From 64c9fd7b88b8465ce1a84c4f8fead01b467739ff Mon Sep 17 00:00:00 2001
From: Daniel Barlow <dan@telent.net>
Date: Sun, 22 Jan 2023 22:05:14 +0000
Subject: [PATCH] handle north pole

---
 rover.fnl | 73 +++++++++++++++++++++++++++++++++++--------------------
 1 file changed, 47 insertions(+), 26 deletions(-)

diff --git a/rover.fnl b/rover.fnl
index 14ccabd..f2335b3 100644
--- a/rover.fnl
+++ b/rover.fnl
@@ -29,39 +29,50 @@
       :w {:x (wrap-longitude (- x distance))}
       :e {:x (wrap-longitude (+ x distance))})))
 
-(fn turn-left [direction]
-  {:direction
-   (-> {
-        :n :w
-        :w :s
-        :s :e
-        :e :n
-        }
-       (. direction))})
+(fn turn-left [{: x : y : direction}]
+  (if (= y 90)
+      {:x (wrap-longitude (- x 90))}
+      {:direction
+       (-> {
+            :n :w
+            :w :s
+            :s :e
+            :e :n
+            }
+           (. direction))}))
 
-(fn turn-right [direction]
-  {:direction
-   (-> {
-        :n :e
-        :e :s
-        :s :w
-        :w :n
-        }
-       (. direction))})
+(fn turn-right [{: x : y : direction}]
+  (if (= y 90)
+      {:x (wrap-longitude (+ x 90))}
+      {:direction
+       (-> {
+            :n :e
+            :e :s
+            :s :w
+            :w :n
+            }
+           (. direction))}))
 
 (fn command [rover string]
   (merge rover
          (match string
            :f (drive rover 1)
            :b (drive rover -1)
-           :r (turn-right rover.direction)
-           :l (turn-left rover.direction)
+           :r (turn-right rover)
+           :l (turn-left rover)
            _ (assert false (. "unrecognised command " string)))))
 
+(fn fudge-for-pole [{: y &as r}]
+  (if (= y 90)
+      (merge r {:direction :s})
+      (= y -90)
+      (merge r {:direction :n})
+      r))
+
 (fn execute [rover cmds]
   (accumulate [rover rover
                _ c (ipairs cmds)]
-    (merge rover (command rover c))))
+    (merge rover (fudge-for-pole (command rover c)))))
 
 (fn rover [x y direction]
   {: x
@@ -155,10 +166,20 @@
         (execute (rover 179 0 :e) [:f :f :f])
         {:x -178 :y 0 :direction :e})
 
-"if we travel west past x=-180, x becomes positive 180"
-"if we travel east past x=180, x becomes -180"
-"if we travel east/west at a non-zero latitude, we don't move a full unit"
-"if we arrive at the north pole, we are {:y 90 :direction :s}"
-"if we arrive at the north pole and turn 90 degrees left, x = x - 90"
+(expect "At the North Pole we always point south"
+        (execute (rover 0 89 :n) [:f])
+        {:x 0 :y 90 :direction :s})
+
+(expect "At the North Pole, turning left affects x not direction"
+        (execute (rover 0 89 :n) [:f :l])
+        {:x -90 :y 90 :direction :s})
+
+(expect "At the North Pole, turning right affects x not direction"
+        (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})
 
 (print "OK")