;; Uses PLT Scheme threads

#lang scheme/base
(require scheme/cmdline)

;; Each thread runs this loop:
(define (run id next)
  (let ([v (thread-receive)])
    (cond
     [(zero? v) ;; Done
      (printf "~a\n" id)
      (exit)]
     [else ;; Keep going
      (thread-send next (sub1 v))
      (run id next)])))
                       

(let ([n (command-line #:args (n) (string->number n))])
  ;; The original thread is #503. Create the rest:
  (let ([t1 (for/fold ([next (current-thread)])
                      ([id (in-range 502 0 -1)])
              (thread (lambda () (run id next))))])
    ;; Start:
    (thread-send t1 n)
    (run 503 t1)))