8 files changed, 657 insertions, 0 deletions
diff --git a/chapter6/resources/body.gif b/chapter6/resources/body.gif
new file mode 100644
index 0000000..94a0956
--- /dev/null
+++ b/chapter6/resources/body.gif
diff --git a/chapter6/resources/goo-red.gif b/chapter6/resources/goo-red.gif
new file mode 100644
index 0000000..bf767b1
--- /dev/null
+++ b/chapter6/resources/goo-red.gif
diff --git a/chapter6/resources/goo.gif b/chapter6/resources/goo.gif
new file mode 100644
index 0000000..cb0d98b
--- /dev/null
+++ b/chapter6/resources/goo.gif
diff --git a/chapter6/resources/head.gif b/chapter6/resources/head.gif
new file mode 100644
index 0000000..664f679
--- /dev/null
+++ b/chapter6/resources/head.gif
diff --git a/chapter6/resources/obstacle.gif b/chapter6/resources/obstacle.gif
new file mode 100644
index 0000000..6ff288e
--- /dev/null
+++ b/chapter6/resources/obstacle.gif
diff --git a/chapter6/resources/tail.gif b/chapter6/resources/tail.gif
new file mode 100644
index 0000000..6fbd317
--- /dev/null
+++ b/chapter6/resources/tail.gif
diff --git a/chapter6/snake.rkt b/chapter6/snake.rkt
new file mode 100644
index 0000000..c57b01c
--- /dev/null
+++ b/chapter6/snake.rkt
@@ -0,0 +1,295 @@
+#lang racket
+(require 2htdp/universe 2htdp/image)
+
+;; data
+(struct pit (snake goos obstacles dinged))
+(struct snake (dir segs))
+(struct goo (loc expire type))
+(struct obstacle (loc expire))
+(struct posn (x y))
+
+;; constants
+(define TICK-RATE 1/10)
+
+(define SIZE 30)
+
+(define SEG-SIZE 15)
+
+(define EXPIRATION-TIME 150)
+(define OBSTACLE-EXPIRATION-TIME 250)
+
+(define WIDTH-PX (* SEG-SIZE 30))
+(define HEIGHT-PX (* SEG-SIZE 30))
+
+(define MT-SCENE (empty-scene WIDTH-PX HEIGHT-PX))
+(define GOO-IMG (bitmap "resources/goo.gif"))
+(define GOO-RED-IMG (bitmap "resources/goo-red.gif"))
+(define OBSTACLE-IMG (bitmap "resources/obstacle.gif"))
+(define SEG-IMG (bitmap "resources/body.gif"))
+(define HEAD-IMG (bitmap "resources/head.gif"))
+
+(define HEAD-LEFT-IMG HEAD-IMG)
+(define HEAD-DOWN-IMG (rotate 90 HEAD-LEFT-IMG))
+(define HEAD-RIGHT-IMG (flip-horizontal HEAD-LEFT-IMG))
+(define HEAD-UP-IMG (flip-vertical HEAD-DOWN-IMG))
+
+(define ENDGAME-TEXT-SIZE 15)
+
+;; main
+(define (start-snake)
+  (big-bang (pit (snake "right" (list (posn 1 1)))
+                 (list (fresh-goo)
+                       (fresh-goo)
+                       (fresh-goo)
+                       (fresh-goo)
+                       (fresh-goo)
+                       (fresh-goo))
+                 (list (fresh-obstacle)
+                       (fresh-obstacle))
+                 0)
+            (on-tick next-pit TICK-RATE)
+            (on-key direct-snake)
+            (to-draw render-pit)
+            (stop-when dead? render-end)))
+
+(define (next-pit w)
+  (define snake (pit-snake w))
+  (define goos (pit-goos w))
+  (define obstacles (pit-obstacles w))
+  (define dinged (pit-dinged w))
+  (define goo-to-eat (can-eat snake goos))
+  (if goo-to-eat
+      (pit (grow-size snake (goo-type goo-to-eat))
+           (age-goo (eat goos goo-to-eat))
+           (age-obstacle obstacles) (+ 1 dinged))
+      (pit (slither snake)
+           (age-goo goos)
+           (age-obstacle obstacles) dinged)))
+
+(define (direct-snake w ke)
+  (cond [(dir? ke) (world-change-dir w ke)]
+        [else w]))
+
+(define (render-pit w)
+  (snake+scene (pit-snake w)
+               (goo-list+scene (pit-goos w)
+                               (obstacle-list+scene
+                                (pit-obstacles w) MT-SCENE))))
+
+(define (dead? w)
+  (define snake (pit-snake w))
+  (or (self-colliding? snake)
+      (wall-colliding? snake)
+      (obstacle-colliding? snake (pit-obstacles w))))
+
+(define (render-end w)
+  (overlay (above (text "Game Over" ENDGAME-TEXT-SIZE "black")
+                  (text (string-append "You dinged "
+                                       (number->string (pit-dinged w))
+                                       " goos.")
