; intersecting number wheels
 
(define-syntax define-generator
  (lambda (x)
    (syntax-case x (lambda)
      ((stx name (lambda formals e0 e1 ...))
         (with-syntax ((yield (datum->syntax (syntax stx) 'yield)))
           (syntax (define name
             (lambda formals
               (let ((resume #f) (return #f))
                 (define yield
                   (lambda args
                     (call-with-current-continuation
                      (lambda (cont)
                        (set! resume cont)
                        (apply return args)))))
                 (lambda ()
                   (call-with-current-continuation
                    (lambda (cont)
                      (set! return cont)
                      (cond (resume (resume))
                      (else (let () e0 e1 ...)
                            (error 'name "unexpected return"))))))))))))
        ((stx (name . formals) e0 e1 ...)
          (syntax (stx name (lambda formals e0 e1 ...)))))))
 
(define (cycle xs)
  (define (last-pair xs)
    (if (null? (cdr xs)) xs
      (last-pair (cdr xs))))
  (set-cdr! (last-pair xs) xs) xs)
 
(define (take-gen n gen)
  (let loop ((n n) (gs (list)))
    (if (zero? n) (reverse gs)
      (loop (- n 1) (cons (gen) gs)))))
 
(define-generator (wheel xs)
  (let loop ((ws (cycle xs)))
    (if (integer? (car ws))
        (yield (car ws))
        (yield ((eval (car ws)))))
    (loop (cdr ws))))
 
(define a (wheel '(1 2 3)))
(display (take-gen 20 a)) (newline)
 
(define a (wheel '(1 b 2)))
(define b (wheel '(3 4)))
(display (take-gen 20 a)) (newline)
 
(define a (wheel '(1 d d)))
(define d (wheel '(6 7 8)))
(display (take-gen 20 a)) (newline)
 
(define a (wheel '(1 b c)))
(define b (wheel '(3 4)))
(define c (wheel '(5 b)))
(display (take-gen 20 a)) (newline)

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