random.c 11 KB

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  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. * This is derived from the Berkeley source:
  19. * @(#)random.c 5.5 (Berkeley) 7/6/88
  20. * It was reworked for the GNU C Library by Roland McGrath.
  21. * Rewritten to use reentrant functions by Ulrich Drepper, 1995.
  22. */
  23. #define _GNU_SOURCE
  24. #include <features.h>
  25. #include <limits.h>
  26. #include <stddef.h>
  27. #include <stdlib.h>
  28. libc_hidden_proto(random_r)
  29. libc_hidden_proto(srandom_r)
  30. libc_hidden_proto(setstate_r)
  31. libc_hidden_proto(initstate_r)
  32. #ifdef __UCLIBC_HAS_THREADS__
  33. # include <pthread.h>
  34. /* POSIX.1c requires that there is mutual exclusion for the `rand' and
  35. `srand' functions to prevent concurrent calls from modifying common
  36. data. */
  37. static pthread_mutex_t lock = PTHREAD_RECURSIVE_MUTEX_INITIALIZER_NP;
  38. #endif
  39. /* An improved random number generation package. In addition to the standard
  40. rand()/srand() like interface, this package also has a special state info
  41. interface. The initstate() routine is called with a seed, an array of
  42. bytes, and a count of how many bytes are being passed in; this array is
  43. then initialized to contain information for random number generation with
  44. that much state information. Good sizes for the amount of state
  45. information are 32, 64, 128, and 256 bytes. The state can be switched by
  46. calling the setstate() function with the same array as was initialized
  47. with initstate(). By default, the package runs with 128 bytes of state
  48. information and generates far better random numbers than a linear
  49. congruential generator. If the amount of state information is less than
  50. 32 bytes, a simple linear congruential R.N.G. is used. Internally, the
  51. state information is treated as an array of longs; the zeroth element of
  52. the array is the type of R.N.G. being used (small integer); the remainder
  53. of the array is the state information for the R.N.G. Thus, 32 bytes of
  54. state information will give 7 longs worth of state information, which will
  55. allow a degree seven polynomial. (Note: The zeroth word of state
  56. information also has some other information stored in it; see setstate
  57. for details). The random number generation technique is a linear feedback
  58. shift register approach, employing trinomials (since there are fewer terms
  59. to sum up that way). In this approach, the least significant bit of all
  60. the numbers in the state table will act as a linear feedback shift register,
  61. and will have period 2^deg - 1 (where deg is the degree of the polynomial
  62. being used, assuming that the polynomial is irreducible and primitive).
  63. The higher order bits will have longer periods, since their values are
  64. also influenced by pseudo-random carries out of the lower bits. The
  65. total period of the generator is approximately deg*(2**deg - 1); thus
  66. doubling the amount of state information has a vast influence on the
  67. period of the generator. Note: The deg*(2**deg - 1) is an approximation
  68. only good for large deg, when the period of the shift register is the
  69. dominant factor. With deg equal to seven, the period is actually much
  70. longer than the 7*(2**7 - 1) predicted by this formula. */
  71. /* For each of the currently supported random number generators, we have a
  72. break value on the amount of state information (you need at least this many
  73. bytes of state info to support this random number generator), a degree for
  74. the polynomial (actually a trinomial) that the R.N.G. is based on, and
  75. separation between the two lower order coefficients of the trinomial. */
  76. /* Linear congruential. */
  77. #define TYPE_0 0
  78. #define BREAK_0 8
  79. #define DEG_0 0
  80. #define SEP_0 0
  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. /* x**15 + x + 1. */
  87. #define TYPE_2 2
  88. #define BREAK_2 64
  89. #define DEG_2 15
  90. #define SEP_2 1
  91. /* x**31 + x**3 + 1. */
  92. #define TYPE_3 3
  93. #define BREAK_3 128
  94. #define DEG_3 31
  95. #define SEP_3 3
  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. /* Array versions of the above information to make code run faster.
  102. Relies on fact that TYPE_i == i. */
  103. #define MAX_TYPES 5 /* Max number of types above. */
  104. /* Initially, everything is set up as if from:
  105. initstate(1, randtbl, 128);
  106. Note that this initialization takes advantage of the fact that srandom
  107. advances the front and rear pointers 10*rand_deg times, and hence the
  108. rear pointer which starts at 0 will also end up at zero; thus the zeroth
  109. element of the state information, which contains info about the current
  110. position of the rear pointer is just
  111. (MAX_TYPES * (rptr - state)) + TYPE_3 == TYPE_3. */
  112. static int32_t randtbl[DEG_3 + 1] =
  113. {
  114. TYPE_3,
  115. -1726662223, 379960547, 1735697613, 1040273694, 1313901226,
  116. 1627687941, -179304937, -2073333483, 1780058412, -1989503057,
  117. -615974602, 344556628, 939512070, -1249116260, 1507946756,
  118. -812545463, 154635395, 1388815473, -1926676823, 525320961,
  119. -1009028674, 968117788, -123449607, 1284210865, 435012392,
  120. -2017506339, -911064859, -370259173, 1132637927, 1398500161,
  121. -205601318,
  122. };
  123. static struct random_data unsafe_state =
  124. {
  125. /* FPTR and RPTR are two pointers into the state info, a front and a rear
  126. pointer. These two pointers are always rand_sep places aparts, as they
  127. cycle through the state information. (Yes, this does mean we could get
  128. away with just one pointer, but the code for random is more efficient
  129. this way). The pointers are left positioned as they would be from the call:
  130. initstate(1, randtbl, 128);
  131. (The position of the rear pointer, rptr, is really 0 (as explained above
  132. in the initialization of randtbl) because the state table pointer is set
  133. to point to randtbl[1] (as explained below).) */
  134. fptr : &randtbl[SEP_3 + 1],
  135. rptr : &randtbl[1],
  136. /* The following things are the pointer to the state information table,
  137. the type of the current generator, the degree of the current polynomial
  138. being used, and the separation between the two pointers.
