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     1 ;; Random number generators

     2 

     3 ;; by Konrad Hinsen

     4 ;; last updated May 3, 2009

     5 

     6 ;; Copyright (c) Konrad Hinsen, 2009. All rights reserved.  The use

     7 ;; and distribution terms for this software are covered by the Eclipse

     8 ;; Public License 1.0 (http://opensource.org/licenses/eclipse-1.0.php)

     9 ;; which can be found in the file epl-v10.html at the root of this

    10 ;; distribution.  By using this software in any fashion, you are

    11 ;; agreeing to be bound by the terms of this license.  You must not

    12 ;; remove this notice, or any other, from this software.

    13 

    14 (ns

    15   ^{:author "Konrad Hinsen"

    16      :doc "Random number streams

    17 

    18            This library provides random number generators with a common

    19            stream interface. They all produce pseudo-random numbers that are

    20            uniformly distributed in the interval [0, 1), i.e. 0 is a

    21            possible value but 1 isn't. For transformations to other

    22            distributions, see clojure.contrib.probabilities.monte-carlo.

    23 

    24            At the moment, the only generator provided is a rather simple

    25            linear congruential generator."}

    26   clojure.contrib.probabilities.random-numbers

    27   (:refer-clojure :exclude (deftype))

    28   (:use [clojure.contrib.types :only (deftype)])

    29   (:use [clojure.contrib.stream-utils :only (defstream)])

    30   (:use [clojure.contrib.def :only (defvar)]))

    31 

    32 ;; Linear congruential generator

    33 ;; http://en.wikipedia.org/wiki/Linear_congruential_generator

    34 

    35 (deftype ::lcg lcg

    36   "Create a linear congruential generator"

    37   {:arglists '([modulus multiplier increment seed])}

    38   (fn [modulus multiplier increment seed]

    39     {:m modulus :a multiplier :c increment :seed seed})

    40   (fn [s] (map s (list :m :a :c :seed))))

    41 

    42 (defstream ::lcg

    43   [lcg-state]

    44   (let [{m :m a :a c :c seed :seed} lcg-state

    45 	value (/ (float seed) (float m))

    46 	new-seed (rem (+ c (* a seed)) m)]

    47     [value (assoc lcg-state :seed new-seed)]))

    48 

    49 ;; A generator based on Clojure's built-in rand function

    50 ;; (and thus random from java.lang.Math)

    51 ;; Note that this generator uses an internal mutable state.

    52 ;;

    53 ;; The state is *not* stored in the stream object and can thus

    54 ;; *not* be restored!

    55 

    56 (defvar rand-stream (with-meta 'rand {:type ::rand-stream})

    57   "A random number stream based on clojure.core/rand. Note that this

    58    generator uses an internal mutable state. The state is thus not stored

    59    in the stream object and cannot be restored.")

    60 

    61 (defstream ::rand-stream

    62   [dummy-state]

    63   [(rand) dummy-state])
author	Robert McIntyre <rlm@mit.edu>
date	Sat, 21 Aug 2010 06:25:44 -0400
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