libc/stdlib/random.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 use reentrant functions by Ulrich Drepper, 1995.
  23  */
  24
  25 #include <features.h>
  26 #include <limits.h>
  27 #include <stddef.h>
  28 #include <stdlib.h>
  29
  30
  31 /* POSIX.1c requires that there is mutual exclusion for the `rand' and
  32    `srand' functions to prevent concurrent calls from modifying common
  33    data.  */
  34 #include <bits/uClibc_mutex.h>
  35 __UCLIBC_MUTEX_STATIC(mylock, PTHREAD_RECURSIVE_MUTEX_INITIALIZER_NP);
  36
  37
  38 /* An improved random number generation package.  In addition to the standard
  39    rand()/srand() like interface, this package also has a special state info
  40    interface.  The initstate() routine is called with a seed, an array of
  41    bytes, and a count of how many bytes are being passed in; this array is
  42    then initialized to contain information for random number generation with
  43    that much state information.  Good sizes for the amount of state
  44    information are 32, 64, 128, and 256 bytes.  The state can be switched by
  45    calling the setstate() function with the same array as was initialized
  46    with initstate().  By default, the package runs with 128 bytes of state
  47    information and generates far better random numbers than a linear
  48    congruential generator.  If the amount of state information is less than
  49    32 bytes, a simple linear congruential R.N.G. is used.  Internally, the
  50    state information is treated as an array of longs; the zeroth element of
  51    the array is the type of R.N.G. being used (small integer); the remainder
  52    of the array is the state information for the R.N.G.  Thus, 32 bytes of
  53    state information will give 7 longs worth of state information, which will
  54    allow a degree seven polynomial.  (Note: The zeroth word of state
  55    information also has some other information stored in it; see setstate
  56    for details).  The random number generation technique is a linear feedback
  57    shift register approach, employing trinomials (since there are fewer terms
  58    to sum up that way).  In this approach, the least significant bit of all
  59    the numbers in the state table will act as a linear feedback shift register,
  60    and will have period 2^deg - 1 (where deg is the degree of the polynomial
  61    being used, assuming that the polynomial is irreducible and primitive).
  62    The higher order bits will have longer periods, since their values are
  63    also influenced by pseudo-random carries out of the lower bits.  The
  64    total period of the generator is approximately deg*(2**deg - 1); thus
  65    doubling the amount of state information has a vast influence on the
  66    period of the generator.  Note: The deg*(2**deg - 1) is an approximation
  67    only good for large deg, when the period of the shift register is the
  68    dominant factor.  With deg equal to seven, the period is actually much
  69    longer than the 7*(2**7 - 1) predicted by this formula.  */
  70
  71
  72
  73 /* Keep constants in sync with random_r.c */
  74
  75 /* Linear congruential.  */
  76 #define TYPE_0          0
  77 #define BREAK_0         8
  78 #define DEG_0           0
  79 #define SEP_0           0
  80
  81 /* x**7 + x**3 + 1.  */
  82 #define TYPE_1          1
  83 #define BREAK_1         32
  84 #define DEG_1           7
  85 #define SEP_1           3
  86
  87 /* x**15 + x + 1.  */
  88 #define TYPE_2          2
  89 #define BREAK_2         64
  90 #define DEG_2           15
  91 #define SEP_2           1
  92
  93 /* x**31 + x**3 + 1.  */
  94 #define TYPE_3          3
  95 #define BREAK_3         128
  96 #define DEG_3           31
  97 #define SEP_3           3
  98
  99 /* x**63 + x + 1.  */
 100 #define TYPE_4          4
 101 #define BREAK_4         256
 102 #define DEG_4           63
 103 #define SEP_4           1
 104
 105 #define MAX_TYPES       5       /* Max number of types above.  */
 106
 107 /* Initially, everything is set up as if from:
 108         initstate(1, randtbl, 128);
 109    Note that this initialization takes advantage of the fact that srandom
 110    advances the front and rear pointers 10*rand_deg times, and hence the
 111    rear pointer which starts at 0 will also end up at zero; thus the zeroth
 112    element of the state information, which contains info about the current
 113    position of the rear pointer is just
 114         (MAX_TYPES * (rptr - state)) + TYPE_3 == TYPE_3.  */
 115
 116 static int32_t randtbl[DEG_3 + 1] =
 117 {
 118     TYPE_3,
 119
 120     -1726662223, 379960547, 1735697613, 1040273694, 1313901226,
 121     1627687941, -179304937, -2073333483, 1780058412, -1989503057,
 122     -615974602, 344556628, 939512070, -1249116260, 1507946756,
 123     -812545463, 154635395, 1388815473, -1926676823, 525320961,
 124     -1009028674, 968117788, -123449607, 1284210865, 435012392,
 125     -2017506339, -911064859, -370259173, 1132637927, 1398500161,
 126     -205601318,
 127 };
 128
 129
 130 static struct random_data unsafe_state =
 131 {
 132     /* FPTR and RPTR are two pointers into the state info, a front and a rear
 133        pointer.  These two pointers are always rand_sep places aparts, as they
 134        cycle through the state information.  (Yes, this does mean we could get
 135        away with just one pointer, but the code for random is more efficient
 136        this way).  The pointers are left positioned as they would be from the call:
 137        initstate(1, randtbl, 128);
 138        (The position of the rear pointer, rptr, is really 0 (as explained above
 139        in the initialization of randtbl) because the state table pointer is set
 140        to point to randtbl[1] (as explained below).)  */
 141
 142     fptr : &randtbl[SEP_3 + 1],
 143     rptr : &randtbl[1],
 144
 145     /* The following things are the pointer to the state information table,
 146        the type of the current generator, the degree of the current polynomial
 147        being used, and the separation between the two pointers.
