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