;; The first three lines of this file were inserted by DrRacket. They record metadata
;; about the language level of this file in a form that our tools can easily process.
#reader(lib "htdp-intermediate-lambda-reader.ss" "lang")((modname today-server) (read-case-sensitive #t) (teachpacks ()) (htdp-settings #(#t constructor repeating-decimal #f #t none #f ())))
(require 2htdp/universe)

; A Bundle is
;   (make-bundle Universe [Listof Mail] [Listof IWorld])
 
; A Mail is a (make-mail IWorld SExp)

;; Universe IWorld -> Bundle
(define (join u iw)
  (make-bundle (make-pair (pair-count u) (cons iw (pair-worlds u)))
               empty 
               empty))

;; [Listof IWorld] -> [Listof Mail]
(define (broadcast-reset iws)
  (map (λ (iw) (make-mail iw 'reset!)) iws))

;; Universe IWorld Msg -> Bundle
(define (process u iw msg)
  (cond [(> (pair-count u) 10)
         (make-bundle (make-pair 0 (pair-worlds u))
                      (broadcast-reset (pair-worlds u))
                      empty)]
        [else 
         (make-bundle (make-pair (add1 (pair-count u))
                                 (pair-worlds u))                                 
                      empty empty)]))


;; A Universe is a (make-pair Natural [Listof IWorld])
(define-struct pair (count worlds))

(universe (make-pair 0 empty)
          (on-new join)
          (on-msg process))