sicp-ex-3.81

  
  
  
  
 (define (random-update x) 
         (remainder (+ (* 13 x) 5) 24)) 
 (define random-init (random-update (expt 2 32))) 
  
 ;; assume the operation 'generator and 'reset is a stream,  
 ;; and if the command is 'generator, the element of 
 ;; stream is a string, if the command is 'reset,  
 ;; it is a pair whose first element is 'reset,  
 ;; the other element is the reset value. 
 (define (random-number-generator command-stream) 
         (define random-number 
                 (cons-stream random-init 
                                 (stream-map (lambda (number command)  
                                                                 (cond ((null? command) the-empty-stream) 
                                                                           ((eq? command 'generator) 
                                                                            (random-update number)) 
                                                                           ((and (pair? command)  
                                                                                    (eq? (car command) 'reset)) 
                                                                            (cdr command)) 
                                                                           (else  
                                                                              (error "bad command -- " commmand)))) 
                                                          random-number 
                                                          command-stream))) 
         random-number)