.TH RAND 2
.SH NAME
rand, lrand, frand, nrand, lnrand, srand \- random number generator
.SH SYNOPSIS
.B #include <u.h>
.br
.B #include <libc.h>
.PP
.ta \w'\fLdouble 'u
.B
int	rand(void)
.PP
.B
long	lrand(void)
.PP
.B
double	frand(void)
.PP
.B
int	nrand(int val)
.PP
.B
long	lnrand(long val)
.PP
.B
void	srand(long seed)
.SH DESCRIPTION
.I Rand
returns a uniform pseudo-random
number
.IR x ,
.RI 0≤ x <2\u\s715\s10\d.
.PP
.I Lrand
returns a uniform
.B long
.IR x ,
.RI 0≤ x <2\u\s731\s10\d.
.PP
.I Frand
returns a uniform
.B double
.IR x ,
.RI 0.0≤ x <1.0,
This function calls
.I lrand
twice to generate a number with as many as 62 significant bits of mantissa.
.PP
.I Nrand
returns a uniform integer
.IR x ,
.RI 0≤ x < val.
.I Lnrand
is the same, but returns a
.BR long .
.PP
The algorithm is additive feedback with:
.IP
x[n] = (x[n\(mi273] + x[n\(mi607]) mod
.if t 2\u\s731\s0\d
.if n 2^31
.LP
giving a period of
.if t 2\u\s730\s10\d \(mu (2\u\s7607\s10\d \- 1).
.if n 2^30 × (2^607 - 1).
.PP
The generators are initialized by calling
.I srand
with whatever you like as argument.
To get a different starting value each time,
.IP
.L
srand(time(0))
.LP
will work as long as it is not called more often
than once per second.
Calling
.IP
.L
srand(1)
.LP
will initialize the generators to their
starting state.
.SS Alef
There is no
.I rand
in Alef.
.SH SOURCE
.B /sys/src/libc/port/rand.c