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+;; Random number generators
+
+;; by Konrad Hinsen
+;; last updated April 16, 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 clojure.contrib.probabilities.random-numbers
+  "Random number streams
+
+   This library provides various 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."
+  (: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])