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 static const int degrees[MAX_TYPES] = { DEG_0, DEG_1, DEG_2, DEG_3, DEG_4 };
 109 static const int seps[MAX_TYPES] = { SEP_0, SEP_1, SEP_2, SEP_3, SEP_4 };
 110
 111
 112
 113 \f
 114 /* Initialize the random number generator based on the given seed.  If the
 115    type is the trivial no-state-information type, just remember the seed.
 116    Otherwise, initializes state[] based on the given "seed" via a linear
 117    congruential generator.  Then, the pointers are set to known locations
 118    that are exactly rand_sep places apart.  Lastly, it cycles the state
 119    information a given number of times to get rid of any initial dependencies
 120    introduced by the L.C.R.N.G.  Note that the initialization of randtbl[]
 121    for default usage relies on values produced by this routine.  */
 122 int
 123 __srandom_r (x, buf)
 124      unsigned int x;
 125      struct random_data *buf;
 126 {
 127   if (buf == NULL || buf->rand_type < TYPE_0 || buf->rand_type > TYPE_4)
 128     return -1;
 129
 130   /* We must make sure the seed is not 0.  Take arbitrarily 1 in this case.  */
 131   buf->state[0] = x ? x : 1;
 132   if (buf->rand_type != TYPE_0)
 133     {
 134       long int i;
 135       for (i = 1; i < buf->rand_deg; ++i)
 136         {
 137           /* This does:
 138                state[i] = (16807 * state[i - 1]) % 2147483647;
 139              but avoids overflowing 31 bits.  */
 140           long int hi = buf->state[i - 1] / 127773;
 141           long int lo = buf->state[i - 1] % 127773;
 142           long int test = 16807 * lo - 2836 * hi;
 143           buf->state[i] = test + (test < 0 ? 2147483647 : 0);
 144         }
 145       buf->fptr = &buf->state[buf->rand_sep];
 146       buf->rptr = &buf->state[0];
 147       for (i = 0; i < 10 * buf->rand_deg; ++i)
 148         {
 149           int32_t discard;
 150           (void) __random_r (buf, &discard);
 151         }
 152     }
 153
 154   return 0;
 155 }
 156
 157 weak_alias (__srandom_r, srandom_r)
 158 \f
 159 /* Initialize the state information in the given array of N bytes for
 160    future random number generation.  Based on the number of bytes we
 161    are given, and the break values for the different R.N.G.'s, we choose
 162    the best (largest) one we can and set things up for it.  srandom is
 163    then called to initialize the state information.  Note that on return
 164    from srandom, we set state[-1] to be the type multiplexed with the current
 165    value of the rear pointer; this is so successive calls to initstate won't
 166    lose this information and will be able to restart with setstate.
 167    Note: The first thing we do is save the current state, if any, just like
 168    setstate so that it doesn't matter when initstate is called.
 169    Returns a pointer to the old state.  */
 170 int
 171 __initstate_r (seed, arg_state, n, buf)
 172      unsigned int seed;
 173      void *arg_state;
 174      size_t n;
 175      struct random_data *buf;
 176 {
 177   if (buf == NULL)
 178     return -1;
 179
 180   if (n < BREAK_1)
 181     {
 182       if (n < BREAK_0)
 183         {
 184           __set_errno (EINVAL);
 185           return -1;
 186         }
 187       buf->rand_type = TYPE_0;
 188       buf->rand_deg = DEG_0;
 189       buf->rand_sep = SEP_0;
 190     }
 191   else if (n < BREAK_2)
 192     {
 193       buf->rand_type = TYPE_1;
 194       buf->rand_deg = DEG_1;
 195       buf->rand_sep = SEP_1;
 196     }
 197   else if (n < BREAK_3)
 198     {
 199       buf->rand_type = TYPE_2;
 200       buf->rand_deg = DEG_2;
 201       buf->rand_sep = SEP_2;
 202     }
 203   else if (n < BREAK_4)
 204     {
 205       buf->rand_type = TYPE_3;
 206       buf->rand_deg = DEG_3;
 207       buf->rand_sep = SEP_3;
 208     }
 209   else
 210     {
 211       buf->rand_type = TYPE_4;
 212       buf->rand_deg = DEG_4;
 213       buf->rand_sep = SEP_4;
 214     }
 215
 216   buf->state = &((int32_t *) arg_state)[1];     /* First location.  */
 217   /* Must set END_PTR before srandom.  */
 218   buf->end_ptr = &buf->state[buf->rand_deg];
 219
 220   __srandom_r (seed, buf);
 221
 222   if (buf->rand_type == TYPE_0)
 223     buf->state[-1] = buf->rand_type;
 224   else
 225     buf->state[-1] = (MAX_TYPES * (buf->rptr - buf->state)) + buf->rand_type;
 226
 227   return 0;
 228 }
 229
 230 weak_alias (__initstate_r, initstate_r)
 231 \f
 232 /* Restore the state from the given state array.
