           ++rptr;
 364           if (rptr >= end_ptr)
 365             rptr = state;
 366         }
 367       buf->fptr = fptr;
 368       buf->rptr = rptr;
 369     }
 370   return 0;
 371
 372  fail:
 373   __set_errno (EINVAL);
 374   return -1;
 375 }
 376
 377 weak_alias (__random_r, random_r)