+                        ENDGAME-TEXT-SIZE "black"))
+           (render-pit w)))
+
+
+;; clock
+(define (can-eat snake goos)
+  (cond [(empty? goos) #f]
+        [else (if (close? (snake-head snake) (first goos))
+                  (first goos)
+                  (can-eat snake (rest goos)))]))
+
+(define (eat goos goo-to-eat)
+  (append (list (fresh-goo)) (remove goo-to-eat goos)))
+
+(define (close? s g)
+  (posn=? s (goo-loc g)))
+
+(define (grow-size sn size)
+  (cond [(= size 0) sn]
+        [else (grow-size (grow sn) (- size 1))]))
+
+(define (grow sn)
+  (snake (snake-dir sn)
+         (cons (next-head sn) (snake-segs sn))))
+
+(define (slither sn)
+  (snake (snake-dir sn)
+         (cons (next-head sn) (all-but-last (snake-segs sn)))))
+
+(define (next-head sn)
+  (define head (snake-head sn))
+  (define dir (snake-dir sn))
+  (cond [(string=? dir "up") (posn-move head 0 -1)]
+        [(string=? dir "down") (posn-move head 0 1)]
+        [(string=? dir "left") (posn-move head -1 0)]
+        [(string=? dir "right") (posn-move head 1 0)]))
+
+(define (posn-move p dx dy)
+  (posn (+ (posn-x p) dx)
+        (+ (posn-y p) dy)))
+
+(define (all-but-last segs)
+  (cond [(empty? (rest segs)) empty]
+        [else (cons (first segs) (all-but-last (rest segs)))]))
+
+(define (age-goo goos)
+  (rot (renew goos)))
+
+(define (renew goos)
+  (cond [(empty? goos) empty]
+        [(rotten? (first goos))
+         (append (fresh-goos) (renew (rest goos)))]
+        [else
+         (append (list (first goos)) (renew (rest goos)))]))
+
+(define (rot goos)
+  (cond [(empty? goos) empty]
+        [else (cons (decay (first goos)) (rot (rest goos)))]))
+
+(define (rotten? g)
+  (zero? (goo-expire g)))
+
+(define (decay g)
+  (goo (goo-loc g) (sub1 (goo-expire g)) (goo-type g)))
+
+(define (fresh-goo)
+  (goo (posn (add1 (random (sub1 SIZE)))
+             (add1 (random (sub1 SIZE))))
+       EXPIRATION-TIME
+       (random 1 3)))
+
+(define (fresh-goos)
+  (define (gen-goos n)
+    (cond [(= n 0) empty]
+          [else (cons (fresh-goo) (gen-goos (- n 1)))]))
+  (let ((n (random 3)))
+    (gen-goos n)))
+
+(define (age-obstacle obstacles)
+  (rot-obstacles (renew-obstacles obstacles)))
+
+(define (renew-obstacles obstacles)
+  (cond [(empty? obstacles)  empty]
+        [(obstacle-expired? (first obstacles))
+         (cons (fresh-obstacle) (renew-obstacles (rest obstacles)))]
+        [else
+         (cons (first obstacles) (renew-obstacles (rest obstacles)))]))
+
+(define (rot-obstacles obstacles)
+  (cond [(empty? obstacles) empty]
+        [else (cons (decay-obstacle (first obstacles))
+                    (rot-obstacles (rest obstacles)))]))
+
+(define (obstacle-expired? obs)
+  (zero? (obstacle-expire obs)))
+
+(define (decay-obstacle obs)
