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;; Random number generators
;; by Konrad Hinsen
;; last updated May 3, 2009
;; Copyright (c) Konrad Hinsen, 2009. All rights reserved. The use
;; and distribution terms for this software are covered by the Eclipse
;; Public License 1.0 (http://opensource.org/licenses/eclipse-1.0.php)
;; which can be found in the file epl-v10.html at the root of this
;; distribution. By using this software in any fashion, you are
;; agreeing to be bound by the terms of this license. You must not
;; remove this notice, or any other, from this software.
(ns
#^{:author "Konrad Hinsen"
:doc "Random number streams
This library provides random number generators with a common
stream interface. They all produce pseudo-random numbers that are
uniformly distributed in the interval [0, 1), i.e. 0 is a
possible value but 1 isn't. For transformations to other
distributions, see clojure.contrib.probabilities.monte-carlo.
At the moment, the only generator provided is a rather simple
linear congruential generator."}
clojure.contrib.probabilities.random-numbers
(:refer-clojure :exclude (deftype))
(:use [clojure.contrib.types :only (deftype)])
(:use [clojure.contrib.stream-utils :only (defstream)])
(:use [clojure.contrib.def :only (defvar)]))
;; Linear congruential generator
;; http://en.wikipedia.org/wiki/Linear_congruential_generator
(deftype ::lcg lcg
"Create a linear congruential generator"
{:arglists '([modulus multiplier increment seed])}
(fn [modulus multiplier increment seed]
{:m modulus :a multiplier :c increment :seed seed})
(fn [s] (map s (list :m :a :c :seed))))
(defstream ::lcg
[lcg-state]
(let [{m :m a :a c :c seed :seed} lcg-state
value (/ (float seed) (float m))
new-seed (rem (+ c (* a seed)) m)]
[value (assoc lcg-state :seed new-seed)]))
;; A generator based on Clojure's built-in rand function
;; (and thus random from java.lang.Math)
;; Note that this generator uses an internal mutable state.
;;
;; The state is *not* stored in the stream object and can thus
;; *not* be restored!
(defvar rand-stream (with-meta 'rand {:type ::rand-stream})
"A random number stream based on clojure.core/rand. Note that this
generator uses an internal mutable state. The state is thus not stored
in the stream object and cannot be restored.")
(defstream ::rand-stream
[dummy-state]
[(rand) dummy-state])
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