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

;;; SNAKE WORLD 

(define-struct world (snake food))
(define-struct snake (dir segs))

;;; A World is a (make-world Snake Food).
;;; Food = Posn
;;; A Snake is a (make-snake Direction Segs)
;;;   A snake's Segs may not be empty.
;;;   The first element of the list is the head.
;;; Direction is one of: 'up 'down 'left 'right
;;; Segs is one of:
;;;  -- empty
;;;  -- (cons Posn Segs)
;;; Coordinates are in "grid" units, with X running left-to-right,
;;; and Y running bottom-to-top.

;;; Start with the world, when you make up a wish list,
;;; the others will fall out. Organise wish list into groups
;;; of functions.

;;; Rendering subystem
;;;   world->scene : World -> Scene
;;;   food+scene   : Food Scene -> Scene 
;;;   snake+scene  : Snake Scene -> Scene

;;; Other big-bang handlers:
;;;   next-world   : World -> World
;;;   key-handler  : World Key-event -> World

;;; World simulation:
;;;   snake-slither : Snake -> Snake
;;;   new-direction : Snake Direction -> Snake
;;;   snake-eat : World -> World
;;;   snake-grow : Snake -> Snake

;;; Collision detection of various kinds:
;;;   snake-dead? : Snake -> Boolean
;;;     snake-self-collide? : Snake -> Boolean
;;;     snake-wall-collide? : Snake -> Boolean
;;;   snake-eating? : World -> Boolean

;; Add purpose statements. Then pick a place and start. 

;; --- CONSTANTS : DESCRIBE PROPERTIES THAT ARE ALWAYS THE SAME 

(define GRID-SIZE 10) ; width of a game-board square
(define BOARD-HEIGHT 20) ; height in grid squares
(define BOARD-WIDTH  30) ; width  in grid squares
(define BOARD-HEIGHT-PIXELS (* GRID-SIZE BOARD-HEIGHT))
(define BOARD-WIDTH-PIXELS  (* GRID-SIZE BOARD-WIDTH))

(define BACKGROUND (empty-scene BOARD-WIDTH-PIXELS BOARD-HEIGHT-PIXELS))

(define SEGMENT-RADIUS (quotient GRID-SIZE 2))
(define SEGMENT-IMAGE  (circle SEGMENT-RADIUS 'solid 'red))
(define FOOD-RADIUS (floor (* 0.9 SEGMENT-RADIUS)))
(define FOOD-IMAGE  (circle FOOD-RADIUS 'solid 'green))

(define Snake1 (make-snake 'right (cons (make-posn 5 3) empty)))
(define Food1  (make-posn 8 12))
(define World1 (make-world Snake1 Food1))
(define World2 (make-world Snake1 (make-posn 5 3))) ; An eating scenario

;; --- FUNCTIONS 

;;; Image-painting functions
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

;;; place-image/grid Image Number Number Scene
;;; Just like PLACE-IMAGE, but use grid coordinates.
(define (place-image-on-grid img x y scene)
  (place-image img
               (* GRID-SIZE x)
	       ;; We have to round-off because of the 1/2:
               (round (- BOARD-HEIGHT-PIXELS 
			 (* GRID-SIZE (+ 1/2 y))))
               scene))

(check-expect (place-image-on-grid FOOD-IMAGE 10 10 BACKGROUND)
	      (place-image FOOD-IMAGE 
			   (* 10 GRID-SIZE)
			   (- BOARD-HEIGHT-PIXELS (* GRID-SIZE 21/2))
			   BACKGROUND))