1 /* Copyright (C) 1995, 2004 Free Software Foundation
2
3 The GNU C Library is free software; you can redistribute it and/or
4 modify it under the terms of the GNU Lesser General Public
5 License as published by the Free Software Foundation; either
6 version 2.1 of the License, or (at your option) any later version.
7
8 The GNU C Library is distributed in the hope that it will be useful,
9 but WITHOUT ANY WARRANTY; without even the implied warranty of
10 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
11 Lesser General Public License for more details.
12
13 You should have received a copy of the GNU Lesser General Public
14 License along with the GNU C Library; if not, write to the Free
15 Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
16 02110-1301 USA. */
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 /*
26 Copyright (C) 1983 Regents of the University of California.
27 All rights reserved.
28
29 Redistribution and use in source and binary forms, with or without
30 modification, are permitted provided that the following conditions
31 are met:
32
33 1. Redistributions of source code must retain the above copyright
34 notice, this list of conditions and the following disclaimer.
35 2. Redistributions in binary form must reproduce the above copyright
36 notice, this list of conditions and the following disclaimer in the
37 documentation and/or other materials provided with the distribution.
38 4. Neither the name of the University nor the names of its contributors
39 may be used to endorse or promote products derived from this software
40 without specific prior written permission.
41
42 THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
43 ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
44 IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
45 ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
46 FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
47 DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
48 OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
49 HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
50 LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
51 OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
52 SUCH DAMAGE.*/
53
54 #include <limits.h>
55 #include <stdlib.h>
56 #include "generate-random.h"
57
58
59 /* An improved random number generation package. In addition to the standard
60 rand()/srand() like interface, this package also has a special state info
61 interface. The initstate() routine is called with a seed, an array of
62 bytes, and a count of how many bytes are being passed in; this array is
63 then initialized to contain information for random number generation with
64 that much state information. Good sizes for the amount of state
65 information are 32, 64, 128, and 256 bytes. The state can be switched by
66 calling the setstate() function with the same array as was initialized
67 with initstate(). By default, the package runs with 128 bytes of state
68 information and generates far better random numbers than a linear
69 congruential generator. If the amount of state information is less than
70 32 bytes, a simple linear congruential R.N.G. is used. Internally, the
71 state information is treated as an array of longs; the zeroth element of
72 the array is the type of R.N.G. being used (small integer); the remainder
73 of the array is the state information for the R.N.G. Thus, 32 bytes of
74 state information will give 7 longs worth of state information, which will
75 allow a degree seven polynomial. (Note: The zeroth word of state
76 information also has some other information stored in it; see setstate
77 for details). The random number generation technique is a linear feedback
78 shift register approach, employing trinomials (since there are fewer terms
79 to sum up that way). In this approach, the least significant bit of all
80 the numbers in the state table will act as a linear feedback shift register,
81 and will have period 2^deg - 1 (where deg is the degree of the polynomial
82 being used, assuming that the polynomial is irreducible and primitive).
83 The higher order bits will have longer periods, since their values are
84 also influenced by pseudo-random carries out of the lower bits. The
85 total period of the generator is approximately deg*(2**deg - 1); thus
86 doubling the amount of state information has a vast influence on the
87 period of the generator. Note: The deg*(2**deg - 1) is an approximation
88 only good for large deg, when the period of the shift register is the
89 dominant factor. With deg equal to seven, the period is actually much
90 longer than the 7*(2**7 - 1) predicted by this formula. */
91
92
93
94 /* For each of the currently supported random number generators, we have a
95 break value on the amount of state information (you need at least this many
96 bytes of state info to support this random number generator), a degree for
97 the polynomial (actually a trinomial) that the R.N.G. is based on, and
98 separation between the two lower order coefficients of the trinomial. */
99
100 /* Linear congruential. */
101 #define TYPE_0 0
102 #define BREAK_0 8
103 #define DEG_0 0
104 #define SEP_0 0
105
106 /* x**7 + x**3 + 1. */
107 #define TYPE_1 1
108 #define BREAK_1 32
109 #define DEG_1 7
110 #define SEP_1 3
111
112 /* x**15 + x + 1. */
113 #define TYPE_2 2
114 #define BREAK_2 64
115 #define DEG_2 15
116 #define SEP_2 1
117
118 /* x**31 + x**3 + 1. */
119 #define TYPE_3 3
120 #define BREAK_3 128
121 #define DEG_3 31
122 #define SEP_3 3
123
124 /* x**63 + x + 1. */
125 #define TYPE_4 4
126 #define BREAK_4 256
127 #define DEG_4 63
128 #define SEP_4 1
129
130
131 /* Array versions of the above information to make code run faster.
