From 1047a8d0c88fc686fb3aba8495167b97a80bab8d Mon Sep 17 00:00:00 2001
From: Leah Neukirchen <leah@vuxu.org>
Date: Fri, 25 Dec 2020 14:35:55 +0100
Subject: day24

---
 day24.clj | 57 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 57 insertions(+)
 create mode 100644 day24.clj

(limited to 'day24.clj')

diff --git a/day24.clj b/day24.clj
new file mode 100644
index 0000000..0e57d2a
--- /dev/null
+++ b/day24.clj
@@ -0,0 +1,57 @@
+(def data
+  (->> (slurp "day24")
+       (clojure.string/split-lines)))
+
+;     nw ne        1,-1   1,0
+;    w  .  e     0,-1  0,0  0,1
+;     sw se        -1,0  -1,1
+
+(def dirs
+  {"e"  [0 1]
+   "se" [-1 1]
+   "sw" [-1 0]
+   "w"  [0 -1]
+   "nw" [1 -1]
+   "ne" [1 0]})
+
+(defn pos [l]
+  (->> l
+       (re-seq #"e|se|sw|w|nw|ne")
+       (map dirs)
+       (apply (partial mapv +))))
+
+(def black
+  (set
+   (map first (filter (fn [[k v]] (odd? v)) (frequencies (map pos data))))))
+
+(count black)
+;; => 386
+
+
+(defn all-neighbours [cell]
+  (for [offset (vals dirs)]
+    (map + offset cell)))
+
+(defn alive-neighbours [cells cell]
+  (filter cells (all-neighbours cell)))
+
+(defn dead-neighbours [cells cell]
+  (filter (complement cells) (all-neighbours cell)))
+
+(defn regulate [cells]
+  (filter #(#{1 2} (count (alive-neighbours cells %))) cells))
+
+(defn dead-neighbour-cells [cells]
+  (reduce clojure.set/union (map (partial dead-neighbours cells) cells)))
+
+(defn reproduce [cells]
+  (filter #(= 2 (count (alive-neighbours cells %))) (dead-neighbour-cells cells)))
+
+(defn tick [cells] 
+  (clojure.set/union (set (reproduce cells)) (set (regulate cells))))
+
+(->> black
+     (iterate tick)
+     (#(nth % 100))
+     count)
+;; => 4214
-- 
cgit 1.4.1