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[glibc.git] / stdlib / random_r.c
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1 /*
2 * Copyright (c) 1983 Regents of the University of California.
3 * All rights reserved.
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.
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 be reentrant by Ulrich Drepper, 1995
25 #include <errno.h>
26 #include <limits.h>
27 #include <stddef.h>
28 #include <stdlib.h>
31 /* An improved random number generation package. In addition to the standard
32 rand()/srand() like interface, this package also has a special state info
33 interface. The initstate() routine is called with a seed, an array of
34 bytes, and a count of how many bytes are being passed in; this array is
35 then initialized to contain information for random number generation with
36 that much state information. Good sizes for the amount of state
37 information are 32, 64, 128, and 256 bytes. The state can be switched by
38 calling the setstate() function with the same array as was initialized
39 with initstate(). By default, the package runs with 128 bytes of state
40 information and generates far better random numbers than a linear
41 congruential generator. If the amount of state information is less than
42 32 bytes, a simple linear congruential R.N.G. is used. Internally, the
43 state information is treated as an array of longs; the zeroth element of
44 the array is the type of R.N.G. being used (small integer); the remainder
45 of the array is the state information for the R.N.G. Thus, 32 bytes of
46 state information will give 7 longs worth of state information, which will
47 allow a degree seven polynomial. (Note: The zeroth word of state
48 information also has some other information stored in it; see setstate
49 for details). The random number generation technique is a linear feedback
50 shift register approach, employing trinomials (since there are fewer terms
51 to sum up that way). In this approach, the least significant bit of all
52 the numbers in the state table will act as a linear feedback shift register,
53 and will have period 2^deg - 1 (where deg is the degree of the polynomial
54 being used, assuming that the polynomial is irreducible and primitive).
55 The higher order bits will have longer periods, since their values are
56 also influenced by pseudo-random carries out of the lower bits. The
57 total period of the generator is approximately deg*(2**deg - 1); thus
58 doubling the amount of state information has a vast influence on the
59 period of the generator. Note: The deg*(2**deg - 1) is an approximation
60 only good for large deg, when the period of the shift register is the
61 dominant factor. With deg equal to seven, the period is actually much
62 longer than the 7*(2**7 - 1) predicted by this formula. */
66 /* For each of the currently supported random number generators, we have a
67 break value on the amount of state information (you need at least this many
68 bytes of state info to support this random number generator), a degree for
69 the polynomial (actually a trinomial) that the R.N.G. is based on, and
70 separation between the two lower order coefficients of the trinomial. */
72 /* Linear congruential. */
73 #define TYPE_0 0
74 #define BREAK_0 8
75 #define DEG_0 0
76 #define SEP_0 0
78 /* x**7 + x**3 + 1. */
79 #define TYPE_1 1
80 #define BREAK_1 32
81 #define DEG_1 7
82 #define SEP_1 3
84 /* x**15 + x + 1. */
85 #define TYPE_2 2
86 #define BREAK_2 64
87 #define DEG_2 15
88 #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
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
103 /* Array versions of the above information to make code run faster.
104 Relies on fact that TYPE_i == i. */
106 #define MAX_TYPES 5 /* Max number of types above. */
108 struct random_poly_info
110 int seps[MAX_TYPES];
111 int degrees[MAX_TYPES];
114 static const struct random_poly_info random_poly_info =
116 { SEP_0, SEP_1, SEP_2, SEP_3, SEP_4 },
117 { DEG_0, DEG_1, DEG_2, DEG_3, DEG_4 }
123 /* Initialize the random number generator based on the given seed. If the
124 type is the trivial no-state-information type, just remember the seed.
125 Otherwise, initializes state[] based on the given "seed" via a linear
126 congruential generator. Then, the pointers are set to known locations
127 that are exactly rand_sep places apart. Lastly, it cycles the state
128 information a given number of times to get rid of any initial dependencies
129 introduced by the L.C.R.N.G. Note that the initialization of randtbl[]
130 for default usage relies on values produced by this routine. */
132 __srandom_r (seed, buf)
133 unsigned int seed;
134 struct random_data *buf;
136 int type;
137 int32_t *state;
138 long int i;
139 long int word;
140 int32_t *dst;
141 int kc;
143 if (buf == NULL)
144 goto fail;
145 type = buf->rand_type;
146 if ((unsigned int) type >= MAX_TYPES)
147 goto fail;
149 state = buf->state;
150 /* We must make sure the seed is not 0. Take arbitrarily 1 in this case. */
151 if (seed == 0)
152 seed = 1;
153 state[0] = seed;
154 if (type == TYPE_0)
155 goto done;
157 dst = state;
158 word = seed;
159 kc = buf->rand_deg;
160 for (i = 1; i < kc; ++i)
162 /* This does:
163 state[i] = (16807 * state[i - 1]) % 2147483647;
164 but avoids overflowing 31 bits. */
165 long int hi = word / 127773;
166 long int lo = word % 127773;
167 word = 16807 * lo - 2836 * hi;
168 if (word < 0)
169 word += 2147483647;
170 *++dst = word;
173 buf->fptr = &state[buf->rand_sep];
174 buf->rptr = &state[0];
175 kc *= 10;
176 while (--kc >= 0)
178 int32_t discard;
179 (void) __random_r (buf, &discard);
182 done:
183 return 0;
185 fail:
186 return -1;
189 weak_alias (__srandom_r, srandom_r)
191 /* Initialize the state information in the given array of N bytes for
192 future random number generation. Based on the number of bytes we
193 are given, and the break values for the different R.N.G.'s, we choose
194 the best (largest) one we can and set things up for it. srandom is
195 then called to initialize the state information. Note that on return
196 from srandom, we set state[-1] to be the type multiplexed with the current
197 value of the rear pointer; this is so successive calls to initstate won't
198 lose this information and will be able to restart with setstate.
