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1.1 root 1: .\" Copyright (c) 1983 Regents of the University of California.
2: .\" All rights reserved. The Berkeley software License Agreement
3: .\" specifies the terms and conditions for redistribution.
4: .\"
5: .\" @(#)random.3 6.2 (Berkeley) 9/29/85
6: .\"
7: .TH RANDOM 3 "September 29, 1985"
8: .UC 5
9: .SH NAME
10: random, srandom, initstate, setstate \- better random number generator; routines for changing generators
11: .SH SYNOPSIS
12: .nf
13: .B long random()
14: .PP
15: .B srandom(seed)
16: .B int seed;
17: .PP
18: .B char *initstate(seed, state, n)
19: .B unsigned seed;
20: .B char *state;
21: .B int n;
22: .PP
23: .B char *setstate(state)
24: .B char *state;
25: .fi
26: .SH DESCRIPTION
27: .PP
28: .I Random
29: uses a non-linear additive feedback random number generator employing a
30: default table of size 31 long integers to return successive pseudo-random
31: numbers in the range from 0 to
32: .if t 2\u\s731\s10\d\(mi1.
33: .if n (2**31)\(mi1.
34: The period of this random number generator is very large, approximately
35: .if t 16\(mu(2\u\s731\s10\d\(mi1).
36: .if n 16*((2**31)\(mi1).
37: .PP
38: .I Random/srandom
39: have (almost) the same calling sequence and initialization properties as
40: .I rand/srand.
41: The difference is that
42: .IR rand (3)
43: produces a much less random sequence \(em in fact, the low dozen bits
44: generated by rand go through a cyclic pattern. All the bits generated by
45: .I random
46: are usable. For example, ``random()&01'' will produce a random binary
47: value.
48: .PP
49: Unlike
50: .IR srand ,
51: .I srandom
52: does not return the old seed; the reason for this is that the amount of
53: state information used is much more than a single word. (Two other
54: routines are provided to deal with restarting/changing random
55: number generators). Like
56: .IR rand (3),
57: however,
58: .I random
59: will by default produce a sequence of numbers that can be duplicated
60: by calling
61: .I srandom
62: with
63: .I 1
64: as the seed.
65: .PP
66: The
67: .I initstate
68: routine allows a state array, passed in as an argument, to be initialized
69: for future use. The size of the state array (in bytes) is used by
70: .I initstate
71: to decide how sophisticated a random number generator it should use -- the
72: more state, the better the random numbers will be.
73: (Current "optimal" values for the amount of state information are
74: 8, 32, 64, 128, and 256 bytes; other amounts will be rounded down to
75: the nearest known amount. Using less than 8 bytes will cause an error).
76: The seed for the initialization (which specifies a starting point for
77: the random number sequence, and provides for restarting at the same
78: point) is also an argument.
79: .I Initstate
80: returns a pointer to the previous state information array.
81: .PP
82: Once a state has been initialized, the
83: .I setstate
84: routine provides for rapid switching between states.
85: .I Setstate
86: returns a pointer to the previous state array; its
87: argument state array is used for further random number generation
88: until the next call to
89: .I initstate
90: or
91: .I setstate.
92: .PP
93: Once a state array has been initialized, it may be restarted at a
94: different point either by calling
95: .I initstate
96: (with the desired seed, the state array, and its size) or by calling
97: both
98: .I setstate
99: (with the state array) and
100: .I srandom
101: (with the desired seed).
102: The advantage of calling both
103: .I setstate
104: and
105: .I srandom
106: is that the size of the state array does not have to be remembered after
107: it is initialized.
108: .PP
109: With 256 bytes of state information, the period of the random number
110: generator is greater than
111: .if t 2\u\s769\s10\d,
112: .if n 2**69
113: which should be sufficient for most purposes.
114: .SH AUTHOR
115: Earl T. Cohen
116: .SH DIAGNOSTICS
117: .PP
118: If
119: .I initstate
120: is called with less than 8 bytes of state information, or if
121: .I setstate
122: detects that the state information has been garbled, error
123: messages are printed on the standard error output.
124: .SH "SEE ALSO"
125: rand(3)
126: .SH BUGS
127: About 2/3 the speed of
128: .IR rand (3C).
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