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.TH RAND 2
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.SH NAME
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rand, lrand, frand, nrand, lnrand, srand, truerand, ntruerand, genrandom, prng, fastrand, nfastrand \- random number generators
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.SH SYNOPSIS
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.B #include <u.h>
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.br
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.B #include <libc.h>
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.PP
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.ta \w'\fLdouble 'u
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.B
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int	rand(void)
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.PP
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.B
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long	lrand(void)
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.PP
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.B
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double	frand(void)
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.PP
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.B
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int	nrand(int val)
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.PP
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.B
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long	lnrand(long val)
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.PP
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.B
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void	srand(long seed)
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.PP
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.B
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ulong	truerand(void)
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.PP
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.B
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ulong	ntruerand(ulong val)
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.sp
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.B #include <mp.h>
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.br
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.B #include <libsec.h>
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.PP
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.B
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void	genrandom(uchar *buf, int nbytes)
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.PP
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.B
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void	prng(uchar *buf, int nbytes)
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.PP
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.B
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ulong	fastrand(void)
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.PP
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.B
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ulong	nfastrand(ulong val)
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.SH DESCRIPTION
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.I Rand
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returns a uniform pseudo-random
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number
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.IR x ,
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.RI 0≤ x <2\u\s715\s10\d.
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.PP
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.I Lrand
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returns a uniform
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.B long
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.IR x ,
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.RI 0≤ x <2\u\s731\s10\d.
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.PP
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.I Frand
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returns a uniform
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.B double
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.IR x ,
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.RI 0.0≤ x <1.0,
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This function calls
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.I lrand
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twice to generate a number with as many as 62 significant bits of mantissa.
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.PP
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.I Nrand
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returns a uniform integer
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.IR x ,
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.RI 0≤ x < val.
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.I Lnrand
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is the same, but returns a
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.BR long .
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.PP
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The algorithm is additive feedback with:
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.IP
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x[n] = (x[n\(mi273] + x[n\(mi607]) mod
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.if t 2\u\s731\s0\d
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.if n 2^31
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.LP
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giving a period of
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.if t 2\u\s730\s10\d \(mu (2\u\s7607\s10\d \- 1).
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.if n 2^30 × (2^607 - 1).
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.PP
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The generators are initialized by calling
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.I srand
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with whatever you like as argument.
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To get a different starting value each time,
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.IP
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.L
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srand(time(0))
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.LP
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will work as long as it is not called more often
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than once per second.
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Calling
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.IP
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.L
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srand(1)
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.LP
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will initialize the generators to their
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starting state.
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.PP
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.I Truerand
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returns a random unsigned long read from
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.BR /dev/random .
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Due to the nature of
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.BR /dev/random ,
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truerand can only return a few hundred bits a
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second.
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.PP
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.I Ntruerand
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returns a uniform random integer
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.IR x ,
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.RI 0≤ x < val ≤ 2\u\s732\s10\d-1.
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.PP
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.I Genrandom
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fills a buffer with bytes from the X9.17 pseudo-random
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number generator.  The X9.17 generator is seeded by 24
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truly random bytes read from
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.BR /dev/random .
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.PP
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.I Prng
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uses the native
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.IR rand (2)
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pseudo-random number generator to fill the buffer.  Used with
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.IR srand ,
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this function can produce a reproducible stream of pseudo random
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numbers useful in testing.
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.PP
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Both
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.I genrandom
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and
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.I prng
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may be passed to
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.I mprand
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(see
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.IR mp (2)).
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.PP
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.I Fastrand
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uses
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.I genrandom
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to return a uniform
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.B "unsigned long
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.IR x ,
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.RI 0≤ x < 2\u\s732\s10\d-1.
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.PP
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.I Nfastrand
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uses
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.I genrandom
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to return a uniform
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.B "unsigned long
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.IR x ,
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.RI 0≤ x < val ≤ 2\u\s732\s10\d-1.
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.SH SOURCE
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.B /sys/src/libc/port/*rand.c
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.br
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.B /sys/src/libc/9sys/truerand.c
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.br
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.B /sys/src/libsec/port/genrandom.c
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.br
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.B /sys/src/libsec/port/prng.c
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.br
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.B /sys/src/libsec/port/*fastrand.c
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.SH "SEE ALSO
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.IR cons (3),
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.IR mp (2)
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.SH BUGS
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.I Truerand
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and
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.I ntruerand
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maintain a static file descriptor.