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 random_r __random_r
  24. #define srandom_r __srandom_r
  25. #define setstate_r __setstate_r
  26. #define _GNU_SOURCE
  27. #include <features.h>
  28. #include <limits.h>
  29. #include <stddef.h>
  30. #include <stdlib.h>
  31. #ifdef __UCLIBC_HAS_THREADS__
  32. # include <pthread.h>
  33. /* POSIX.1c requires that there is mutual exclusion for the `rand' and
  34. `srand' functions to prevent concurrent calls from modifying common
  35. data. */
  36. static pthread_mutex_t lock = PTHREAD_RECURSIVE_MUTEX_INITIALIZER_NP;
  37. #endif
  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. /* For each of the currently supported random number generators, we have a
  71. break value on the amount of state information (you need at least this many
  72. bytes of state info to support this random number generator), a degree for
  73. the polynomial (actually a trinomial) that the R.N.G. is based on, and
  74. separation between the two lower order coefficients of the trinomial. */
  75. /* Linear congruential. */
  76. #define TYPE_0 0
  77. #define BREAK_0 8
  78. #define DEG_0 0
  79. #define SEP_0 0
  80. /* x**7 + x**3 + 1. */
  81. #define TYPE_1 1
  82. #define BREAK_1 32
  83. #define DEG_1 7
  84. #define SEP_1 3
  85. /* x**15 + x + 1. */
  86. #define TYPE_2 2
  87. #define BREAK_2 64
  88. #define DEG_2 15
  89. #define SEP_2 1
  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. /* x**63 + x + 1. */
  96. #define TYPE_4 4
  97. #define BREAK_4 256
  98. #define DEG_4 63
  99. #define SEP_4 1
  100. /* Array versions of the above information to make code run faster.
  101. Relies on fact that TYPE_i == i. */
  102. #define MAX_TYPES 5 /* Max number of types above. */
  103. /* Initially, everything is set up as if from:
  104. initstate(1, randtbl, 128);
  105. Note that this initialization takes advantage of the fact that srandom
  106. advances the front and rear pointers 10*rand_deg times, and hence the
  107. rear pointer which starts at 0 will also end up at zero; thus the zeroth
  108. element of the state information, which contains info about the current
  109. position of the rear pointer is just
  110. (MAX_TYPES * (rptr - state)) + TYPE_3 == TYPE_3. */
  111. static int32_t randtbl[DEG_3 + 1] =
  112. {
  113. TYPE_3,
  114. -1726662223, 379960547, 1735697613, 1040273694, 1313901226,
  115. 1627687941, -179304937, -2073333483, 1780058412, -1989503057,
  116. -615974602, 344556628, 939512070, -1249116260, 1507946756,
  117. -812545463, 154635395, 1388815473, -1926676823, 525320961,
  118. -1009028674, 968117788, -123449607, 1284210865, 435012392,
  119. -2017506339, -911064859, -370259173, 1132637927, 1398500161,
  120. -205601318,
  121. };
  122. static struct random_data unsafe_state =
  123. {
  124. /* FPTR and RPTR are two pointers into the state info, a front and a rear
  125. pointer. These two pointers are always rand_sep places aparts, as they
  126. cycle through the state information. (Yes, this does mean we could get
  127. away with just one pointer, but the code for random is more efficient
  128. this way). The pointers are left positioned as they would be from the call:
  129. initstate(1, randtbl, 128);
  130. (The position of the rear pointer, rptr, is really 0 (as explained above
  131. in the initialization of randtbl) because the state table pointer is set
  132. to point to randtbl[1] (as explained below).) */
  133. fptr : &randtbl[SEP_3 + 1],
  134. rptr : &randtbl[1],
  135. /* The following things are the pointer to the state information table,
  136. the type of the current generator, the degree of the current polynomial
  137. being used, and the separation between the two pointers.
  138. Note that for efficiency of random, we remember the first location of
  139. the state information, not the zeroth. Hence it is valid to access
  140. state[-1], which is used to store the type of the R.N.G.
