198 lines
5.8 KiB
Groff
198 lines
5.8 KiB
Groff
.\" Copyright (c) 1983, 1991, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)random.3 8.1 (Berkeley) 6/4/93
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.\" $FreeBSD$
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.\"
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.Dd April 2, 2013
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.Dt RANDOM 3
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.Os
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.Sh NAME
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.Nm random ,
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.Nm srandom ,
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.Nm srandomdev ,
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.Nm initstate ,
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.Nm setstate
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.Nd better random number generator; routines for changing generators
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In stdlib.h
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.Ft long
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.Fn random void
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.Ft void
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.Fn srandom "unsigned long seed"
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.Ft void
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.Fn srandomdev void
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.Ft char *
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.Fn initstate "unsigned long seed" "char *state" "long n"
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.Ft char *
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.Fn setstate "char *state"
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.Sh DESCRIPTION
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.Bf -symbolic
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The functions described in this manual page are not secure.
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Applications which require unpredictable random numbers should use
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.Xr arc4random 3
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instead.
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.Ef
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.Pp
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The
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.Fn random
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function
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uses a non-linear additive feedback random number generator employing a
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default table of size 31 long integers to return successive pseudo-random
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numbers in the range from 0 to
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.if t 2\u\s731\s10\d\(mi1.
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.if n (2**31)\(mi1.
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The period of this random number generator is very large, approximately
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.if t 16\(mu(2\u\s731\s10\d\(mi1).
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.if n 16*((2**31)\(mi1).
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.Pp
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The
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.Fn random
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and
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.Fn srandom
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functions have (almost) the same calling sequence and initialization properties as the
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.Xr rand 3
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and
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.Xr srand 3
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functions.
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The difference is that
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.Xr rand 3
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produces a much less random sequence \(em in fact, the low dozen bits
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generated by rand go through a cyclic pattern.
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All the bits generated by
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.Fn random
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are usable.
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For example,
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.Sq Li random()&01
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will produce a random binary
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value.
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.Pp
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Like
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.Xr rand 3 ,
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.Fn random
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will by default produce a sequence of numbers that can be duplicated
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by calling
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.Fn srandom
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with
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.Ql 1
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as the seed.
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.Pp
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The
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.Fn srandomdev
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routine initializes a state array using
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pseudo-random numbers obtained from the kernel.
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Note that this particular seeding
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procedure can generate states which are impossible to reproduce by
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calling
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.Fn srandom
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with any value, since the succeeding terms in the
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state buffer are no longer derived from the LC algorithm applied to
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a fixed seed.
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.Pp
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The
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.Fn initstate
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routine allows a state array, passed in as an argument, to be initialized
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for future use.
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The size of the state array (in bytes) is used by
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.Fn initstate
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to decide how sophisticated a random number generator it should use \(em the
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more state, the better the random numbers will be.
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(Current "optimal" values for the amount of state information are
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8, 32, 64, 128, and 256 bytes; other amounts will be rounded down to
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the nearest known amount.
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Using less than 8 bytes will cause an error.)
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The seed for the initialization (which specifies a starting point for
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the random number sequence, and provides for restarting at the same
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point) is also an argument.
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The
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.Fn initstate
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function
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returns a pointer to the previous state information array.
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.Pp
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Once a state has been initialized, the
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.Fn setstate
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routine provides for rapid switching between states.
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The
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.Fn setstate
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function
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returns a pointer to the previous state array; its
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argument state array is used for further random number generation
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until the next call to
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.Fn initstate
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or
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.Fn setstate .
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.Pp
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Once a state array has been initialized, it may be restarted at a
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different point either by calling
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.Fn initstate
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(with the desired seed, the state array, and its size) or by calling
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both
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.Fn setstate
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(with the state array) and
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.Fn srandom
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(with the desired seed).
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The advantage of calling both
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.Fn setstate
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and
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.Fn srandom
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is that the size of the state array does not have to be remembered after
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it is initialized.
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.Pp
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With 256 bytes of state information, the period of the random number
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generator is greater than
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.if t 2\u\s769\s10\d,
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.if n 2**69
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which should be sufficient for most purposes.
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.Sh DIAGNOSTICS
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If
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.Fn initstate
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is called with less than 8 bytes of state information, or if
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.Fn setstate
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detects that the state information has been garbled,
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NULL is returned.
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.Sh SEE ALSO
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.Xr arc4random 3 ,
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.Xr lrand48 3 ,
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.Xr rand 3 ,
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.Xr random 4
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.Sh HISTORY
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These
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functions appeared in
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.Bx 4.2 .
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.Sh AUTHORS
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.An Earl T. Cohen
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.Sh BUGS
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About 2/3 the speed of
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.Xr rand 3 .
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.Pp
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The historical implementation used to have a very weak seeding; the
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random sequence did not vary much with the seed.
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The current implementation employs a better pseudo-random number
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generator for the initial state calculation.
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