199 Note: The first thing we do is save the current state, if any, just like
200 setstate so that it doesn't matter when initstate is called.
201 Returns a pointer to the old state. */
203 __initstate_r (seed, arg_state, n, buf)
204 unsigned int seed;
205 void *arg_state;
206 size_t n;
207 struct random_data *buf;
209 int type;
210 int degree;
211 int separation;
212 int32_t *state;
214 if (buf == NULL)
215 goto fail;
217 if (n >= BREAK_3)
218 type = n < BREAK_4 ? TYPE_3 : TYPE_4;
219 else if (n < BREAK_1)
221 if (n < BREAK_0)
223 __set_errno (EINVAL);
224 goto fail;
226 type = TYPE_0;
228 else
229 type = n < BREAK_2 ? TYPE_1 : TYPE_2;
231 degree = random_poly_info.degrees[type];
232 separation = random_poly_info.seps[type];
234 buf->rand_type = type;
235 buf->rand_sep = separation;
236 buf->rand_deg = degree;
237 state = &((int32_t *) arg_state)[1]; /* First location. */
238 /* Must set END_PTR before srandom. */
239 buf->end_ptr = &state[degree];
241 buf->state = state;
243 __srandom_r (seed, buf);
245 state[-1] = TYPE_0;
246 if (type != TYPE_0)
247 state[-1] = (buf->rptr - state) * MAX_TYPES + type;
249 return 0;
251 fail:
252 return -1;
255 weak_alias (__initstate_r, initstate_r)
257 /* Restore the state from the given state array.
258 Note: It is important that we also remember the locations of the pointers
259 in the current state information, and restore the locations of the pointers
260 from the old state information. This is done by multiplexing the pointer
261 location into the zeroth word of the state information. Note that due
262 to the order in which things are done, it is OK to call setstate with the
263 same state as the current state
264 Returns a pointer to the old state information. */
266 __setstate_r (arg_state, buf)
267 void *arg_state;
268 struct random_data *buf;
270 int32_t *new_state = (int32_t *) arg_state;
271 int type;
272 int old_type;
273 int32_t *old_state;
274 int degree;
275 int separation;
277 if (buf == NULL)
278 goto fail;
280 old_type = buf->rand_type;
281 old_state = buf->state;
282 if (old_type == TYPE_0)
283 old_state[-1] = TYPE_0;
284 else
285 old_state[-1] = (MAX_TYPES * (buf->rptr - old_state)) + old_type;
287 type = new_state[0] % MAX_TYPES;
288 if (type < TYPE_0 || type >= TYPE_4)
289 goto fail;
291 buf->rand_deg = degree = random_poly_info.degrees[type];
292 buf->rand_sep = separation = random_poly_info.seps[type];
293 buf->rand_type = type;
295 if (type != TYPE_0)
297 int rear = new_state[0] / MAX_TYPES;
298 buf->rptr = &new_state[rear];
299 buf->fptr = &new_state[(rear + separation) % degree];
301 buf->state = &new_state[1];
302 /* Set end_ptr too. */
303 buf->end_ptr = &new_state[degree];
305 return 0;
307 fail:
308 __set_errno (EINVAL);
309 return -1;
312 weak_alias (__setstate_r, setstate_r)
314 /* If we are using the trivial TYPE_0 R.N.G., just do the old linear
315 congruential bit. Otherwise, we do our fancy trinomial stuff, which is the
316 same in all the other cases due to all the global variables that have been
317 set up. The basic operation is to add the number at the rear pointer into
318 the one at the front pointer. Then both pointers are advanced to the next
319 location cyclically in the table. The value returned is the sum generated,
320 reduced to 31 bits by throwing away the "least random" low bit.
321 Note: The code takes advantage of the fact that both the front and
322 rear pointers can't wrap on the same call by not testing the rear
323 pointer if the front one has wrapped. Returns a 31-bit random number. */
326 __random_r (buf, result)
327 struct random_data *buf;
328 int32_t *result;
330 int32_t *state;
332 if (buf == NULL || result == NULL)
333 goto fail;
335 state = buf->state;
337 if (buf->rand_type == TYPE_0)
339 int32_t val = state[0];
340 val = ((state[0] * 1103515245) + 12345) & 0x7fffffff;
341 state[0] = val;
342 *result = val;
344 else
346 int32_t *fptr = buf->fptr;
347 int32_t *rptr = buf->rptr;
348 int32_t *end_ptr = buf->end_ptr;
349 int32_t val;
351 val = *fptr += *rptr;
352 /* Chucking least random bit. */
353 *result = (val >> 1) & 0x7fffffff;
354 ++fptr;
355 if (fptr >= end_ptr)
357 fptr = state;
358 ++rptr;
360 else
362 ++rptr;
363 if (rptr >= end_ptr)
364 rptr = state;
366 buf->fptr = fptr;
367 buf->rptr = rptr;
369 return 0;
371 fail:
372 return -1;
375 weak_alias (__random_r, random_r)