+  (obstacle (obstacle-loc obs) (sub1 (obstacle-expire obs))))
+
+(define (fresh-obstacle)
+  (obstacle (posn (add1 (random (sub1 SIZE)))
+                  (add1 (random (sub1 SIZE))))
+            OBSTACLE-EXPIRATION-TIME))
+
+;; keys
+(define (dir? x)
+  (or (key=? x "up")
+      (key=? x "down")
+      (key=? x "left")
+      (key=? x "right")))
+
+(define (world-change-dir w d)
+  (define the-snake (pit-snake w))
+  (cond [(and (opposite-dir? (snake-dir the-snake) d)
+              (cons? (rest (snake-segs the-snake))))
+         (stop-with w)]
+        [else
+         (pit (snake-change-dir the-snake d)
+              (pit-goos w)
+              (pit-obstacles w)
+              (pit-dinged w))]))
+
+(define (opposite-dir? d1 d2)
+  (cond [(string=? d1 "up") (string=? d2 "down")]
+        [(string=? d1 "down") (string=? d2 "up")]
+        [(string=? d1 "left") (string=? d2 "right")]
+        [(string=? d1 "right") (string=? d2 "left")]))
+
+
+;; render
+(define (snake+scene snake scene)
+  (define snake-body-scene
+    (img-list+scene (snake-body snake) SEG-IMG scene))
+  (define dir (snake-dir snake))
+  (img+scene (snake-head snake)
+             (cond [(string=? "up" dir) HEAD-UP-IMG]
+                   [(string=? "down" dir) HEAD-DOWN-IMG]
+                   [(string=? "left" dir) HEAD-LEFT-IMG]
+                   [(string=? "right" dir) HEAD-RIGHT-IMG])
+             snake-body-scene))
+
+(define (goo-list+scene goos scene)
+  (define (get-posns-from-goo goos type)
+    (cond [(empty? goos) empty]
+          [(= (goo-type (first goos)) type)
+           (cons (goo-loc (first goos))
+                 (get-posns-from-goo (rest goos) type))]
+          [else (get-posns-from-goo (rest goos) type)]))
+  (img-list+scene (get-posns-from-goo goos 1) GOO-IMG
+                  (img-list+scene (get-posns-from-goo goos 2)
+                                  GOO-RED-IMG scene)))
+
+(define (obstacle-list+scene obstacles scene)
+  (define (get-posns-from-obstacle obstacles)
+    (cond [(empty? obstacles) empty]
+          [else (cons (obstacle-loc (first obstacles))
+                      (get-posns-from-obstacle (rest obstacles)))]))
+  (img-list+scene (get-posns-from-obstacle obstacles)
+                  OBSTACLE-IMG scene))
+
+(define (img-list+scene posns img scene)
+  (cond [(empty? posns) scene]
+        [else (img+scene
+               (first posns)
+               img
+               (img-list+scene (rest posns) img scene))]))
+
+(define (img+scene posn img scene)
+  (place-image img
+               (* (posn-x posn) SEG-SIZE)
+               (* (posn-y posn) SEG-SIZE)
+               scene))
+
+
+;; end game
+(define (self-colliding? snake)
+  (cons? (member (snake-head snake) (snake-body snake))))
+
+(define (wall-colliding? snake)
+  (define x (posn-x (snake-head snake)))
