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MIT/GNU Scheme provides a facility for random number generation. The current implementation uses the ChaCha stream cipher, reseeding itself at each request so that past outputs cannot be distinguished from uniform random even if the state of memory is compromised in the future.
The interface described here is a mixture of the Common Lisp and SRFI 27 systems.
The argument m must be either an exact positive integer, or an inexact positive real.
random returns an
exact nonnegative integer below m with uniform distribution.
random returns an
inexact real in the interval [0, m) with uniform distribution.
If state is given and not #f, it must be a random-state
object; otherwise, it defaults to the default-random-source.
This object is used to maintain the state of the pseudorandom number
generator and is altered as a side effect of the random
procedure.
Use of the default random state requires synchronization between threads, so it is better for multithreaded programs to use explicit states.
(random 1.0) ⇒ .32744744667719056 (random 1.0) ⇒ .01668326768172354 (random 10) ⇒ 3 (random 10) ⇒ 8 (random 100) ⇒ 38 (random 100) ⇒ 63
State must be a random-state object.
Flo:random-unit-closed returns a flonum in the closed interval
[0,1] with uniform distribution.
In practical terms, the result is in the half-closed interval
(0,1] because the probability of returning 0 is
2^{-1075}, far below the standard probability 2^{-128}
that means “never” in cryptographic engineering terms.
Flo:random-unit-open is like flo:random-unit-closed, but
it explicitly rejects 0.0 and 1.0 as outputs, so that
the result is a floating-point number in the open interval
(0,1).
(flo:random-unit-open) is equivalent (random 1.), except
that it is faster.
Callers should generally use flo:random-unit-closed, because
for the uniform distribution on the interval [0,1] of real
numbers, the probability of a real number that is rounded to the
floating-point 1.0 is the small but nonnegligible
2^{-54}, and arithmetic downstream should be prepared to handle
results that are rounded to 1.0 much more readily than results
that are rounded to 0.0 — in other words, a requirement to
use flo:random-unit-open is evidence of bad numerics
downstream.
Deprecated alias for flo:random-unit-open.
Returns a bytevector of n bytes drawn independently uniformly at random from state.
Replaces the bytes in bytevector from start to end by bytes drawn independently uniformly at random from state.
The next three definitions concern random-state objects. In addition to
these definitions, it is important to know that random-state objects are
specifically designed so that they can be saved to disk using the
fasdump procedure, and later restored using the fasload
procedure. This allows a particular random-state object to be saved in
order to replay a particular pseudorandom sequence.
This variable is deprecated; pass an explicit state instead.
This procedure returns a new random-state object, suitable for use as
as the state argument to random. If state is not
given or #f, make-random-state returns a copy of
default-random-source. If state is a random-state
object, a copy of that object is returned. If state is
#t, then a new random-state object is returned that has been
“randomly” initialized by some means (such as by a time-of-day
clock).
Returns #t if object is a random-state object, otherwise
returns #f.
Export-random-state returns an external representation of a
random state — an object that can be safely read and written with
read and write, consisting only of nested lists,
vectors, symbols, and small exact integers.
Import-random-state creates a random state from its external
representation.
In the MIT/GNU Scheme implementation of the SRFI 27 API, a “random source” happens to be the same as a random state, but users should not rely on this.
[SRFI 27]
Returns a random source.
Every random source created by make-random-source returns the
same sequence of outputs unless modified by
random-source-state-set!, random-source-randomize!, or
random-source-pseudo-randomize!.
[SRFI 27]
Returns #t if object is a random source, otherwise
returns #f.
[SRFI 27]
The default random source, used by the various random procedures if no
explicit state is specified and *random-state* is false.
[SRFI 27]
Random-source-state-ref returns an external representation of a
random source — an object that can be safely read and written with
read and write, consisting only of nested lists,
vectors, symbols, and small exact integers.
Random-source-state-set! replaces the innards of source
by the source represented by exported-state from
random-source-state-ref.
[SRFI 27] Loads entropy from the environment into source so that its subsequent outputs are nondeterministic.
Warning: Most implementations of SRFI 27 do not make subsequent outputs unpredictable with cryptography, so don’t rely on this.
[SRFI 27] The arguments i and j must be exact nonnegative integers below 2^{128}. This procedure sets source to generate one of 2^{256} distinct possible streams of output, so that if i and j are chosen uniformly at random, it is hard to distinguish the outputs of the source from uniform random.
Warning: Most implementations of SRFI 27 do not make it hard to distinguish the outputs of the source from uniform random even if the indices i and j are uniform random, so don’t rely on this.
[SRFI 27] Returns an exact nonnegative integer below n chosen uniformly at random.
Equivalent to:
((random-source-make-integers default-random-source) n)
[SRFI 27] Returns an inexact real in the open interval (0, 1) with uniform distribution.
Equivalent to:
((random-source-make-reals default-random-source))
[SRFI 27] Returns a procedure of one argument, n, that deterministically draws from source a exact nonnegative integer below n with uniform distribution.
[SRFI 27] Returns a procedure of zero arguments that deterministically draws from source an inexact real in the interval (0,1) with uniform distribution. If unit is specified, the results are instead uniform random integral multiples of unit in (0,1) and of the same exactness as unit.
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