  141. Also, we remember the last location, since this is more efficient than
  142. indexing every time to find the address of the last element to see if
  143. the front and rear pointers have wrapped. */
  144. state : &randtbl[1],
  145. rand_type : TYPE_3,
  146. rand_deg : DEG_3,
  147. rand_sep : SEP_3,
  148. end_ptr : &randtbl[sizeof (randtbl) / sizeof (randtbl[0])]
  149. };
  150. /* Initialize the random number generator based on the given seed. If the
  151. type is the trivial no-state-information type, just remember the seed.
  152. Otherwise, initializes state[] based on the given "seed" via a linear
  153. congruential generator. Then, the pointers are set to known locations
  154. that are exactly rand_sep places apart. Lastly, it cycles the state
  155. information a given number of times to get rid of any initial dependencies
  156. introduced by the L.C.R.N.G. Note that the initialization of randtbl[]
  157. for default usage relies on values produced by this routine. */
  158. void srandom (unsigned int x)
  159. {
  160. __pthread_mutex_lock(&lock);
  161. srandom_r (x, &unsafe_state);
  162. __pthread_mutex_unlock(&lock);
  163. }
  164. weak_alias (srandom, srand)
  165. /* Initialize the state information in the given array of N bytes for
  166. future random number generation. Based on the number of bytes we
  167. are given, and the break values for the different R.N.G.'s, we choose
  168. the best (largest) one we can and set things up for it. srandom is
  169. then called to initialize the state information. Note that on return
  170. from srandom, we set state[-1] to be the type multiplexed with the current
  171. value of the rear pointer; this is so successive calls to initstate won't
  172. lose this information and will be able to restart with setstate.
  173. Note: The first thing we do is save the current state, if any, just like
  174. setstate so that it doesn't matter when initstate is called.
  175. Returns a pointer to the old state. */
  176. char * initstate (unsigned int seed, char *arg_state, size_t n)
  177. {
  178. int32_t *ostate;
  179. __pthread_mutex_lock(&lock);
  180. ostate = &unsafe_state.state[-1];
  181. initstate_r (seed, arg_state, n, &unsafe_state);
  182. __pthread_mutex_unlock(&lock);
  183. return (char *) ostate;
  184. }
  185. /* Restore the state from the given state array.
  186. Note: It is important that we also remember the locations of the pointers
  187. in the current state information, and restore the locations of the pointers
  188. from the old state information. This is done by multiplexing the pointer
  189. location into the zeroth word of the state information. Note that due
  190. to the order in which things are done, it is OK to call setstate with the
  191. same state as the current state
  192. Returns a pointer to the old state information. */
  193. char * setstate (char *arg_state)
  194. {
  195. int32_t *ostate;
  196. __pthread_mutex_lock(&lock);
  197. ostate = &unsafe_state.state[-1];
  198. if (setstate_r (arg_state, &unsafe_state) < 0)
  199. ostate = NULL;
  200. __pthread_mutex_unlock(&lock);
  201. return (char *) ostate;
  202. }
  203. /* If we are using the trivial TYPE_0 R.N.G., just do the old linear
  204. congruential bit. Otherwise, we do our fancy trinomial stuff, which is the
  205. same in all the other cases due to all the global variables that have been
  206. set up. The basic operation is to add the number at the rear pointer into
  207. the one at the front pointer. Then both pointers are advanced to the next
  208. location cyclically in the table. The value returned is the sum generated,
  209. reduced to 31 bits by throwing away the "least random" low bit.
  210. Note: The code takes advantage of the fact that both the front and
  211. rear pointers can't wrap on the same call by not testing the rear
  212. pointer if the front one has wrapped. Returns a 31-bit random number. */
  213. long int attribute_hidden __random (void)
  214. {
  215. int32_t retval;
  216. __pthread_mutex_lock(&lock);
  217. random_r (&unsafe_state, &retval);
  218. __pthread_mutex_unlock(&lock);
  219. return retval;
  220. }
  221. strong_alias(__random,random)