Discussion on: AoC Day 5: Alchemical Reduction

(ns aoc.aoc5)

(defn polymer-drop [[c1 c2]]
  (cond
    (= c1 c2) false
    (or (nil? c1) (nil? c2)) false
    (= (Character/toLowerCase c1) (Character/toLowerCase c2)) true
    :else false))

(defn shrink [input]
  (loop [shrunk [] chars-to-test (take 2 input) left (drop 2 input)]
    (cond
      (and (empty? left) (every? nil? chars-to-test)) (apply str shrunk)
      (nil? (first chars-to-test)) (recur shrunk [(last chars-to-test) (first left)] (rest left))
      (polymer-drop chars-to-test) (if (empty? shrunk)
                                     (recur shrunk (take 2 left) (drop 2 left))
                                     (recur (pop shrunk) [(last shrunk) (first left)] (rest left)))
      :else (recur (conj shrunk (first chars-to-test)) [(last chars-to-test) (first left)] (rest left)))))

(defn remove-char
  "Remove all instances of a character (case-insensitive)
   from a string"
  [string chr]
  (apply str (remove #(or (= % (Character/toUpperCase chr)) (= % (Character/toLowerCase chr))) string)))

(defn char-range
  [start end]
  (map char (range (int start) (inc (int end)))))

(defn find-shortest-polymer [input-string]
  (apply min (pmap #(-> input-string
                        (remove-char %)
                        (shrink)
                        (count)) (char-range \a \z))))