spacing

2025/src/day09.clj

@@ -21,30 +21,31 @@
      last
      (apply area))
 
-;; wikipedia maths
 (defn normalize [x] (min 1 (max -1 x)))
 (defn orientation
-  "returns a value indicating if point A is to the left or right (or on) BC"
+  "returns a value indicating if point A is to the left, right, or on the line BC"
   [[ax ay] [bx by] [cx cy]]
   (normalize (- (* (- by ay) (- cx bx))
                 (* (- bx ax) (- cy by)))))
 
-(defn intersect? [[ax ay] [bx by]]
+(defn intersect?
-  (let [o1 (orientation ax ay bx)
+  "Lines AB and CD intersect if points A and B falls on different sides of CD
-        o2 (orientation ax ay by)
+  and if C and D fall on different sides of AB"
-        o3 (orientation bx by ax)
+  [[a b] [c d]]
-        o4 (orientation bx by ay)]
+  (let [o1 (orientation a b c)
+        o2 (orientation a b d)
+        o3 (orientation c d a)
+        o4 (orientation c d b)]
     (and (not= o1 o2) (not= o3 o4))))
 
 ;; part 2
 (def line-segments (map vector input (conj (vec (rest input)) (first input))))
 (->> furthest-points
      (filter (fn [[a b]]
-               (let [segments (filter (fn [[seg-a seg-b]]
+               (let [segments (filter (fn [[seg-a seg-b]] (and (not= a seg-a)
-                                        (and (not= a seg-a)
+                                                               (not= a seg-b)
-                                             (not= a seg-b)
+                                                               (not= b seg-a)
-                                             (not= b seg-a)
+                                                               (not= b seg-b)))
-                                             (not= b seg-b)))
                                       line-segments)
                      [[cx cy] [dx dy]] [a b]]
                  (and (not (some #(intersect? [a b] %) segments))