implement collision detection

rover.fnl

@@ -1,21 +1,23 @@
 (local { : view } (require :fennel))
-(local { : merge } (require :lume))
+(local { : merge : any } (require :lume))
 
 (fn wrap-longitude [x]
   (if (<= x -180) (wrap-longitude (+ x 360))
       (< 180 x)   (wrap-longitude (- x 360))
       x))
 
-(fn drive [{: x : y : direction} distance]
-  (let [distance
-        (if (. {:w true :e true} direction)
-            (/  distance (math.cos (/ (* math.pi y) 180)))
-            distance)]
-    (match direction
-      :n {:y (+ y distance)}
-      :s {:y (- y distance)}
-      :w {:x (wrap-longitude (- x distance))}
-      :e {:x (wrap-longitude (+ x distance))})))
+(fn drive [{: x : y : direction : stopped &as r} distance]
+  (if stopped
+      r
+      (let [distance
+            (if (. {:w true :e true} direction)
+                (/  distance (math.cos (/ (* math.pi y) 180)))
+                distance)]
+        (match direction
+          :n {:y (+ y distance)}
+          :s {:y (- y distance)}
+          :w {:x (wrap-longitude (- x distance))}
+          :e {:x (wrap-longitude (+ x distance))}))))
 
 (fn turn-left [{: x : y : direction}]
   (if (= y 90)
@@ -41,14 +43,22 @@
             }
            (. direction))}))
 
+(fn at-obstacle? [{: x : y : obstacles}]
+  (any obstacles
+       (fn [[ox oy]] (and (= ox x) (= oy y)))))
+
 (fn command [rover string]
-  (merge rover
-         (match string
-           :f (drive rover 1)
-           :b (drive rover -1)
-           :r (turn-right rover)
-           :l (turn-left rover)
-           _ (assert false (. "unrecognised command " string)))))
+  (let [next-move
+        (merge rover
+               (match string
+                 :f (drive rover 1)
+                 :b (drive rover -1)
+                 :r (turn-right rover)
+                 :l (turn-left rover)
+                 _ (assert false (. "unrecognised command " string))))]
+    (if (at-obstacle? next-move)
+        (merge rover {:stopped true})
+        next-move)))
 
 (fn fudge-for-pole [{: y &as r}]
   (if (= y 90)
@@ -62,10 +72,11 @@
                _ c (ipairs cmds)]
     (merge rover (fudge-for-pole (command rover c)))))
 
-(fn rover [x y direction]
+(fn rover [x y direction ?obstacles]
   {: x
    : y
    : direction
+   :obstacles (or ?obstacles [])
    })
 
 ;;;; TESTS
@@ -179,9 +190,10 @@
         (execute (rover 0 89 :n) [:f :r])
         {:x 90 :y 90 :direction :s})
 
-(expect "Near the North Pole, I can travel 90 degree in three steps"
-        (execute (rover 0 89 :n) [:f :r :f])
-        {:x 90 :y 89 :direction :s})
+(expect "It does not move past obstacles"
+        (let [obstacles [[5 5] [7 5]]]
+          (execute (rover 5 3 :n obstacles) [:f :f :f :f :f :f]))
+        {:x 5 :y 4 :direction :n :stopped true})
 
 
 ;; "TODO: deal with the south pole special casing"