+  (define y (posn-y (snake-head snake)))
+  (or (= 0 x) (= x SIZE)
+      (= 0 y) (= y SIZE)))
+
+(define (obstacle-colliding? snake obstacles)
+  (cond [(empty? obstacles) #f]
+        [(posn=? (snake-head snake)
+                 (obstacle-loc (first obstacles))) #t]
+        [else (obstacle-colliding? snake (rest obstacles))]))
+
+;; aux
+(define (posn=? p1 p2)
+  (and (= (posn-x p1) (posn-x p2))
+       (= (posn-y p1) (posn-y p2))))
+
+(define (snake-head sn)
+  (first (snake-segs sn)))
+
+(define (snake-body sn)
+  (rest (snake-segs sn)))
+
+(define (snake-tail sn)
+  (last (snake-segs sn)))
+
+(define (snake-change-dir sn d)
+  (snake d (snake-segs sn)))
diff --git a/chapter6/snakes.rkt b/chapter6/snakes.rkt
new file mode 100644
index 0000000..dae3468
--- /dev/null
+++ b/chapter6/snakes.rkt
@@ -0,0 +1,362 @@
+#lang racket
+(require 2htdp/universe 2htdp/image)
+
+;; data
+(struct pit (snake-1 snake-2 goos obstacles dinged))
+(struct snake (dir segs))
+(struct goo (loc expire type))
+(struct obstacle (loc expire))
+(struct posn (x y))
+
+;; constants
+(define TICK-RATE 1/10)
+
+(define SIZE 30)
+
+(define SEG-SIZE 15)
+
+(define EXPIRATION-TIME 150)
+(define OBSTACLE-EXPIRATION-TIME 250)
+
+(define WIDTH-PX (* SEG-SIZE 30))
+(define HEIGHT-PX (* SEG-SIZE 30))
+
+(define MT-SCENE (empty-scene WIDTH-PX HEIGHT-PX))
+(define GOO-IMG (bitmap "resources/goo.gif"))
+(define GOO-RED-IMG (bitmap "resources/goo-red.gif"))
+(define OBSTACLE-IMG (bitmap "resources/obstacle.gif"))
+(define SEG-IMG (bitmap "resources/body.gif"))
+(define HEAD-IMG (bitmap "resources/head.gif"))
+
+(define HEAD-LEFT-IMG HEAD-IMG)
+(define HEAD-DOWN-IMG (rotate 90 HEAD-LEFT-IMG))
+(define HEAD-RIGHT-IMG (flip-horizontal HEAD-LEFT-IMG))
+(define HEAD-UP-IMG (flip-vertical HEAD-DOWN-IMG))
+
+(define ENDGAME-TEXT-SIZE 15)
+
+;; main
+(define (start-snakes)
+  (big-bang (pit (snake "right" (list (posn 1 1)))
+                 (snake "d" (list (posn 1 10)))
+                 (list (fresh-goo)
+                       (fresh-goo)
+                       (fresh-goo)
+                       (fresh-goo)
+                       (fresh-goo)
+                       (fresh-goo))
+                 (list (fresh-obstacle)
+                       (fresh-obstacle))
+                 0)
+            (on-tick next-pit TICK-RATE)
+            (on-pad direct-snakes)
+            (to-draw render-pit)
+            (stop-when dead? render-end)))
+
+(define (next-pit w)
+  (define snake-1 (pit-snake-1 w))
+  (define snake-2 (pit-snake-2 w))
+  (define goos (pit-goos w))
+  (define obstacles (pit-obstacles w))
+  (define dinged (pit-dinged w))
+  (define goo-to-eat-sn1 (can-eat snake-1 goos))
+  (define goo-to-eat-sn2 (can-eat snake-2 goos))
+  (cond [(and goo-to-eat-sn1 goo-to-eat-sn2) ; sn1 and sn2 dinged.
