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(require '[clojure.string :as str])
(def input (->> (slurp "./in")
str/split-lines
((juxt
#(let [init-polymer (first %)]
(for [i (range 0 (dec (count init-polymer)))]
(subs init-polymer i (+ i 2))))
(fn [lines]
(->> lines
(drop 2)
(map #(str/split % #" -> "))
(map (fn [[[a b] c]]
{(str/join [a b]) (map str/join [[a c] [c b]])}))
(reduce into)
))))))
(def blank-map (zipmap (keys (second input)) (repeat 0)))
(defn grow-polymer [polymer insertion-rules]
(reduce
#(let [[p1 p2] (insertion-rules (key %2)) v (val %2)]
(-> %1 (update p1 + v) (update p2 + v)))
blank-map
(filter (comp pos? val) polymer)))
(def growth-seq
(iterate #(grow-polymer % (second input))
(reduce #(update %1 %2 inc) blank-map (first input))))
(let [final-polymer (nth growth-seq 40)
letter-counts (reduce
(fn [r [k v]] (-> r (update (first k) + v) (update (second k) + v)))
(zipmap (str/join (keys final-polymer)) (repeat 0))
final-polymer)
unique-counts (filter pos? (map #(Math/ceil (/ (val %) 2)) letter-counts))]
(println (- (apply max unique-counts) (apply min unique-counts))))
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