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(random-integer
n)
produces the next random integer in {0, ..., n-1} and
(random-real)
produces the next random
real number between zero and one.
The details of how these random values are produced may not be
very relevant, as long as they appear to be sufficiently random.
Once random sources provide the infrastructure to obtain random bits, these can be used to construct other random deviates. Most important are floating point numbers of various distributions and random discrete structures, such as permutations or graphs. As there is an essentially unlimited number of such objects (with limited use elsewhere), we do not include them in this SRFI. In other words, this SRFI is not about making all sorts of random objects---it is about obtaining random bits in a portable, flexible, reliable, and efficient way.
The design aims at sufficient flexibility to cover the usage patterns of many applications as diverse as discrete structures, numerical simulations, and cryptographic protocols. At the same time, the interface aims at simplicity, which is important for occasional use. As there is no "one size fits all" random number generator, the design necessarily represents some form of compromise between the needs of the various applications.
Although strictly speaking not part of the specification, the emphasis of this proposal is on high quality random numbers and on high performance. As the state of the art in pseudo random number generators is still advancing considerably, the choice of method for the reference implementation should essentially be considered preliminary.
(random-integer
n) ->
x
default-random-source
.
Subsequent results of this procedure appear to be independent
uniformly distributed over the range {0, ..., n-1}.
The argument n must be a positive integer,
otherwise an error is signalled.
(random-real) ->
x
default-random-source
.
Subsequent results of this procedure appear to be
independent uniformly distributed.
The numerical type of the results and the
quantization of the output range depend on the implementation;
refer to random-source-make-reals
for details.
default-random-source
random-integer
and
random-real
have been derived using
random-source-make-integers
and
random-source-make-reals
.
Note that an assignment to default-random-source
does not change random
or random-real
;
it is also strongly recommended not to assign a new value.
(make-random-source) ->
s
make-random-source
represents a deterministic stream of random bits generated
by some form of pseudo random number generator.
Each random source obtained as (make-random-source)
generates the same stream of values, unless the state is modified
with one of the procedures below.
(random-source?
obj) ->
bool
(random-source-state-ref
s) ->
state
(random-source-state-set!
s
state)
random-source-state-set!
.
It is, however, required that a state possess an external
representation.
(random-source-randomize!
s)
(random-source-pseudo-randomize!
s i j)
random-source-randomize!
,
this procedure is entirely deterministic.
(random-source-make-integers
s) ->
rand
If an application obtains and uses several generators for the same random source s, a call to any of these generators advances the state of s. Hence, the generators do not produce the same sequence of random integers each but rather share a state. This also holds for all other types of generators derived from a fixed random sources. Implementations that support concurrency make sure that the state of a generator is properly advanced.
(random-source-make-reals
s) ->
rand
(random-source-make-reals
s
unit) ->
rand
The optional parameter unit determines the type of numbers being produced by rand and the quantization of the output. Unit must be a number such that 0 < unit < 1. The numbers created by rand are of the same numerical type as unit and the potential output values are spaced by at most unit. One can imagine rand to create numbers as x*unit where x is a random integer in {1, ..., floor(1/unit)-1}. Note, however, that this need not be the way the values are actually created and that the actual resolution of rand can be much higher than unit. In case unit is absent it defaults to a reasonably small value (related to the width of the mantissa of an efficient number format).
random-integer
and
random-real
?random-integer
and random-real
.
In addition, is saves the user from the pitfall of changing
the range with a simple modulo
-computation
which may substantially reduce the quality of the
numbers being produced.The design of the interface is based on three prototype applications:
random-integer
accepts a range argument n in every call.
The implementation should try to avoid boxing/unboxing of values
if the ranges fit into immediate integers.
float
and double
representations.
Therefore,
random-real
does not accept any parameters but
the procedure random-source-make-reals
creates
a properly configured random-real
procedure.
random-real
?random-real
does not return
x = 0 or x = 1 in order to allow
(log
x)
and
(log (- 1
x))
without the danger of a numerical exception.
For this reason, my initial proposal was George Marsaglia's COMBO generator, which is the combination of a 32-bit multiplicative lagged Fibonacci-generator with a 16-bit multiply with carry generator. The COMBO generator passes all tests of Marsaglia's DIEHARD testsuite for random number generators and has a period of order 2^60.
As an improvement, Brad Lucier suggested suggested Pierre L'Ecuyer's MRG32k3a generator which is combination of two recursive generators of degree three, both of which fit into 54-bit arithmetics. The MRG32k3a generator also passes DIEHARD and in addition, has desireable spectral properties and a period in the order of 2^191. As a matter of fact, multiple recursive generators (MRGs) are theoretically much better understood than special constructions as the COMBO generator. This is the reason why the implementations provided here implements the MRG32k3a generator. When implemented in floating point arithmetics with sufficient mantissa-width, this generator is also very fast.
As performance is critical to many applications, one might want to implement the actual generator itself in native code. For this reason, I provide three different implementations of the backbone generator as a source of inspiration. See the code below.
