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1.1 root 1: .\" Copyright (c) 1983 The Regents of the University of California.
2: .\" All rights reserved.
3: .\"
4: .\" Redistribution and use in source and binary forms are permitted provided
5: .\" that: (1) source distributions retain this entire copyright notice and
6: .\" comment, and (2) distributions including binaries display the following
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8: .\" University of California, Berkeley and its contributors'' in the
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14: .\" THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR IMPLIED
15: .\" WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF
16: .\" MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
17: .\"
18: .\" @(#)random.3 6.4 (Berkeley) 6/23/90
19: .\"
20: .UC 7
21: .TH RANDOM 3 "June 23, 1990"
22: .UC 5
23: .SH NAME
24: random, srandom, initstate, setstate \- better random number generator; routines for changing generators
25: .SH SYNOPSIS
26: .nf
27: .B long random()
28: .PP
29: .B srandom(seed)
30: .B int seed;
31: .PP
32: .B char *initstate(seed, state, n)
33: .B unsigned seed;
34: .B char *state;
35: .B int n;
36: .PP
37: .B char *setstate(state)
38: .B char *state;
39: .fi
40: .SH DESCRIPTION
41: .PP
42: .I Random
43: uses a non-linear additive feedback random number generator employing a
44: default table of size 31 long integers to return successive pseudo-random
45: numbers in the range from 0 to
46: .if t 2\u\s731\s10\d\(mi1.
47: .if n (2**31)\(mi1.
48: The period of this random number generator is very large, approximately
49: .if t 16\(mu(2\u\s731\s10\d\(mi1).
50: .if n 16*((2**31)\(mi1).
51: .PP
52: .I Random/srandom
53: have (almost) the same calling sequence and initialization properties as
54: .I rand/srand.
55: The difference is that
56: .IR rand (3)
57: produces a much less random sequence \(em in fact, the low dozen bits
58: generated by rand go through a cyclic pattern. All the bits generated by
59: .I random
60: are usable. For example, ``random()&01'' will produce a random binary
61: value.
62: .PP
63: Unlike
64: .IR srand ,
65: .I srandom
66: does not return the old seed; the reason for this is that the amount of
67: state information used is much more than a single word. (Two other
68: routines are provided to deal with restarting/changing random
69: number generators). Like
70: .IR rand (3),
71: however,
72: .I random
73: will by default produce a sequence of numbers that can be duplicated
74: by calling
75: .I srandom
76: with
77: .I 1
78: as the seed.
79: .PP
80: The
81: .I initstate
82: routine allows a state array, passed in as an argument, to be initialized
83: for future use. The size of the state array (in bytes) is used by
84: .I initstate
85: to decide how sophisticated a random number generator it should use -- the
86: more state, the better the random numbers will be.
87: (Current "optimal" values for the amount of state information are
88: 8, 32, 64, 128, and 256 bytes; other amounts will be rounded down to
89: the nearest known amount. Using less than 8 bytes will cause an error).
90: The seed for the initialization (which specifies a starting point for
91: the random number sequence, and provides for restarting at the same
92: point) is also an argument.
93: .I Initstate
94: returns a pointer to the previous state information array.
95: .PP
96: Once a state has been initialized, the
97: .I setstate
98: routine provides for rapid switching between states.
99: .I Setstate
100: returns a pointer to the previous state array; its
101: argument state array is used for further random number generation
102: until the next call to
103: .I initstate
104: or
105: .I setstate.
106: .PP
107: Once a state array has been initialized, it may be restarted at a
108: different point either by calling
109: .I initstate
110: (with the desired seed, the state array, and its size) or by calling
111: both
112: .I setstate
113: (with the state array) and
114: .I srandom
115: (with the desired seed).
116: The advantage of calling both
117: .I setstate
118: and
119: .I srandom
120: is that the size of the state array does not have to be remembered after
121: it is initialized.
122: .PP
123: With 256 bytes of state information, the period of the random number
124: generator is greater than
125: .if t 2\u\s769\s10\d,
126: .if n 2**69
127: which should be sufficient for most purposes.
128: .SH AUTHOR
129: Earl T. Cohen
130: .SH DIAGNOSTICS
131: .PP
132: If
133: .I initstate
134: is called with less than 8 bytes of state information, or if
135: .I setstate
136: detects that the state information has been garbled, error
137: messages are printed on the standard error output.
138: .SH "SEE ALSO"
139: rand(3)
140: .SH BUGS
141: About 2/3 the speed of
142: .IR rand (3C).
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