#lang racket

(require 2htdp/image)
(require 2htdp/universe)

;; structure used
(struct crcl (radius color))

;; constants
(define SIZE 500)
(define COUNT 5)
(define CENTER (/ SIZE 2))
(define RADIUS (/ (* SIZE (sqrt 1/2)) COUNT))
(define FPS 60)
(define STEP (/ RADIUS FPS))
(define BACKGROUND (square SIZE 'solid 'white))

;; creates a new crcl struct with a random color
;; num -> crcl
(define (new-circle r)
  (crcl (* r RADIUS)  (color (random 256) (random 256) (random 256))))

;; renders a list of circle structs
;; list-of-crcl -> image
(define (render circle-list)
  (foldr (lambda (x y) (place-image (circle (crcl-radius x) 'solid (crcl-color x)) CENTER CENTER y))
         BACKGROUND circle-list))

;; moves the circles on each tick (and creates/deletes ones when necessary)
;; list-of-crcls -> list-of-crcls
(define (move-circles crcls)
  (if (>= (crcl-radius (first crcls)) RADIUS) (cons (new-circle 0) (drop-right crcls 1))
      (map (lambda (x) (struct-copy crcl x [radius (+ STEP (crcl-radius x))])) crcls)))

;; big-bang function call that runs this infinitely
(big-bang (build-list (add1 COUNT) new-circle)
  [name "Circles"]
  [on-tick move-circles (/ 1 FPS)]
  [to-draw render])