1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
|
/*
* This is the ``Mersenne Twister'' random number generator MT19937, which
* generates pseudorandom integers uniformly distributed in 0..(2^32 - 1)
* starting from any odd seed in 0..(2^32 - 1). This version is a recode
* by Shawn Cokus (Cokus@math.washington.edu) on March 8, 1998 of a version by
* Takuji Nishimura (who had suggestions from Topher Cooper and Marc Rieffel in
* July-August 1997).
*
* Effectiveness of the recoding (on Goedel2.math.washington.edu, a DEC Alpha
* running OSF/1) using GCC -O3 as a compiler: before recoding: 51.6 sec. to
* generate 300 million random numbers; after recoding: 24.0 sec. for the same
* (i.e., 46.5% of original time), so speed is now about 12.5 million random
* number generations per second on this machine.
*
* According to the URL <http://www.math.keio.ac.jp/~matumoto/emt.html>
* (and paraphrasing a bit in places), the Mersenne Twister is ``designed
* with consideration of the flaws of various existing generators,'' has
* a period of 2^19937 - 1, gives a sequence that is 623-dimensionally
* equidistributed, and ``has passed many stringent tests, including the
* die-hard test of G. Marsaglia and the load test of P. Hellekalek and
* S. Wegenkittl.'' It is efficient in memory usage (typically using 2506
* to 5012 bytes of static data, depending on data type sizes, and the code
* is quite short as well). It generates random numbers in batches of 624
* at a time, so the caching and pipelining of modern systems is exploited.
* It is also divide- and mod-free.
*
* This library is free software; you can redistribute it and/or modify it
* under the terms of the GNU Library General Public License as published by
* the Free Software Foundation (either version 2 of the License or, at your
* option, any later version). This library is distributed in the hope that
* it will be useful, but WITHOUT ANY WARRANTY, without even the implied
* warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See
* the GNU Library General Public License for more details. You should have
* received a copy of the GNU Library General Public License along with this
* library; if not, write to the Free Software Foundation, Inc., 59 Temple
* Place, Suite 330, Boston, MA 02111-1307, USA.
*
* The code as Shawn received it included the following notice:
*
* Copyright (C) 1997 Makoto Matsumoto and Takuji Nishimura. When
* you use this, send an e-mail to <matumoto@math.keio.ac.jp> with
* an appropriate reference to your work.
*
* It would be nice to CC: <Cokus@math.washington.edu> when you write.
*
*/
#include <stdlib.h>
#define N (624) /* length of state vector */
#define M (397) /* a period parameter */
#define K (0x9908B0DFU) /* a magic constant */
#define hiBit(u) ((u) & 0x80000000U) /* mask all but highest bit of u */
#define loBit(u) ((u) & 0x00000001U) /* mask all but lowest bit of u */
#define loBits(u) ((u) & 0x7FFFFFFFU) /* mask the highest bit of u */
#define mixBits(u, v) (hiBit(u)|loBits(v)) /* move highest bit of u to
highest bit of v */
static unsigned int state[N+1]; /* state vector + 1 to not violate ANSI C */
static unsigned int *next; /* next random value is computed from here */
static int left = -1; /* can *next++ this many times before reloading */
void srand(unsigned int seed)
{
/*
* We initialize state[0..(N-1)] via the generator
*
* x_new = (69069 * x_old) mod 2^32
*
* from Line 15 of Table 1, p. 106, Sec. 3.3.4 of Knuth's
* _The Art of Computer Programming_, Volume 2, 3rd ed.
*
* Notes (SJC): I do not know what the initial state requirements
* of the Mersenne Twister are, but it seems this seeding generator
* could be better. It achieves the maximum period for its modulus
* (2^30) iff x_initial is odd (p. 20-21, Sec. 3.2.1.2, Knuth); if
* x_initial can be even, you have sequences like 0, 0, 0, ...;
* 2^31, 2^31, 2^31, ...; 2^30, 2^30, 2^30, ...; 2^29, 2^29 + 2^31,
* 2^29, 2^29 + 2^31, ..., etc. so I force seed to be odd below.
*
* Even if x_initial is odd, if x_initial is 1 mod 4 then
*
* the lowest bit of x is always 1,
* the next-to-lowest bit of x is always 0,
* the 2nd-from-lowest bit of x alternates ... 0 1 0 1 0 1 0 1 ... ,
* the 3rd-from-lowest bit of x 4-cycles ... 0 1 1 0 0 1 1 0 ... ,
* the 4th-from-lowest bit of x has the 8-cycle ... 0 0 0 1 1 1 1 0 ... ,
* ...
*
* and if x_initial is 3 mod 4 then
*
* the lowest bit of x is always 1,
* the next-to-lowest bit of x is always 1,
* the 2nd-from-lowest bit of x alternates ... 0 1 0 1 0 1 0 1 ... ,
* the 3rd-from-lowest bit of x 4-cycles ... 0 0 1 1 0 0 1 1 ... ,
* the 4th-from-lowest bit of x has the 8-cycle ... 0 0 1 1 1 1 0 0 ... ,
* ...
*
* The generator's potency (min. s>=0 with (69069-1)^s = 0 mod 2^32) is
* 16, which seems to be alright by p. 25, Sec. 3.2.1.3 of Knuth. It
* also does well in the dimension 2..5 spectral tests, but it could be
* better in dimension 6 (Line 15, Table 1, p. 106, Sec. 3.3.4, Knuth).
*
* Note that the random number user does not see the values generated
* here directly since reloadMT() will always munge them first, so maybe
* none of all of this matters. In fact, the seed values made here could
* even be extra-special desirable if the Mersenne Twister theory says
* so-- that's why the only change I made is to restrict to odd seeds.
*/
unsigned int x = (seed | 1U) & 0xFFFFFFFFU, *s = state;
int j;
for(left=0, *s++=x, j=N; --j;
*s++ = (x*=69069U) & 0xFFFFFFFFU);
}
static int rand_reload(void)
{
unsigned int *p0=state, *p2=state+2, *pM=state+M, s0, s1;
int j;
if(left < -1)
srand(4357U);
left=N-1, next=state+1;
for(s0=state[0], s1=state[1], j=N-M+1; --j; s0=s1, s1=*p2++)
*p0++ = *pM++ ^ (mixBits(s0, s1) >> 1) ^ (loBit(s1) ? K : 0U);
for(pM=state, j=M; --j; s0=s1, s1=*p2++)
*p0++ = *pM++ ^ (mixBits(s0, s1) >> 1) ^ (loBit(s1) ? K : 0U);
s1=state[0], *p0 = *pM ^ (mixBits(s0, s1) >> 1) ^ (loBit(s1) ? K : 0U);
s1 ^= (s1 >> 11);
s1 ^= (s1 << 7) & 0x9D2C5680U;
s1 ^= (s1 << 15) & 0xEFC60000U;
return (int)s1 ^ (s1 >> 18);
}
int rand(void)
{
int y;
if(--left < 0) {
y = rand_reload();
}
else {
y = *next++;
y ^= (y >> 11);
y ^= (y << 7) & 0x9D2C5680U;
y ^= (y << 15) & 0xEFC60000U;
y ^= (y >> 18);
}
return (y & 0xfffffffe) >> 1; /* 31-bit limit by Björn Stenberg*/
}
|