  139. Note that for efficiency of random, we remember the first location of
  140. the state information, not the zeroth. Hence it is valid to access
  141. state[-1], which is used to store the type of the R.N.G.
  142. Also, we remember the last location, since this is more efficient than
  143. indexing every time to find the address of the last element to see if
  144. the front and rear pointers have wrapped. */
  145. state : &randtbl[1],
  146. rand_type : TYPE_3,
  147. rand_deg : DEG_3,
  148. rand_sep : SEP_3,
  149. end_ptr : &randtbl[sizeof (randtbl) / sizeof (randtbl[0])]
  150. };
  151. /* Initialize the random number generator based on the given seed. If the
  152. type is the trivial no-state-information type, just remember the seed.
  153. Otherwise, initializes state[] based on the given "seed" via a linear
  154. congruential generator. Then, the pointers are set to known locations
  155. that are exactly rand_sep places apart. Lastly, it cycles the state
  156. information a given number of times to get rid of any initial dependencies
  157. introduced by the L.C.R.N.G. Note that the initialization of randtbl[]
  158. for default usage relies on values produced by this routine. */
  159. void srandom (unsigned int x)
  160. {
  161. __pthread_mutex_lock(&lock);
  162. srandom_r (x, &unsafe_state);
  163. __pthread_mutex_unlock(&lock);
  164. }
  165. strong_alias(srandom,srand)
  166. /* Initialize the state information in the given array of N bytes for
  167. future random number generation. Based on the number of bytes we
  168. are given, and the break values for the different R.N.G.'s, we choose
  169. the best (largest) one we can and set things up for it. srandom is
  170. then called to initialize the state information. Note that on return
  171. from srandom, we set state[-1] to be the type multiplexed with the current
  172. value of the rear pointer; this is so successive calls to initstate won't
  173. lose this information and will be able to restart with setstate.
  174. Note: The first thing we do is save the current state, if any, just like
  175. setstate so that it doesn't matter when initstate is called.
  176. Returns a pointer to the old state. */
  177. char * initstate (unsigned int seed, char *arg_state, size_t n)
  178. {
  179. int32_t *ostate;
  180. __pthread_mutex_lock(&lock);
  181. ostate = &unsafe_state.state[-1];
  182. initstate_r (seed, arg_state, n, &unsafe_state);
  183. __pthread_mutex_unlock(&lock);
  184. return (char *) ostate;
  185. }
  186. /* Restore the state from the given state array.
  187. Note: It is important that we also remember the locations of the pointers
  188. in the current state information, and restore the locations of the pointers
  189. from the old state information. This is done by multiplexing the pointer
  190. location into the zeroth word of the state information. Note that due
  191. to the order in which things are done, it is OK to call setstate with the
  192. same state as the current state
  193. Returns a pointer to the old state information. */
  194. char * setstate (char *arg_state)
  195. {
  196. int32_t *ostate;
  197. __pthread_mutex_lock(&lock);
  198. ostate = &unsafe_state.state[-1];
  199. if (setstate_r (arg_state, &unsafe_state) < 0)
  200. ostate = NULL;
  201. __pthread_mutex_unlock(&lock);
  202. return (char *) ostate;
  203. }
  204. /* If we are using the trivial TYPE_0 R.N.G., just do the old linear
  205. congruential bit. Otherwise, we do our fancy trinomial stuff, which is the
  206. same in all the other cases due to all the global variables that have been
  207. set up. The basic operation is to add the number at the rear pointer into
  208. the one at the front pointer. Then both pointers are advanced to the next
  209. location cyclically in the table. The value returned is the sum generated,
  210. reduced to 31 bits by throwing away the "least random" low bit.
  211. Note: The code takes advantage of the fact that both the front and
  212. rear pointers can't wrap on the same call by not testing the rear
  213. pointer if the front one has wrapped. Returns a 31-bit random number. */
  214. libc_hidden_proto(random)
  215. long int random (void)
  216. {
  217. int32_t retval;
  218. __pthread_mutex_lock(&lock);
  219. random_r (&unsafe_state, &retval);
  220. __pthread_mutex_unlock(&lock);
  221. return retval;
  222. }
  223. libc_hidden_def(random)