+         (pit (grow-size snake-1 (goo-type goo-to-eat-sn1))
+              (grow-size snake-2 (goo-type goo-to-eat-sn2))
+              (age-goo (eat (eat goos goo-to-eat-sn1) goo-to-eat-sn2))
+              (age-obstacle obstacles) (+ 2 dinged))]
+        [(and goo-to-eat-sn1 (not goo-to-eat-sn2)) ; sn1 dinged.
+         (pit (grow-size snake-1 (goo-type goo-to-eat-sn1))
+              (slither  snake-2)
+              (age-goo (eat goos goo-to-eat-sn1))
+              (age-obstacle obstacles) (+ 1 dinged))]
+        [(and (not goo-to-eat-sn1) goo-to-eat-sn2) ; sn2 dinged.
+         (pit (slither  snake-1)
+              (grow-size snake-2 (goo-type goo-to-eat-sn2))
+              (age-goo (eat goos goo-to-eat-sn2))
+              (age-obstacle obstacles) (+ 1 dinged))]
+        [else ; none dinged.
+         (pit (slither snake-1)
+              (slither snake-2)
+              (age-goo goos)
+              (age-obstacle obstacles) dinged)]))
+
+
+(define (direct-snakes w ke)
+  (cond [(arrow-key? ke) (direct-snake-1 w ke)]
+        [(wasd-key? ke) (direct-snake-2 w ke)]
+        [else w]))
+
+(define (direct-snake-1 w ke)
+  (world-change-dir 1 w ke))
+
+(define (direct-snake-2 w ke)
+  (world-change-dir 2 w ke))
+
+(define (render-pit w)
+  (snake+scene (pit-snake-1 w)
+               (snake+scene (pit-snake-2 w)
+                            (goo-list+scene (pit-goos w)
+                                            (obstacle-list+scene
+                                             (pit-obstacles w) MT-SCENE)))))
+
+(define (dead? w)
+  (define snake-1 (pit-snake-1 w))
+  (define snake-2 (pit-snake-2 w))
+  (define (colliding? sn sn-other)
+    (or (self-colliding? sn)
+        (wall-colliding? sn)
+        (obstacle-colliding? sn (pit-obstacles w))
+        (snake-colliding? sn sn-other)))
+  (or (colliding? snake-1 snake-2) (colliding? snake-2 snake-1)))
+
+(define (render-end w)
+  (overlay (above (text "Game Over" ENDGAME-TEXT-SIZE "black")
+                  (text (string-append "Dinged "
+                                       (number->string (pit-dinged w))
+                                       " goos.")
+                        ENDGAME-TEXT-SIZE "black"))
+           (render-pit w)))
+
+
+;; clock
+(define (can-eat snake goos)
+  (cond [(empty? goos) #f]
+        [else (if (close? (snake-head snake) (first goos))
+                  (first goos)
+                  (can-eat snake (rest goos)))]))
+
+(define (eat goos goo-to-eat)
+  (append (list (fresh-goo)) (remove goo-to-eat goos)))
+
+(define (close? s g)
+  (posn=? s (goo-loc g)))
+
+(define (grow-size sn size)
+  (cond [(= size 0) sn]
+        [else (grow-size (grow sn) (- size 1))]))
+
+(define (grow sn)
+  (snake (snake-dir sn)
+         (cons (next-head sn) (snake-segs sn))))
+
+(define (slither sn)
+  (snake (snake-dir sn)
+         (cons (next-head sn) (all-but-last (snake-segs sn)))))
+
+(define (next-head sn)
+  (define head (snake-head sn))
+  (define dir (snake-dir sn))
+  (cond [(or (string=? dir "up") (string=? dir "w")) (posn-move head 0 -1)]
+        [(or (string=? dir "down") (string=? dir "s")) (posn-move head 0 1)]
