| [9] | 1 | .\" Copyright (c) 1983 Regents of the University of California.
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| 2 | .\" All rights reserved. The Berkeley software License Agreement
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| 3 | .\" specifies the terms and conditions for redistribution.
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| 4 | .\"
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| 5 | .\" @(#)random.3 6.2 (Berkeley) 9/29/85
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| 6 | .\"
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| 7 | .TH RANDOM 3 "September 29, 1985"
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| 8 | .UC 5
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| 9 | .SH NAME
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| 10 | random, srandom, initstate, setstate \- better random number generator; routines for changing generators
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| 11 | .SH SYNOPSIS
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| 12 | .nf
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| 13 | .ft B
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| 14 | #include <stdlib.h>
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| 15 |
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| 16 | long random(void)
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| 17 | void srandom(unsigned \fIseed\fP)
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| 18 | char *initstate(unsigned \fIseed\fP, char *\fIstate\fP, int \fIn\fP)
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| 19 | char *setstate(char *\fIstate\fP)
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| 20 | .ft R
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| 21 | .fi
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| 22 | .SH DESCRIPTION
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| 23 | .PP
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| 24 | .B Random
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| 25 | uses a non-linear additive feedback random number generator employing a
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| 26 | default table of size 31 long integers to return successive pseudo-random
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| 27 | numbers in the range from 0 to
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| 28 | .if t 2\u\s731\s10\d\(mi1.
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| 29 | .if n (2**31)\(mi1.
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| 30 | The period of this random number generator is very large, approximately
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| 31 | .if t 16\(mu(2\u\s731\s10\d\(mi1).
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| 32 | .if n 16*((2**31)\(mi1).
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| 33 | .PP
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| 34 | .B Random/srandom
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| 35 | have (almost) the same calling sequence and initialization properties as
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| 36 | .B rand/srand.
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| 37 | The difference is that
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| 38 | .BR rand (3)
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| 39 | produces a much less random sequence \(em in fact, the low dozen bits
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| 40 | generated by rand go through a cyclic pattern. All the bits generated by
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| 41 | .B random
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| 42 | are usable. For example, ``random()&01'' will produce a random binary
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| 43 | value.
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| 44 | .PP
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| 45 | Unlike
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| 46 | .BR srand ,
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| 47 | .B srandom
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| 48 | does not return the old seed; the reason for this is that the amount of
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| 49 | state information used is much more than a single word. (Two other
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| 50 | routines are provided to deal with restarting/changing random
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| 51 | number generators). Like
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| 52 | .BR rand (3),
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| 53 | however,
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| 54 | .B random
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| 55 | will by default produce a sequence of numbers that can be duplicated
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| 56 | by calling
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| 57 | .B srandom
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| 58 | with
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| 59 | .B 1
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| 60 | as the seed.
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| 61 | .PP
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| 62 | The
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| 63 | .B initstate
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| 64 | routine allows a state array, passed in as an argument, to be initialized
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| 65 | for future use. The size of the state array (in bytes) is used by
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| 66 | .B initstate
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| 67 | to decide how sophisticated a random number generator it should use -- the
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| 68 | more state, the better the random numbers will be.
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| 69 | (Current "optimal" values for the amount of state information are
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| 70 | 8, 32, 64, 128, and 256 bytes; other amounts will be rounded down to
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| 71 | the nearest known amount. Using less than 8 bytes will cause an error).
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| 72 | The seed for the initialization (which specifies a starting point for
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| 73 | the random number sequence, and provides for restarting at the same
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| 74 | point) is also an argument.
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| 75 | .B Initstate
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| 76 | returns a pointer to the previous state information array.
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| 77 | .PP
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| 78 | Once a state has been initialized, the
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| 79 | .B setstate
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| 80 | routine provides for rapid switching between states.
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| 81 | .B Setstate
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| 82 | returns a pointer to the previous state array; its
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| 83 | argument state array is used for further random number generation
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| 84 | until the next call to
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| 85 | .B initstate
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| 86 | or
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| 87 | .BR setstate .
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| 88 | .PP
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| 89 | Once a state array has been initialized, it may be restarted at a
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| 90 | different point either by calling
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| 91 | .B initstate
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| 92 | (with the desired seed, the state array, and its size) or by calling
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| 93 | both
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| 94 | .B setstate
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| 95 | (with the state array) and
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| 96 | .B srandom
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| 97 | (with the desired seed).
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| 98 | The advantage of calling both
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| 99 | .B setstate
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| 100 | and
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| 101 | .B srandom
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| 102 | is that the size of the state array does not have to be remembered after
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| 103 | it is initialized.
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| 104 | .PP
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| 105 | With 256 bytes of state information, the period of the random number
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| 106 | generator is greater than
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| 107 | .if t 2\u\s769\s10\d,
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| 108 | .if n 2**69
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| 109 | which should be sufficient for most purposes.
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| 110 | .SH AUTHOR
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| 111 | Earl T. Cohen
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| 112 | .SH DIAGNOSTICS
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| 113 | .PP
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| 114 | If
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| 115 | .B initstate
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| 116 | is called with less than 8 bytes of state information, or if
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| 117 | .B setstate
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| 118 | detects that the state information has been garbled, error
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| 119 | messages are printed on the standard error output.
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| 120 | .SH "SEE ALSO"
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| 121 | .BR rand (3).
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| 122 | .SH NOTES
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| 123 | .B initstate
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| 124 | and
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| 125 | .B setstate
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| 126 | are not declared in
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| 127 | .IR <stdlib.h> ,
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| 128 | programmers must provide their own declarations.
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| 129 | .SH BUGS
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| 130 | About 2/3 the speed of
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| 131 | .BR rand (3).
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