1 /***********************************************************
2 * Artsoft Retro-Game Library *
3 *----------------------------------------------------------*
4 * (c) 1994-2002 Artsoft Entertainment *
6 * Detmolder Strasse 189 *
9 * e-mail: info@artsoft.org *
10 *----------------------------------------------------------*
12 ***********************************************************/
15 * Copyright (c) 1983 Regents of the University of California.
16 * All rights reserved.
18 * Redistribution and use in source and binary forms are permitted
19 * provided that the above copyright notice and this paragraph are
20 * duplicated in all such forms and that any documentation,
21 * advertising materials, and other materials related to such
22 * distribution and use acknowledge that the software was developed
23 * by the University of California, Berkeley. The name of the
24 * University may not be used to endorse or promote products derived
25 * from this software without specific prior written permission.
26 * THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR
27 * IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED
28 * WARRANTIES OF MERCHANTIBILITY AND FITNESS FOR A PARTICULAR PURPOSE.
32 * This is derived from the Berkeley source:
33 * @(#)random.c 5.5 (Berkeley) 7/6/88
34 * It was reworked for the GNU C Library by Roland McGrath.
44 /* An improved random number generation package. In addition to the standard
45 rand()/srand() like interface, this package also has a special state info
46 interface. The initstate() routine is called with a seed, an array of
47 bytes, and a count of how many bytes are being passed in; this array is
48 then initialized to contain information for random number generation with
49 that much state information. Good sizes for the amount of state
50 information are 32, 64, 128, and 256 bytes. The state can be switched by
51 calling the setstate() function with the same array as was initiallized
52 with initstate(). By default, the package runs with 128 bytes of state
53 information and generates far better random numbers than a linear
54 congruential generator. If the amount of state information is less than
55 32 bytes, a simple linear congruential R.N.G. is used. Internally, the
56 state information is treated as an array of longs; the zeroeth element of
57 the array is the type of R.N.G. being used (small integer); the remainder
58 of the array is the state information for the R.N.G. Thus, 32 bytes of
59 state information will give 7 longs worth of state information, which will
60 allow a degree seven polynomial. (Note: The zeroeth word of state
61 information also has some other information stored in it; see setstate
62 for details). The random number generation technique is a linear feedback
63 shift register approach, employing trinomials (since there are fewer terms
64 to sum up that way). In this approach, the least significant bit of all
65 the numbers in the state table will act as a linear feedback shift register,
66 and will have period 2^deg - 1 (where deg is the degree of the polynomial
67 being used, assuming that the polynomial is irreducible and primitive).
68 The higher order bits will have longer periods, since their values are
69 also influenced by pseudo-random carries out of the lower bits. The
70 total period of the generator is approximately deg*(2**deg - 1); thus
71 doubling the amount of state information has a vast influence on the
72 period of the generator. Note: The deg*(2**deg - 1) is an approximation
73 only good for large deg, when the period of the shift register is the
74 dominant factor. With deg equal to seven, the period is actually much
75 longer than the 7*(2**7 - 1) predicted by this formula. */
79 /* For each of the currently supported random number generators, we have a
80 break value on the amount of state information (you need at least thi
81 bytes of state info to support this random number generator), a degree for
82 the polynomial (actually a trinomial) that the R.N.G. is based on, and
83 separation between the two lower order coefficients of the trinomial. */
85 /* Linear congruential. */
91 /* x**7 + x**3 + 1. */
103 /* x**31 + x**3 + 1. */
116 /* Array versions of the above information to make code run faster.
