;;; rand.scm
;;; Created by Robert R. Snapp, Copyright (C) 2004
;;;
;;; This file contains pseudo-random number generators that use the linear congruence method:
;;;
;;;    x' = (a*x + b) mod m
;;;

;; The random number generator rand is implemented using "global variables"


(define a 123456789) ; a global definition
(define b 9874923843) ; a global definition
(define m 10000000) ; a global definition
(define x 234987)    ; another global definition

(define (rand)
    (set! x (remainder (+ (* a x) b) m))  ; remainder impliments modulo division
     x)                              ; now return the new random number


;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

;; The random number generator newrand uses "local variables" using a device known as a "closure"

(define newrand '*)  ; a "dummy" definition.

(let ((a 249834723)  ; let defines local variables
     (b 987438243)  ; another local definition
     (m 100000000)
     (x 234984734))
  (set! newrand      ; We redefine newrand so that it generates random numbers using the local variables.
     (lambda ()
       (set! x (remainder (+ (* a x) b) m))
       x)))