aoc2021-day14b Clojure/Babashka program

; SPDX-License-Identifier: MIT
; Copyright (C) 2021 Tito Sacchi <tito@tilde.team>
; WARNING: These solutions were written while I was still learning Clojure and
; should by no means be taken as examples of good programming practice or fast
; implementations.

(ns aoc.2021.14b
  (:require
    [clojure.string :as str]
    [clojure.java.io :as io]))

(defn parse-rule [line]
  (let [[in out] (str/split line #" -> ")]
    [(vec in) (first out)]))
(def input *in*)
(def reader (io/reader input))
(def polymer-template (first (line-seq reader)))
(def insertion-rules
  (persistent!
    (reduce #(conj! %1 (parse-rule %2))
            (transient {})
            (rest (line-seq reader)))))

; Transducer that construct sliding windows of size 2 with step 1
(defn adjacent-pairs []
  (fn [xf]
    (let [l (volatile! nil)]
      (fn
        ([]         (xf))
        ([result]   (xf (xf result [@l])))
        ([result x] (let [l- @l]
                      (vreset! l x)
                      (if l- (xf result [l- x]) result)))))))
(defn apply-rule [[a b]]
  (let [c (insertion-rules [a b])]
    (if c [a c] [a])))
(def substitute-polymers
  (comp
    (adjacent-pairs)
    (mapcat apply-rule)))

(def evolve-pair
  (memoize
    (fn [input n]
      (let [notlast (drop-last input)
            freqs   (frequencies notlast)]
        (if (= n 0)
          [freqs]
          (into [freqs]
                (apply mapv (partial merge-with +)
                       (for [pair (eduction
                                    (comp substitute-polymers (adjacent-pairs))
                                    input)]
                         (evolve-pair pair (dec n))))))))))
(defn evolve-string [input n]
  (apply mapv
         (partial merge-with +)
         (map #(evolve-pair % n) (eduction (adjacent-pairs) input))))

(println
  (let [starting (conj (vec polymer-template) ::none)
        final    (peek (evolve-string starting 40))
        freqs    (vals final)]
    (- (apply max freqs) (apply min freqs))))
    

NOTES:
Linux


Sun, 23 Jan 2022 21:52:53 GMT

COMMAND LINE:
bb -f aoc2021_day14b.clj_babashka-1.clj_babashka 0 < aoc2021_day14b-input2.txt

PROGRAM OUTPUT:
2265039461737