#lang scheme


(define-struct robot
	(continuation continuation-tag) #:mutable)

(define (create-robot)
	(make-robot (void) (make-continuation-prompt-tag)))


(define (step-robots! robots)
	
	(for ((robot (in-list robots)))
		(cond
			((void? (robot-continuation robot))
				(start-robot! robot))
			((continuation? (robot-continuation robot))
				(enter-robot! robot))
			(else
				(display 'died)
				(newline))))
	
	(step-robots! robots))

(define (start-robot! robot)
	
	(define continuation-tag (robot-continuation-tag robot))
	
	(set-robot-continuation! robot #f)
	(set-robot-continuation! robot
		(call-with-continuation-prompt
			(lambda ()
				(call-with-current-continuation
					(lambda (new-framework-continuation)
						(set-robot-continuation! robot new-framework-continuation)
						(exit-robot! robot)
						(do-robot! robot)
						(done-robot! robot))
					continuation-tag))
			continuation-tag))
	
	(void))

(define (enter-robot! robot)
	
	(define continuation-tag (robot-continuation-tag robot))
	(define old-behaviour-continuation (robot-continuation robot))
	
	(set-robot-continuation! robot #f)
	(set-robot-continuation! robot
		(call-with-continuation-prompt
			(lambda ()
				(call-with-current-continuation
					old-behaviour-continuation
					continuation-tag))
			continuation-tag))
	
	(void))

(define (exit-robot! robot)
	
	(define continuation-tag (robot-continuation-tag robot))
	(define old-framework-continuation (robot-continuation robot))
	
	(set-robot-continuation! robot #f)
	(set-robot-continuation! robot
		(call-with-current-continuation
			old-framework-continuation
			continuation-tag))
	
	(void))

(define (done-robot! robot)
	
	(define old-framework-continuation (robot-continuation robot))
	
	(set-robot-continuation! robot #f)
	
	(old-framework-continuation #f))

(define (do-robot! robot (steps 10000))
	(unless (zero? steps)
		(exit-robot! robot)
		(do-robot! robot (- steps 1))))


(define robots (list (create-robot) (create-robot) (create-robot)))

(step-robots! robots)