The reference implementations below define a record
type to contain the exported procedures.
The actual state of the generator is stored in the
binding time environment of make-random-source
.
This has the advantage that access to the state is fast
even if the record type would be slow (which need not be
the case).
random-source-randomize!
as
it needs access to a real entropy source.A reasonable choice for such as source is to use the system clock in order to obtain a value for randomization, for example in the way John David Stone recommends (see reference below). This is good enough for most applications with the notable exception of security related programs. One way to obtain the time in Scheme is to use SRFI-19.
For the reference implementation, a relatively large part
of the code deals with the more advanced features of the
MRG32k3a generator,
in particular random-source-pseudo-randomize!
.
This code is inspired by Pierre L'Ecuyer's own implementation
of the MRG32k3a generator.
Another part of this generic code deals with changing the range and quantization of the random numbers and with error checking to detect common mistakes and abuses.
(random-integer 2)
can be computed about x
times a second and (random-real)
about y times a second.
The implementations are
integer
only.
This implementation aims at portability, not at performance
(30000 ints/s, 3000/s reals/s).
(double)
-arithmetics.
The generator is made available in Scheme 48 via the
C/Scheme
interface.
The performance of this generator is good
(160000 ints/s, 180000 reals/s).
flonum
and
54-bit integer
.
This code is inspired by a program by Brad Lucier as
posted
to the discussion archive of this SRFI.
The performance of this generator is good when compiled
(5000 ints/s, 25000/s reals/s when interpreted,
200000 ints/s, 400000/s reals/s when compiled;
see acknowledgements).
vector
of length n for the images of the points.
Observe that the implementation first defines the procedure
random-source-make-permutations
to
turn a random source s into a procedure to generate
permutations of given degree n.
In a second step, this is applied to the default source
to define a ready-to-use procedure for permutations:
(random-permutation
n)
constructs a random permutation of degree n.
(define (random-source-make-permutations s)
(let ((rand (random-source-make-integers s)))
(lambda (n)
(let ((x (make-vector n 0)))
(do ((i 0 (+ i 1)))
((= i n))
(vector-set! x i i))
(do ((k n (- k 1)))
((= k 1) x)
(let* ((i (- k 1))
(j (rand k))
(xi (vector-ref x i))
(xj (vector-ref x j)))
(vector-set! x i xj)
(vector-set! x j xi)))))))
(define random-permutation
(random-source-make-permutations default-random-source))
For the algorithm refer to Knuth's "The Art of Computer Programming",
Vol. II, 2nd ed., Algorithm P of Section 3.4.2.
random-source-make-reals
.
(define (random-source-make-exponentials s . unit)
(let ((rand (apply random-source-make-reals s unit)))
(lambda (mu)
(- (* mu (log (rand)))))))
(define random-exponential
(random-source-make-exponentials default-random-source))
The algorithm is folklore. Refer to Knuth's "The Art of Computer
Programming", Vol. II, 2nd ed., Section 3.4.1.D.
The technical difficulty of the interface addressed here
is that the polar method generates two results per computation.
We return one of the result and store the second one to be
returned by the next call to the procedure.
Note that this implies that random-source-state-set!
(and the other procedures modifying the state) does not necessarily
affect the output of random-normal
immediately!
(define (random-source-make-normals s . unit)
(let ((rand (apply random-source-make-reals s unit))
(next #f))
(lambda (mu sigma)
(if next
(let ((result next))
(set! next #f)
(+ mu (* sigma result)))
(let loop ()
(let* ((v1 (- (* 2 (rand)) 1))
(v2 (- (* 2 (rand)) 1))
(s (+ (* v1 v1) (* v2 v2))))
(if (>= s 1)
(loop)
(let ((scale (sqrt (/ (* -2 (log s)) s))))
(set! next (* scale v2))
(+ mu (* sigma scale v1))))))))))
(define random-normal
(random-source-make-normals default-random-source))
For the algorithm refer to Knuth's "The Art of Computer Programming",
Vol. II, 2nd ed., Algorithm P of Section 3.4.1.C.
random flo:random-unit *random-state* make-random-state
random-state?
http://www.swiss.ai.mit.edu/projects/scheme/documentation/scheme_5.html#SEC53
(A mechanism to run a fixed unspecified generator.)
random *random-state* copy-random-state seed->random-state
make-random-state random:uniform random:exp random:normal-vector!
random-hollow-sphere! random:solid-sphere!
http://swiss.csail.mit.edu/~jaffer/slib_5.html#SEC108
(Uses RC-4.)
make-random make-random-vector
(Internal procedures of Scheme48; a fixed 28-bit generator.)
random random-seed current-pseudo-random-generator
make-pseudo-random-generator pseudo-random-generator?
http://download.plt-scheme.org/doc/200alpha1/html/mzscheme/mzscheme-Z-H-3.html#%_idx_144
(A mechanism to run a generator and to exchange the generator.)
rand
-example shows a textbook way to define a
random number generator.)
Copyright (C) Sebastian Egner (2002). All Rights Reserved.
Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
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