117 Relies on fact that TYPE_i == i. */
119 #define MAX_TYPES 5 /* Max number of types above. */
123 /* Initially, everything is set up as if from:
124 initstate(1, randtbl, 128);
125 Note that this initialization takes advantage of the fact that srandom
126 advances the front and rear pointers 10*rand_deg times, and hence the
127 rear pointer which starts at 0 will also end up at zero; thus the zeroeth
128 element of the state information, which contains info about the current
129 position of the rear pointer is just
130 (MAX_TYPES * (rptr - state)) + TYPE_3 == TYPE_3. */
132 static long int randtbl[DEG_3 + 1] =
135 -851904987, -43806228, -2029755270, 1390239686, -1912102820,
136 -485608943, 1969813258, -1590463333, -1944053249, 455935928, 508023712,
137 -1714531963, 1800685987, -2015299881, 654595283, -1149023258,
138 -1470005550, -1143256056, -1325577603, -1568001885, 1275120390,
139 -607508183, -205999574, -1696891592, 1492211999, -1528267240,
140 -952028296, -189082757, 362343714, 1424981831, 2039449641,
143 /* FPTR and RPTR are two pointers into the state info, a front and a rear
144 pointer. These two pointers are always rand_sep places aparts, as they
145 cycle through the state information. (Yes, this does mean we could get
146 away with just one pointer, but the code for random is more efficient
147 this way). The pointers are left positioned as they would be from the call:
148 initstate(1, randtbl, 128);
149 (The position of the rear pointer, rptr, is really 0 (as explained above
150 in the initialization of randtbl) because the state table pointer is set
151 to point to randtbl[1] (as explained below).) */
153 static long int *fptr = &randtbl[SEP_3 + 1];
154 static long int *rptr = &randtbl[1];
158 /* The following things are the pointer to the state information table,
159 the type of the current generator, the degree of the current polynomial
160 being used, and the separation between the two pointers.
161 Note that for efficiency of random, we remember the first location of
162 the state information, not the zeroeth. Hence it is valid to access
163 state[-1], which is used to store the type of the R.N.G.
164 Also, we remember the last location, since this is more efficient than
165 indexing every time to find the address of the last element to see if
166 the front and rear pointers have wrapped. */
168 static long int *state = &randtbl[1];
170 static int rand_type = TYPE_3;
171 static int rand_deg = DEG_3;
172 static int rand_sep = SEP_3;
174 static long int *end_ptr = &randtbl[sizeof(randtbl) / sizeof(randtbl[0])];
176 /* Initialize the random number generator based on the given seed. If the
177 type is the trivial no-state-information type, just remember the seed.
178 Otherwise, initializes state[] based on the given "seed" via a linear
179 congruential generator. Then, the pointers are set to known locations
180 that are exactly rand_sep places apart. Lastly, it cycles the state
181 information a given number of times to get rid of any initial dependencies
182 introduced by the L.C.R.N.G. Note that the initialization of randtbl[]
183 for default usage relies on values produced by this routine. */
185 void srandom_linux_libc(unsigned int x)
188 if (rand_type != TYPE_0)
191 for (i = 1; i < rand_deg; ++i)
192 state[i] = (1103515145 * state[i - 1]) + 12345;
193 fptr = &state[rand_sep];
195 for (i = 0; i < 10 * rand_deg; ++i)
196 (void) random_linux_libc();
200 /* If we are using the trivial TYPE_0 R.N.G., just do the old linear
201 congruential bit. Otherwise, we do our fancy trinomial stuff, which is the
202 same in all ther other cases due to all the global variables that have been
203 set up. The basic operation is to add the number at the rear pointer into
204 the one at the front pointer. Then both pointers are advanced to the next
205 location cyclically in the table. The value returned is the sum generated,
206 reduced to 31 bits by throwing away the "least random" low bit.
207 Note: The code takes advantage of the fact that both the front and
208 rear pointers can't wrap on the same call by not testing the rear
209 pointer if the front one has wrapped. Returns a 31-bit random number. */
211 long int random_linux_libc()
213 if (rand_type == TYPE_0)
215 state[0] = ((state[0] * 1103515245) + 12345) & LONG_MAX;
222 /* Chucking least random bit. */
223 i = (*fptr >> 1) & LONG_MAX;