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Source for wiki RandomnessCommonLisp version 3
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cowan
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98.14.172.204
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RandomnessCommonLisp
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== Randomness in the style of Common Lisp ==
This proposal for random numbers is not a literal transcription of the Common Lisp interface, but it uses the same concepts, and the text is a heavily edited version of the CLHS. I have added `make-strong-random-source` to what is in CL.
A ''random-source object'' is an encapsulation of the state information used by a pseudo-random number generator. Its state, which is implementation-dependent, can be printed out and successfully read back in by the same implementation, but might not function correctly as a state in another implementation. Random-source objects are a disjoint type.
== Procedures ==
`(make-random-source)`
Constructs and returns a random-source object that has been randomly initialized by some implementation-defined means. The CL equivalent is `(make-random-state t)`.
`(make-strong-random-source)`
Constructs and returns a random-source object whose state has been randomly initialized by some implementation-defined means. This may be the same as `make-random-source`, but should if possible use a truly random source of bits (such as `/dev/random` on certain systems) to initialize its state. The CL equivalent is `(make-random-state t)`.
`(make-random-source-from-state `''state''`)`
Constructs and returns a random-source object whose state is a copy of ''state'', so that mutating ''state'' does not affect the random-source object. The result will generate the same sequence of random numbers that the original random-source object would have generated as of the time `random-source-state` was invoked on it. It is an error to pass a ''state'' object that has been mutated. There is no CL equivalent of this procedure, because CL `random-state` objects are themselves required to be printable and rereadable (they are typically CL `structs`).
`(random-source-state `''random-source''`)`
Returns an implementation-specific object representing a copy of the state encapsulated by ''random-source''. This object must be printable and rereadable. It must also be suitable for passing to `make-random-source-from-state`. Providing this mechanism makes it possible to save and reconstitute a random-source in a file or database, or to pass it across a network to an equivalent implementation. Mutating the result of this procedure does not affect ''random-source''. There is no CL equivalent of this procedure.
`(copy-random-source `''random-source''`)`
Constructs and returns a random-source object whose state is an independent copy of the state of ''random-source''. Calling this procedure is equivalent to `(make-random-source-from-state (random-source-state `''random-source''`))`, but potentially more efficient because it can avoid copying the state twice. The result and ''random-source'' will henceforth return the same sequence of values, allowing the same series of pseudo-random numbers to be generated many times within a single program. The CL equivalent is `make-random-state` with a random-state argument.
`(random-source? `''obj''`)`
Returns `#t` if ''obj'' is a random-source object, and `#f` otherwise. The CL equivalent is `random-state-p`.
`(current-random-source `[''random-source'']`)`
A parameter that returns or sets the default random-source object. Its initial value must be a random-source object, but is implementation-dependent. The CL equivalent is `*random-state*`.
`(random `''limit'' [''random-source'']`)`
Returns a pseudo-random number that is a non-negative number less than ''limit''. ''Limit'' must be an exact integer (in which case `random` returns an exact integer), or an inexact real number (in which case `random` returns an inexact real number).
If ''random-source'' is not specified, the value of `(current-random-source)` is used.
An approximately uniform choice distribution is used. If ''limit'' is an integer, each of the possible results occurs with (approximate) probability 1/''limit''.
The CL equivalent is `random`.
== Examples ==
{{{
(<= 0 (random 1000) 1000) => true
(let ((state1 (copy-random-source (current-random-source)))
(state2 (copy-random-source (current-random-source))))
(= (random 1000 state1) (random 1000 state2))) => true
}}
time
2010-11-03 23:57:19
version
3