 148        Note that for efficiency of random, we remember the first location of
 149        the state information, not the zeroth.  Hence it is valid to access
 150        state[-1], which is used to store the type of the R.N.G.
 151        Also, we remember the last location, since this is more efficient than
 152        indexing every time to find the address of the last element to see if
 153        the front and rear pointers have wrapped.  */
 154
 155     state : &randtbl[1],
 156
 157     rand_type : TYPE_3,
 158     rand_deg : DEG_3,
 159     rand_sep : SEP_3,
 160
 161     end_ptr : &randtbl[sizeof (randtbl) / sizeof (randtbl[0])]
 162 };
 163
 164
 165 /* Initialize the random number generator based on the given seed.  If the
 166    type is the trivial no-state-information type, just remember the seed.
 167    Otherwise, initializes state[] based on the given "seed" via a linear
 168    congruential generator.  Then, the pointers are set to known locations
 169    that are exactly rand_sep places apart.  Lastly, it cycles the state
 170    information a given number of times to get rid of any initial dependencies
 171    introduced by the L.C.R.N.G.  Note that the initialization of randtbl[]
 172    for default usage relies on values produced by this routine.  */
 173 void srandom (unsigned int x)
 174 {
 175     __UCLIBC_MUTEX_LOCK(mylock);
 176     srandom_r (x, &unsafe_state);
 177     __UCLIBC_MUTEX_UNLOCK(mylock);
 178 }
 179 strong_alias(srandom,srand)
 180
 181 /* Initialize the state information in the given array of N bytes for
 182    future random number generation.  Based on the number of bytes we
 183    are given, and the break values for the different R.N.G.'s, we choose
 184    the best (largest) one we can and set things up for it.  srandom is
 185    then called to initialize the state information.  Note that on return
 186    from srandom, we set state[-1] to be the type multiplexed with the current
 187    value of the rear pointer; this is so successive calls to initstate won't
 188    lose this information and will be able to restart with setstate.
 189    Note: The first thing we do is save the current state, if any, just like
 190    setstate so that it doesn't matter when initstate is called.
 191    Returns a pointer to the old state.  */
 192 char * initstate (unsigned int seed, char *arg_state, size_t n)
 193 {
 194     int32_t *ostate;
 195
 196     __UCLIBC_MUTEX_LOCK(mylock);
 197     ostate = &unsafe_state.state[-1];
 198     initstate_r (seed, arg_state, n, &unsafe_state);
 199     __UCLIBC_MUTEX_UNLOCK(mylock);
 200     return (char *) ostate;
 201 }
 202
 203 /* Restore the state from the given state array.
 204    Note: It is important that we also remember the locations of the pointers
 205    in the current state information, and restore the locations of the pointers
 206    from the old state information.  This is done by multiplexing the pointer
 207    location into the zeroth word of the state information. Note that due
 208    to the order in which things are done, it is OK to call setstate with the
 209    same state as the current state
 210    Returns a pointer to the old state information.  */
 211 char * setstate (char *arg_state)
 212 {
 213     int32_t *ostate;
 214
 215     __UCLIBC_MUTEX_LOCK(mylock);
 216     ostate = &unsafe_state.state[-1];
 217     if (setstate_r (arg_state, &unsafe_state) < 0)
 218         ostate = NULL;
 219     __UCLIBC_MUTEX_UNLOCK(mylock);
 220     return (char *) ostate;
 221 }
 222
 223 /* If we are using the trivial TYPE_0 R.N.G., just do the old linear
 224    congruential bit.  Otherwise, we do our fancy trinomial stuff, which is the
 225    same in all the other cases due to all the global variables that have been
 226    set up.  The basic operation is to add the number at the rear pointer into
 227    the one at the front pointer.  Then both pointers are advanced to the next
 228    location cyclically in the table.  The value returned is the sum generated,
 229    reduced to 31 bits by throwing away the "least random" low bit.
 230    Note: The code takes advantage of the fact that both the front and
 231    rear pointers can't wrap on the same call by not testing the rear
 232    pointer if the front one has wrapped.  Returns a 31-bit random number.  */
 233
 234 long int random (void)
 235 {
 236   int32_t retval;
 237
 238   __UCLIBC_MUTEX_LOCK(mylock);
 239   random_r (&unsafe_state, &retval);
 240   __UCLIBC_MUTEX_UNLOCK(mylock);
 241   return retval;
 242 }
 243 libc_hidden_def(random)