lib/random.c

   1 /* Copyright (C) 1995-2024 Free Software Foundation, Inc.
   2
   3    This file is free software: you can redistribute it and/or modify
   4    it under the terms of the GNU Lesser General Public License as
   5    published by the Free Software Foundation, either version 3 of the
   6    License, or (at your option) any later version.
   7
   8    This file is distributed in the hope that it will be useful,
   9    but WITHOUT ANY WARRANTY; without even the implied warranty of
  10    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  11    GNU Lesser General Public License for more details.
  12
  13    You should have received a copy of the GNU Lesser General Public License
  14    along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
  15
  16 /*
  17  * This is derived from the Berkeley source:
  18  *      @(#)random.c    5.5 (Berkeley) 7/6/88
  19  * It was reworked for the GNU C Library by Roland McGrath.
  20  * Rewritten to use reentrant functions by Ulrich Drepper, 1995.
  21  */
  22
  23 /*
  24    Copyright (C) 1983 Regents of the University of California.
  25    All rights reserved.
  26
  27    Redistribution and use in source and binary forms, with or without
  28    modification, are permitted provided that the following conditions
  29    are met:
  30
  31    1. Redistributions of source code must retain the above copyright
  32       notice, this list of conditions and the following disclaimer.
  33    2. Redistributions in binary form must reproduce the above copyright
  34       notice, this list of conditions and the following disclaimer in the
  35       documentation and/or other materials provided with the distribution.
  36    4. Neither the name of the University nor the names of its contributors
  37       may be used to endorse or promote products derived from this software
  38       without specific prior written permission.
  39
  40    THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS "AS IS" AND
  41    ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  42    IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  43    ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
  44    FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  45    DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  46    OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  47    HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  48    LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  49    OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  50    SUCH DAMAGE.*/
  51
  52 #ifndef _LIBC
  53 # include <libc-config.h>
  54 # define __srandom srandom
  55 # define __initstate initstate
  56 # define __setstate setstate
  57 # define __random random
  58 # define __srandom_r srandom_r
  59 # define __initstate_r initstate_r
  60 # define __setstate_r setstate_r
  61 # define __random_r random_r
  62 #endif
  63
  64 /* Specification.  */
  65 #include <stdlib.h>
  66
  67 #ifdef _LIBC
  68 # include <libc-lock.h>
  69 #else
  70 # include "glthread/lock.h"
  71 # define __libc_lock_define_initialized gl_lock_define_initialized
  72 # define __libc_lock_lock gl_lock_lock
  73 # define __libc_lock_unlock gl_lock_unlock
  74 #endif
  75
  76 /* An improved random number generation package.  In addition to the standard
  77    rand()/srand() like interface, this package also has a special state info
  78    interface.  The initstate() routine is called with a seed, an array of
  79    bytes, and a count of how many bytes are being passed in; this array is
  80    then initialized to contain information for random number generation with
  81    that much state information.  Good sizes for the amount of state
  82    information are 32, 64, 128, and 256 bytes.  The state can be switched by
  83    calling the setstate() function with the same array as was initialized
  84    with initstate().  By default, the package runs with 128 bytes of state
  85    information and generates far better random numbers than a linear
  86    congruential generator.  If the amount of state information is less than
  87    32 bytes, a simple linear congruential R.N.G. is used.  Internally, the
  88    state information is treated as an array of longs; the zeroth element of
  89    the array is the type of R.N.G. being used (small integer); the remainder
  90    of the array is the state information for the R.N.G.  Thus, 32 bytes of
  91    state information will give 7 longs worth of state information, which will
  92    allow a degree seven polynomial.  (Note: The zeroth word of state
  93    information also has some other information stored in it; see setstate
  94    for details).  The random number generation technique is a linear feedback
  95    shift register approach, employing trinomials (since there are fewer terms
  96    to sum up that way).  In this approach, the least significant bit of all
  97    the numbers in the state table will act as a linear feedback shift register,
  98    and will have period 2^deg - 1 (where deg is the degree of the polynomial
  99    being used, assuming that the polynomial is irreducible and primitive).
