do day 9

2025/src/day09.clj

@@ -0,0 +1,55 @@
+(ns day09
+  (:require input-manager))
+
+(def input (->> (input-manager/get-input 2025 9)
+                (mapv #(mapv parse-long (re-seq #"\d+" %)))))
+
+(defn area [[ax ay] [bx by]]
+  (* (inc (abs (- ax bx)))
+     (inc (abs (- ay by)))))
+(def furthest-points
+  (->> (for [a input
+             b input
+             :when (not= a b)]
+         #{a b})
+       distinct
+       (map vec)
+       (sort-by #(apply area %))))
+
+;; part 1
+(->> furthest-points
+     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"
+  [[ax ay] [bx by] [cx cy]]
+  (normalize (- (* (- by ay) (- cx bx))
+                (* (- bx ax) (- cy by)))))
+
+(defn intersect? [[ax ay] [bx by]]
+  (let [o1 (orientation ax ay bx)
+        o2 (orientation ax ay by)
+        o3 (orientation bx by ax)
+        o4 (orientation bx by ay)]
+    (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]]
+                                        (and (not= a seg-a)
+                                             (not= a seg-b)
+                                             (not= b seg-a)
+                                             (not= b seg-b)))
+                                      line-segments)
+                     [[cx cy] [dx dy]] [a b]]
+                 (and (not (some #(intersect? [a b] %) segments))
+                      (not (some #(intersect? [[cx dy] [dx cy]] %) segments))))))
+     (sort-by (partial apply area))
+     last
+     (apply area))
+