132 Relies on fact that TYPE_i == i. */
133
134 #define MAX_TYPES 5 /* Max number of types above. */
135
136
137 /* Initially, everything is set up as if from:
138 initstate(1, randtbl, 128);
139 Note that this initialization takes advantage of the fact that srandom
140 advances the front and rear pointers 10*rand_deg times, and hence the
141 rear pointer which starts at 0 will also end up at zero; thus the zeroth
142 element of the state information, which contains info about the current
143 position of the rear pointer is just
144 (MAX_TYPES * (rptr - state)) + TYPE_3 == TYPE_3. */
145
146 static int randtbl[DEG_3 + 1] =
147 {
148 TYPE_3,
149
150 -1726662223, 379960547, 1735697613, 1040273694, 1313901226,
151 1627687941, -179304937, -2073333483, 1780058412, -1989503057,
152 -615974602, 344556628, 939512070, -1249116260, 1507946756,
153 -812545463, 154635395, 1388815473, -1926676823, 525320961,
154 -1009028674, 968117788, -123449607, 1284210865, 435012392,
155 -2017506339, -911064859, -370259173, 1132637927, 1398500161,
156 -205601318,
157 };
158
159
160 static struct generate_random_data unsafe_state =
161 {
162 /* FPTR and RPTR are two pointers into the state info, a front and a rear
163 pointer. These two pointers are always rand_sep places aparts, as they
164 cycle through the state information. (Yes, this does mean we could get
165 away with just one pointer, but the code for random is more efficient
166 this way). The pointers are left positioned as they would be from the call:
167 initstate(1, randtbl, 128);
168 (The position of the rear pointer, rptr, is really 0 (as explained above
169 in the initialization of randtbl) because the state table pointer is set
170 to point to randtbl[1] (as explained below).) */
171
172 &randtbl[SEP_3 + 1], /* fptr */
173 &randtbl[1], /* rptr */
174
175 /* The following things are the pointer to the state information table,
176 the type of the current generator, the degree of the current polynomial
177 being used, and the separation between the two pointers.
178 Note that for efficiency of random, we remember the first location of
179 the state information, not the zeroth. Hence it is valid to access
180 state[-1], which is used to store the type of the R.N.G.
181 Also, we remember the last location, since this is more efficient than
182 indexing every time to find the address of the last element to see if
183 the front and rear pointers have wrapped. */
184
185 &randtbl[1], /* state */
186
187 TYPE_3, /* rand_type */
188 DEG_3, /* rand_deg */
189 SEP_3, /* rand_sep */
190
191 &randtbl[sizeof (randtbl) / sizeof (randtbl[0])] /* end_ptr */
192 };
193
194 /* Initialize the random number generator based on the given seed. If the
195 type is the trivial no-state-information type, just remember the seed.
196 Otherwise, initializes state[] based on the given "seed" via a linear
197 congruential generator. Then, the pointers are set to known locations
198 that are exactly rand_sep places apart. Lastly, it cycles the state
199 information a given number of times to get rid of any initial dependencies
200 introduced by the L.C.R.N.G. Note that the initialization of randtbl[]
201 for default usage relies on values produced by this routine. */
202 void
203 generate_srandom (unsigned int x)
204 {
205 (void) generate_srandom_r (x, &unsafe_state);
206 }
207
208 /* Initialize the state information in the given array of N bytes for
209 future random number generation. Based on the number of bytes we
210 are given, and the break values for the different R.N.G.'s, we choose
211 the best (largest) one we can and set things up for it. srandom is
212 then called to initialize the state information. Note that on return
213 from srandom, we set state[-1] to be the type multiplexed with the current
214 value of the rear pointer; this is so successive calls to initstate won't
215 lose this information and will be able to restart with setstate.
216 Note: The first thing we do is save the current state, if any, just like
217 setstate so that it doesn't matter when initstate is called.
218 Returns a pointer to the old state. */
219 char *
220 generate_initstate (unsigned int seed, char *arg_state, size_t n)
221 {
222 int *ostate;
223
224 ostate = &unsafe_state.state[-1];
225 generate_initstate_r (seed, arg_state, n, &unsafe_state);
226 return (char *) ostate;
227 }
228
229 /* Restore the state from the given state array.
230 Note: It is important that we also remember the locations of the pointers
231 in the current state information, and restore the locations of the pointers
232 from the old state information. This is done by multiplexing the pointer
233 location into the zeroth word of the state information. Note that due
234 to the order in which things are done, it is OK to call setstate with the
235 same state as the current state
236 Returns a pointer to the old state information. */
237 char *
238 generate_setstate (char *arg_state)
239 {
240 int *ostate;
241
242 ostate = &unsafe_state.state[-1];
243 if (generate_setstate_r (arg_state, &unsafe_state) < 0)
244 ostate = NULL;
245 return (char *) ostate;
246 }
247
248 /* If we are using the trivial TYPE_0 R.N.G., just do the old linear
249 congruential bit. Otherwise, we do our fancy trinomial stuff, which is the
250 same in all the other cases due to all the global variables that have been
251 set up. The basic operation is to add the number at the rear pointer into
252 the one at the front pointer. Then both pointers are advanced to the next
253 location cyclically in the table. The value returned is the sum generated,
254 reduced to 31 bits by throwing away the "least random" low bit.
255 Note: The code takes advantage of the fact that both the front and
256 rear pointers can't wrap on the same call by not testing the rear
257 pointer if the front one has wrapped. Returns a 31-bit random number. */
258
259 long int
260 generate_random (void)
261 {
262 int retval;
263 (void) generate_random_r (&unsafe_state, &retval);
264 return retval;
265 }