 100    The higher order bits will have longer periods, since their values are
 101    also influenced by pseudo-random carries out of the lower bits.  The
 102    total period of the generator is approximately deg*(2**deg - 1); thus
 103    doubling the amount of state information has a vast influence on the
 104    period of the generator.  Note: The deg*(2**deg - 1) is an approximation
 105    only good for large deg, when the period of the shift register is the
 106    dominant factor.  With deg equal to seven, the period is actually much
 107    longer than the 7*(2**7 - 1) predicted by this formula.  */
 108
 109
 110
 111 /* For each of the currently supported random number generators, we have a
 112    break value on the amount of state information (you need at least this many
 113    bytes of state info to support this random number generator), a degree for
 114    the polynomial (actually a trinomial) that the R.N.G. is based on, and
 115    separation between the two lower order coefficients of the trinomial.  */
 116
 117 /* Linear congruential.  */
 118 #define TYPE_0          0
 119 #define BREAK_0         8
 120 #define DEG_0           0
 121 #define SEP_0           0
 122
 123 /* x**7 + x**3 + 1.  */
 124 #define TYPE_1          1
 125 #define BREAK_1         32
 126 #define DEG_1           7
 127 #define SEP_1           3
 128
 129 /* x**15 + x + 1.  */
 130 #define TYPE_2          2
 131 #define BREAK_2         64
 132 #define DEG_2           15
 133 #define SEP_2           1
 134
 135 /* x**31 + x**3 + 1.  */
 136 #define TYPE_3          3
 137 #define BREAK_3         128
 138 #define DEG_3           31
 139 #define SEP_3           3
 140
 141 /* x**63 + x + 1.  */
 142 #define TYPE_4          4
 143 #define BREAK_4         256
 144 #define DEG_4           63
 145 #define SEP_4           1
 146
 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.  */
 152
 153
 154 /* Initially, everything is set up as if from:
 155         initstate(1, randtbl, 128);
 156    Note that this initialization takes advantage of the fact that srandom
 157    advances the front and rear pointers 10*rand_deg times, and hence the
 158    rear pointer which starts at 0 will also end up at zero; thus the zeroth
 159    element of the state information, which contains info about the current
 160    position of the rear pointer is just
 161         (MAX_TYPES * (rptr - state)) + TYPE_3 == TYPE_3.  */
 162
 163 static int32_t randtbl[DEG_3 + 1] =
 164   {
 165     TYPE_3,
 166
 167     -1726662223, 379960547, 1735697613, 1040273694, 1313901226,
 168     1627687941, -179304937, -2073333483, 1780058412, -1989503057,
 169     -615974602, 344556628, 939512070, -1249116260, 1507946756,
 170     -812545463, 154635395, 1388815473, -1926676823, 525320961,
 171     -1009028674, 968117788, -123449607, 1284210865, 435012392,
 172     -2017506339, -911064859, -370259173, 1132637927, 1398500161,
 173     -205601318,
 174   };
 175
 176
 177 static struct random_data unsafe_state =
 178   {
 179 /* FPTR and RPTR are two pointers into the state info, a front and a rear
 180    pointer.  These two pointers are always rand_sep places apart, as they
 181    cycle through the state information.  (Yes, this does mean we could get
 182    away with just one pointer, but the code for random is more efficient
 183    this way).  The pointers are left positioned as they would be from the call:
 184         initstate(1, randtbl, 128);
 185    (The position of the rear pointer, rptr, is really 0 (as explained above
 186    in the initialization of randtbl) because the state table pointer is set
 187    to point to randtbl[1] (as explained below).)  */
 188
 189     .fptr = &randtbl[SEP_3 + 1],
 190     .rptr = &randtbl[1],
 191
 192 /* The following things are the pointer to the state information table,
 193    the type of the current generator, the degree of the current polynomial
 194    being used, and the separation between the two pointers.
 195    Note that for efficiency of random, we remember the first location of
 196    the state information, not the zeroth.  Hence it is valid to access
 197    state[-1], which is used to store the type of the R.N.G.
 198    Also, we remember the last location, since this is more efficient than
 199    indexing every time to find the address of the last element to see if
 200    the front and rear pointers have wrapped.  */
 201
 202     .state = &randtbl[1],
 203
 204     .rand_type = TYPE_3,
 205     .rand_deg = DEG_3,
 206     .rand_sep = SEP_3,
 207
 208     .end_ptr = &randtbl[sizeof (randtbl) / sizeof (randtbl[0])]
 209 };
 210 \f
 211 /* POSIX.1c requires that there is mutual exclusion for the 'rand' and
 212    'srand' functions to prevent concurrent calls from modifying common
 213    data.  */
 214 __libc_lock_define_initialized (static, lock)
 215 \f
 216 /* Initialize the random number generator based on the given seed.  If the
 217    type is the trivial no-state-information type, just remember the seed.