 233    Note: It is important that we also remember the locations of the pointers
 234    in the current state information, and restore the locations of the pointers
 235    from the old state information.  This is done by multiplexing the pointer
 236    location into the zeroth word of the state information. Note that due
 237    to the order in which things are done, it is OK to call setstate with the
 238    same state as the current state
 239    Returns a pointer to the old state information.  */
 240 int
 241 __setstate_r (arg_state, buf)
 242      void *arg_state;
 243      struct random_data *buf;
 244 {
 245   int32_t *new_state = (int32_t *) arg_state;
 246   int type = new_state[0] % MAX_TYPES;
 247   int rear = new_state[0] / MAX_TYPES;
 248
 249   if (buf == NULL)
 250     return -1;
 251
 252   if (buf->rand_type == TYPE_0)
 253     buf->state[-1] = buf->rand_type;
 254   else
 255     buf->state[-1] = (MAX_TYPES * (buf->rptr - buf->state)) + buf->rand_type;
 256
 257   switch (type)
 258     {
 259     case TYPE_0:
 260     case TYPE_1:
 261     case TYPE_2:
 262     case TYPE_3:
 263     case TYPE_4:
 264       buf->rand_type = type;
 265       buf->rand_deg = degrees[type];
 266       buf->rand_sep = seps[type];
 267       break;
 268     default:
 269       /* State info munged.  */
 270       __set_errno (EINVAL);
 271       return -1;
 272     }
 273
 274   buf->state = &new_state[1];
 275   if (buf->rand_type != TYPE_0)
 276     {
 277       buf->rptr = &buf->state[rear];
 278       buf->fptr = &buf->state[(rear + buf->rand_sep) % buf->rand_deg];
 279     }
 280   /* Set end_ptr too.  */
 281   buf->end_ptr = &buf->state[buf->rand_deg];
 282
 283   return 0;
 284 }
 285
 286 weak_alias (__setstate_r, setstate_r)
 287 \f
 288 /* If we are using the trivial TYPE_0 R.N.G., just do the old linear
 289    congruential bit.  Otherwise, we do our fancy trinomial stuff, which is the
 290    same in all the other cases due to all the global variables that have been
 291    set up.  The basic operation is to add the number at the rear pointer into
 292    the one at the front pointer.  Then both pointers are advanced to the next
 293    location cyclically in the table.  The value returned is the sum generated,
 294    reduced to 31 bits by throwing away the "least random" low bit.
 295    Note: The code takes advantage of the fact that both the front and
 296    rear pointers can't wrap on the same call by not testing the rear
 297    pointer if the front one has wrapped.  Returns a 31-bit random number.  */
 298
 299 int
 300 __random_r (buf, result)
 301      struct random_data *buf;
 302      int32_t *result;
 303 {
 304   if (buf == NULL || result == NULL)
 305     return -1;
 306
 307   if (buf->rand_type == TYPE_0)
 308     {
 309       buf->state[0] = ((buf->state[0] * 1103515245) + 12345) & 0x7fffffff;
 310       *result = buf->state[0];
 311     }
 312   else
 313     {
 314       *buf->fptr += *buf->rptr;
 315       /* Chucking least random bit.  */
 316       *result = (*buf->fptr >> 1) & 0x7fffffff;
 317       ++buf->fptr;
 318       if (buf->fptr >= buf->end_ptr)
 319         {
 320           buf->fptr = buf->state;
 321           ++buf->rptr;
 322         }
 323       else
 324         {
 325           ++buf->rptr;
 326           if (buf->rptr >= buf->end_ptr)
 327             buf->rptr = buf->state;
 328         }
 329     }
 330   return 0;
 331 }
 332
 333 weak_alias (__random_r, random_r)