+        [(or (string=? dir "left") (string=? dir "a")) (posn-move head -1 0)]
+        [(or (string=? dir "right") (string=? dir "d")) (posn-move head 1 0)]))
+
+(define (posn-move p dx dy)
+  (posn (+ (posn-x p) dx)
+        (+ (posn-y p) dy)))
+
+(define (all-but-last segs)
+  (cond [(empty? (rest segs)) empty]
+        [else (cons (first segs) (all-but-last (rest segs)))]))
+
+(define (age-goo goos)
+  (rot (renew goos)))
+
+(define (renew goos)
+  (cond [(empty? goos) empty]
+        [(rotten? (first goos))
+         (append (fresh-goos) (renew (rest goos)))]
+        [else
+         (append (list (first goos)) (renew (rest goos)))]))
+
+(define (rot goos)
+  (cond [(empty? goos) empty]
+        [else (cons (decay (first goos)) (rot (rest goos)))]))
+
+(define (rotten? g)
+  (zero? (goo-expire g)))
+
+(define (decay g)
+  (goo (goo-loc g) (sub1 (goo-expire g)) (goo-type g)))
+
+(define (fresh-goo)
+  (goo (posn (add1 (random (sub1 SIZE)))
+             (add1 (random (sub1 SIZE))))
+       EXPIRATION-TIME
+       (random 1 3)))
+
+(define (fresh-goos)
+  (define (gen-goos n)
+    (cond [(= n 0) empty]
+          [else (cons (fresh-goo) (gen-goos (- n 1)))]))
+  (let ((n (random 3)))
+    (gen-goos n)))
+
+(define (age-obstacle obstacles)
+  (rot-obstacles (renew-obstacles obstacles)))
+
+(define (renew-obstacles obstacles)
+  (cond [(empty? obstacles)  empty]
+        [(obstacle-expired? (first obstacles))
+         (cons (fresh-obstacle) (renew-obstacles (rest obstacles)))]
+        [else
+         (cons (first obstacles) (renew-obstacles (rest obstacles)))]))
+
+(define (rot-obstacles obstacles)
+  (cond [(empty? obstacles) empty]
+        [else (cons (decay-obstacle (first obstacles))
+                    (rot-obstacles (rest obstacles)))]))
+
+(define (obstacle-expired? obs)
+  (zero? (obstacle-expire obs)))
+
+(define (decay-obstacle obs)
+  (obstacle (obstacle-loc obs) (sub1 (obstacle-expire obs))))
+
+(define (fresh-obstacle)
+  (obstacle (posn (add1 (random (sub1 SIZE)))
+                  (add1 (random (sub1 SIZE))))
+            OBSTACLE-EXPIRATION-TIME))
+
+;; keys
+(define (dir? x)
+  (or (arrow-key? x)
+      (wasd-key? x)))
+
+(define (arrow-key? x)
+  (or (key=? x "up")
+      (key=? x "down")
+      (key=? x "left")
+      (key=? x "right")))
+
+(define (wasd-key? x)
+  (or (key=? x "w")
+      (key=? x "s")
+      (key=? x "a")
+      (key=? x "d")))
+
+(define (world-change-dir sn-number w d)
+  (define snake-1 (pit-snake-1 w))
+  (define snake-2 (pit-snake-2 w))
+  (cond [(and (= sn-number 1) ;snake-1
+              (opposite-dir? (snake-dir snake-1) d)
+              (cons? (rest (snake-segs snake-1))))
+         (stop-with w)]
+        [(and (= sn-number 2) ;snake-2
+              (opposite-dir? (snake-dir snake-2) d)
+              (cons? (rest (snake-segs snake-2))))
+         (stop-with w)]
+        [(= sn-number 1) ;snake-1 change dir.
+         (pit (snake-change-dir snake-1 d)
+              (pit-snake-2 w)
+              (pit-goos w)
+              (pit-obstacles w)
+              (pit-dinged w))]
+        [else ;snake-2 change dir.
