aoc24_d11

d11.clj

(require '[clojure.string :as string])

(def blink
  (memoize
   (fn [times n]
     (if (zero? times)
       1
       (let [l (count n)
             l-half (bit-shift-right l 1)
             even? (= (+ l-half l-half) l)]
         (cond
           (= n "0")
           (blink (dec times) "1")

           even?
           (let [sl (subs n 0 l-half)
                 sr (string/replace-first (subs n l-half) #"^0+" "")
                 sr (if (zero? (count sr)) "0" sr)]
             (+ (blink (dec times) sl)
                (blink (dec times) sr)))

           :else
           (blink (dec times) (str (*' (parse-long n) 2024)))))))))

(defn solve [times numbers]
  (transduce
   (comp (map str)
         (map #(blink times %)))
   +
   numbers))

(comment

  (time
   (dotimes [_ 100000]
     (solve 25 '(125 17))))

  (time
   (dotimes [_ 100000]
     (solve 75 '(64599 31 674832 2659361 1 0 8867 321)))))