snake.rkt

#lang racket
(require 2htdp/universe)
(require 2htdp/image)
(require lang/posn)

(define speed 5)

(define (create-snake-scene snake)
  (place-images (create-rectangles (length snake)) (create-positions snake) (rectangle 500 500 "solid" "white")))

(define (create-positions l)
  (map (lambda (x) (make-posn (car x) (cadr x))) l))

(define (create-rectangles n)
  (build-list n (lambda (x) (rectangle 10 10 "solid" "green"))))

(define snake0 (list (list 40 250 'e) (list 30 250 'e) (list 20 250 'e) (list 10 250 'e) (list 0 250 'e)))

(define (update snake)
  (let ([fst (first snake)])
    (cons (create-dir-snake-piece fst (caddr fst)) (take snake (- (length snake) 1)))))

(define (update-dir snake dir)
  (cons (create-dir-snake-piece (first snake) dir) (take snake (- (length snake) 1))))

(define (create-dir-snake-piece l dir)
  (let ([x (car l)]
        [y (cadr l)])
    (case dir
      ['e (list (+ x speed) y dir)]
      ['w (list (- x speed) y dir)]
      ['s (list x (+ y speed) dir)]
      ['n (list x (- y speed) dir)]
 )))

(define (change-dir snake key)
  (cond
    [(key=? key "left") (update-dir snake 'w)]
    [(key=? key "right")(update-dir snake 'e)]
    [(key=? key "up")(update-dir snake 'n)]
    [(key=? key "down")(update-dir snake 's)]))

(big-bang snake0 (to-draw create-snake-scene) (on-tick update) (on-key change-dir))