+         (pit (pit-snake-1 w)
+              (snake-change-dir snake-2 d)
+              (pit-goos w)
+              (pit-obstacles w)
+              (pit-dinged w))]))
+
+(define (opposite-dir? d1 d2)
+  (cond [(string=? d1 "up") (string=? d2 "down")]
+        [(string=? d1 "down") (string=? d2 "up")]
+        [(string=? d1 "left") (string=? d2 "right")]
+        [(string=? d1 "right") (string=? d2 "left")]
+        [(string=? d1 "w") (string=? d2 "s")]
+        [(string=? d1 "s") (string=? d2 "w")]
+        [(string=? d1 "a") (string=? d2 "d")]
+        [(string=? d1 "d") (string=? d2 "a")]))
+
+
+;; render
+(define (snake+scene snake scene)
+  (define snake-body-scene
+    (img-list+scene (snake-body snake) SEG-IMG scene))
+  (define dir (snake-dir snake))
+  (img+scene (snake-head snake)
+             (cond [(or (string=? "up" dir) (string=? "w" dir))
+                    HEAD-UP-IMG]
+                   [(or (string=? "down" dir) (string=? "s" dir))
+                    HEAD-DOWN-IMG]
+                   [(or (string=? "left" dir) (string=? "a" dir))
+                    HEAD-LEFT-IMG]
+                   [(or (string=? "right" dir) (string=? "d" dir))
+                    HEAD-RIGHT-IMG])
+             snake-body-scene))
+
+(define (goo-list+scene goos scene)
+  (define (get-posns-from-goo goos type)
+    (cond [(empty? goos) empty]
+          [(= (goo-type (first goos)) type)
+           (cons (goo-loc (first goos))
+                 (get-posns-from-goo (rest goos) type))]
+          [else (get-posns-from-goo (rest goos) type)]))
+  (img-list+scene (get-posns-from-goo goos 1) GOO-IMG
+                  (img-list+scene (get-posns-from-goo goos 2)
+                                  GOO-RED-IMG scene)))
+
+(define (obstacle-list+scene obstacles scene)
+  (define (get-posns-from-obstacle obstacles)
+    (cond [(empty? obstacles) empty]
+          [else (cons (obstacle-loc (first obstacles))
+                      (get-posns-from-obstacle (rest obstacles)))]))
+  (img-list+scene (get-posns-from-obstacle obstacles)
+                  OBSTACLE-IMG scene))
+
+(define (img-list+scene posns img scene)
+  (cond [(empty? posns) scene]
+        [else (img+scene
+               (first posns)
+               img
+               (img-list+scene (rest posns) img scene))]))
+
+(define (img+scene posn img scene)
+  (place-image img
+               (* (posn-x posn) SEG-SIZE)
+               (* (posn-y posn) SEG-SIZE)
+               scene))
+
+
+;; end game
+(define (self-colliding? snake)
+  (cons? (member (snake-head snake) (snake-body snake))))
+
+(define (wall-colliding? snake)
+  (define x (posn-x (snake-head snake)))
+  (define y (posn-y (snake-head snake)))
+  (or (= 0 x) (= x SIZE)
+      (= 0 y) (= y SIZE)))
+
+(define (obstacle-colliding? snake obstacles)
+  (cond [(empty? obstacles) #f]
+        [(posn=? (snake-head snake)
+                 (obstacle-loc (first obstacles))) #t]
+        [else (obstacle-colliding? snake (rest obstacles))]))
+
+(define (snake-colliding? snake snake-other)
+  (define (head-in-other sn other)
+    (cond [(empty? other) #f]
+          [(posn=? sn (first other)) #t]
+          [else (head-in-other sn (rest other))]))
+  (head-in-other (snake-head snake) (snake-segs snake-other)))
+
+;; aux
+(define (posn=? p1 p2)
+  (and (= (posn-x p1) (posn-x p2))
+       (= (posn-y p1) (posn-y p2))))
+
+(define (snake-head sn)
+  (first (snake-segs sn)))
+
+(define (snake-body sn)
+  (rest (snake-segs sn)))
+
+(define (snake-tail sn)
+  (last (snake-segs sn)))
+
+(define (snake-change-dir sn d)
+  (snake d (snake-segs sn)))