 218    Otherwise, initializes state[] based on the given "seed" via a linear
 219    congruential generator.  Then, the pointers are set to known locations
 220    that are exactly rand_sep places apart.  Lastly, it cycles the state
 221    information a given number of times to get rid of any initial dependencies
 222    introduced by the L.C.R.N.G.  Note that the initialization of randtbl[]
 223    for default usage relies on values produced by this routine.  */
 224 void
 225 __srandom (unsigned int x)
 226 {
 227   __libc_lock_lock (lock);
 228   (void) __srandom_r (x, &unsafe_state);
 229   __libc_lock_unlock (lock);
 230 }
 231
 232 weak_alias (__srandom, srandom)
 233 weak_alias (__srandom, srand)
 234 \f
 235 /* Initialize the state information in the given array of N bytes for
 236    future random number generation.  Based on the number of bytes we
 237    are given, and the break values for the different R.N.G.'s, we choose
 238    the best (largest) one we can and set things up for it.  srandom is
 239    then called to initialize the state information.  Note that on return
 240    from srandom, we set state[-1] to be the type multiplexed with the current
 241    value of the rear pointer; this is so successive calls to initstate won't
 242    lose this information and will be able to restart with setstate.
 243    Note: The first thing we do is save the current state, if any, just like
 244    setstate so that it doesn't matter when initstate is called.
 245    Returns a pointer to the old state.  */
 246 char *
 247 __initstate (unsigned int seed, char *arg_state, size_t n)
 248 {
 249   int32_t *ostate;
 250   int ret;
 251
 252   __libc_lock_lock (lock);
 253
 254   ostate = &unsafe_state.state[-1];
 255
 256   ret = __initstate_r (seed, arg_state, n, &unsafe_state);
 257
 258   __libc_lock_unlock (lock);
 259
 260   return ret == -1 ? NULL : (char *) ostate;
 261 }
 262
 263 weak_alias (__initstate, initstate)
 264 \f
 265 /* Restore the state from the given state array.
 266    Note: It is important that we also remember the locations of the pointers
 267    in the current state information, and restore the locations of the pointers
 268    from the old state information.  This is done by multiplexing the pointer
 269    location into the zeroth word of the state information. Note that due
 270    to the order in which things are done, it is OK to call setstate with the
 271    same state as the current state
 272    Returns a pointer to the old state information.  */
 273 char *
 274 __setstate (char *arg_state)
 275 {
 276   int32_t *ostate;
 277
 278   __libc_lock_lock (lock);
 279
 280   ostate = &unsafe_state.state[-1];
 281
 282   if (__setstate_r (arg_state, &unsafe_state) < 0)
 283     ostate = NULL;
 284
 285   __libc_lock_unlock (lock);
 286
 287   return (char *) ostate;
 288 }
 289
 290 weak_alias (__setstate, setstate)
 291 \f
 292 /* If we are using the trivial TYPE_0 R.N.G., just do the old linear
 293    congruential bit.  Otherwise, we do our fancy trinomial stuff, which is the
 294    same in all the other cases due to all the global variables that have been
 295    set up.  The basic operation is to add the number at the rear pointer into
 296    the one at the front pointer.  Then both pointers are advanced to the next
 297    location cyclically in the table.  The value returned is the sum generated,
 298    reduced to 31 bits by throwing away the "least random" low bit.
 299    Note: The code takes advantage of the fact that both the front and
 300    rear pointers can't wrap on the same call by not testing the rear
 301    pointer if the front one has wrapped.  Returns a 31-bit random number.  */
 302
 303 long int
 304 __random (void)
 305 {
 306   int32_t retval;
 307
 308   __libc_lock_lock (lock);
 309
 310   (void) __random_r (&unsafe_state, &retval);
 311
 312   __libc_lock_unlock (lock);
 313
 314   return retval;
 315 }
 316
 317 weak_alias (__random, random)